Capacitors, including electrolytic and ceramic types, are among the most commonly used and easily aged components in electronic devices. A faulty capacitor can cause minor malfunctions or even serious damage, such as burning out the motherboard or power supply.

For beginners, mastering scientific capacitor testing methods—using simple tools like a multimeter or LCR meter—can quickly locate faults and avoid the costly mistake of blindly replacing components.

This guide provides step-by-step instructions on capacitor testing for beginners, helping you identify capacitor problems efficiently and safely—whether you are repairing electronics at home or in a professional workshop.

Before You Test a Capacitor: Safety First

When learning how to test capacitors, safety must come first. Always ensure the capacitor is fully discharged to prevent residual charge from causing electric shock or damaging your multimeter.

Disconnect at least one lead of the capacitor from the circuit to avoid interference from parallel paths. For polarized capacitors (electrolytic or tantalum), double-check the positive (+) and negative (−) terminals to prevent damage. Large-capacity capacitors can be discharged through a resistor, while small capacitors can be safely shorted with a wire.

Never apply voltage exceeding the capacitor’s rated voltage, as this can cause bursting, electrolyte leakage, or fire. When handling high-voltage or high-capacitance capacitors, wear protective gear such as insulated gloves and safety goggles to ensure safe testing.

How to Test Capacitors: Visual Inspection Method (Fast & Beginner-Friendly)

Visual inspection is the fastest way to test capacitors without tools and is widely used as the first step in capacitor testing. It helps quickly identify visible damage in electrolytic, ceramic, and film capacitors, making it ideal for beginners and initial troubleshooting. A capacitor is generally considered normal if it has an intact casing, no bulging, no leakage, clean leads without oxidation, and clear printed specifications. However, visual inspection only checks external condition and cannot confirm full functionality.

Common signs of a bad capacitor include bulging tops, broken vents, or leakage in electrolytic capacitors, and cracked casing, loose leads, or severe oxidation in all capacitor types. These defects indicate internal failure or physical damage, and the capacitor should be replaced immediately. Keep in mind that this method cannot detect issues such as capacitance loss, high ESR, or leakage current, so it should be combined with multimeter testing or LCR measurement for accurate results.

How to Test a Capacitor with a Digital Multimeter (DMM)

Testing capacitors with a digital multimeter is a simple and reliable method for checking capacitor health in electronic circuits. This method works for electrolytic, ceramic, and film capacitors and is suitable for both beginners and electronics enthusiasts.

Capacitance Mode (Precise Measurement)

This is the most accurate and commonly used method for testing capacitors. It directly measures capacitance and works for electrolytic, ceramic, film, and tantalum capacitors.

1. Fully discharge the capacitor to avoid electric shock and protect the multimeter.
2. Set your digital multimeter (DMM) to capacitance mode (F, μF, nF).
3. Connect the probes: red to positive (+), black to negative (−) for polarized capacitors.
4. Read the capacitance value and compare it with the rated capacitance:

• Value close to the rated capacitance → capacitor is good
• Significant deviation → capacitor may be faulty

⚠️ Capacitance mode provides accurate measurements and is recommended for precise capacitor testing.

Resistance (Ω) Mode (Quick Check)

Use this method when your multimeter does not have capacitance mode. It cannot measure capacitance but can detect faults.

1. Fully discharge the capacitor.
2. Set the DMM to high resistance mode (×1k or ×10k Ω).
3. Connect the probes and observe the resistance:

• Resistance rises from low to high → capacitor is good
• Constant low → capacitor is shorted
• Constant high or no change → capacitor is open or degraded

⚠️ Resistance mode is suitable for rough checks only and cannot measure exact capacitance. It is useful for quickly identifying faulty capacitors.

How to Test Capacitors Using a Digital Bridge (LCR Meter)

A digital bridge (LCR meter) provides the most accurate way to test capacitors, measuring not only capacitance but also ESR (Equivalent Series Resistance) and dissipation factor. This method is ideal for professional diagnostics, detecting hidden faults in electrolytic, ceramic, film, and tantalum capacitors, and is especially useful in high-frequency circuits like switching power supplies and motherboards, where capacitor performance is critical.

Steps to Test a Capacitor with a Digital Bridge

1. Discharge the Capacitor – Ensure the capacitor is fully discharged to prevent damage or inaccurate readings.
2. Set the LCR Meter – Select capacitance mode and choose an appropriate test frequency (e.g., 100Hz or 1kHz).
3. Connect the Capacitor – Attach the capacitor to the meter’s test terminals, ensuring correct polarity for polarized capacitors.
4. Read and Interpret Results ：

• Capacitance within tolerance → capacitor is normal
• High ESR or unstable readings → capacitor is degraded or damaged

Compared with multimeter testing, a digital bridge provides more precise and comprehensive analysis, making it the preferred choice for advanced troubleshooting and quality inspection.

FAQ – How to Test Capacitors

Q1: Can I test a capacitor without a multimeter?
A: You can do a rough visual check for bulging, leakage, cracks, or corrosion, but a multimeter or ESR tester is needed for accurate testing.

Q2: How do I safely discharge a high-voltage capacitor?
A: Use a series resistor, insulated gloves, and tools; never short high-voltage capacitors directly.

Q3: What is ESR and why is it important?
A: ESR (Equivalent Series Resistance) affects capacitor performance and can cause overheating or circuit failure even if capacitance seems normal.

Q4: Can I reverse the probes on a polarized capacitor?
A: No; always connect red → positive (+), black → negative (−) to avoid damage.

Q5: Why does my multimeter show “0” or “OL”?
A: “0” means a shorted capacitor, “OL” means open or not charging; both indicate a faulty capacitor.

The post How to test capacitors? 4 practical capacitor testing methods that even beginners can learn (super detailed) appeared first on Dongguan Xuanxuan Electrolytic Technology Co,.ltd.

This type of circuit automatically generates rectangular pulses after the power is switched on, without requiring an external trigger pulse, such as the sequential LED multivibrator circuit shown in Figure 1.1. It also works using the RC charging principle: once the power supply VCC is connected, due to the inherent differences in the components, one transistor will always conduct first. If transistor VT3 conducts first, then the potential at the base of VT3 (VB3) rises → the potential at the collector of VT3 (Vc3) falls → VD1 is forward biased and emits light → the potential at the positive terminal (left side) of capacitor C1 approaches zero. Because the voltage across capacitor C1 cannot change instantaneously → the potential at the base of VT4 approaches zero potential → VT4 is in the cutoff state and the potential at its collector (Vc4) is high → VD2 is in the cutoff state and does not emit light, as shown in Figure 1.2.

As the power supply VCC charges capacitor C1 through resistor R2, the potential at the base of VT4 (VB4) rises, causing VT4 to conduct. This causes the potential at the collector of VT4 (Vc4) to decrease, and VD2 conducts and emits light. Similarly, because the voltage across capacitor C2 cannot change instantaneously, the potential at the base of VT3 decreases, causing VT3 to turn off. VT3 is in the cutoff state, and the potential at its collector (Vc3) is high, causing VD1 to be in the cutoff state and extinguished, as shown in Figure 1.3.

This cycle continues, with VT3 and VT4 alternately turning on and off, causing VD1 and VD2 to repeatedly light up. By changing the capacitance of C1 and C2 (or the resistances R2 and R3), the on/off cycle speed of the two LEDs can be adjusted. The oscillation period of the circuit is approximately 1.4_Ttimes the RC time constant. This behavior directly reflects the capacitor charging and discharging characteristics.

We performed the simulation using the circuit parameters shown in Figure 1.4.

   Figure 1.3. Circuit state after transistor VT4 is turned on.

Figure 1.4 RC multivibrator circuit

The output waveform of the oscillating circuit is shown in Figure 1.5 (the resonant frequency is approximately 3.9 Hz).

Figure 1.5 Output waveform of the oscillating circuit

We will rearrange the layout of the circuit shown in Figure 1.4, as shown in Figure 1.6.

Figure 1.6 RC multivibrator circuit after layout modification

It is essentially just two inverters connected in series through two RC circuits. We can also use two inverters to build an oscillator, whose basic structure is shown in Figure 1.7. The circuit operation relies on the capacitor charging and discharging characteristics of the RC networks to generate the oscillation.

Figure 1.7 Multivibrator circuit constructed using inverter gates.

Working principle of RC multivibrator circuit explained in detail, showing how capacitor charging and discharging with positive feedback generates square wave oscillations for LED running lights and electronic circuit design. Click here for more information about capacitors  https://xuansncapacitor.com

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Width spread refers to the change in the width of a rolled piece during the rolling process, also known as transverse spread. It affects the non-uniformity of deformation in the width direction. There are two methods to represent width spread: absolute width spread and relative width spread. Absolute width spread is the difference in width of the rolled piece before and after rolling, i.e., ∆b = b₁ – b₀; relative width spread is the ratio of the difference in width of the rolled piece before and after rolling to the width before rolling, i.e., ∆b/b₀. This article uses the capacitor material metal produced by XUANSN as an example for explanation.

1.1 Components of Width Expansion

Width expansion is a complex deformation process, comprising the following three components:

1.1.1 Slip Width Expansion. When a rolled piece is compressed upwards, if there is no friction at the contact surfaces at both ends, the metal will slide in various directions; transverse sliding constitutes slip width expansion.

1.1.2 Lateral Transfer Width Expansion If the frictional force on the contact surfaces is high, sliding cannot occur between the two contact surfaces, forcing the metal on the side to transfer and become a new contact surface, resulting in lateral transfer widening.

1.1.3 Lateral Deformation Widening. Due to the frictional resistance at the contact surfaces, the metal near the contact surface lags behind the metal farther from the contact surface, resulting in lateral deformation widening.

The sum of these three factors constitutes the entire widening.

In Figure 1, ∆b₁‘, ∆b₁” and ∆b₁”’ represent the slip width, lateral transfer width, and lateral deformation width, respectively. The total width A6 should be the sum of the above three, that is:

                                                                                (1)

Figure 1 Composition diagram of width

1.2 Factors affecting width

Various conditions during rolling affect width. The main influencing factors include reduction, height and width of the rolled piece, roll diameter, chemical composition of the rolled piece and rolls, surface condition of the rolls, rolling temperature, and lubrication conditions during rolling, as shown in Figure 2. As shown in Figure 2, these influencing factors are particularly crucial for controlling the final width spread when processing materials with high dimensional stability requirements, such as capacitor material metals. XUANSN strictly controls these factors during the production process.

Figure 2: Influence of various factors on width spread

a- Influence of processing rate; b- Influence of roll diameter D; c- Influence of workpiece width b; d- Influence of friction coefficient μ; e- Influence of workpiece chemical composition; f- Influence of tension

As shown in the figure, when the workpiece height decreases, the width and length inevitably increase. Therefore, the greater the processing rate during rolling, the greater the width spread of the workpiece. When other conditions are the same, an increase in roll diameter, an increase in bite arc length, and an increase in resistance along the extension direction of the metal result in a corresponding increase in width spread.

The influence of the friction coefficient varies with the shape of the deformation zone. Figure 3 shows two shapes of the deformation zone during rolling. The deformation zone for profile rolling and narrow strip rolling is narrow and long, as shown in Figure 3a; the deformation zone for plate rolling is wide and short, as shown in Figure 3a. As shown in b. When the coefficient of friction increases, the P_y force in the narrow and long deformation zone increases, forcing the metal to expand more to both sides, i.e., the width increases; for the wide and short deformation zone, the P_z force increases, and therefore the width expands less. So when rolling wide plates, the width expands very little and can be ignored.

Figure 3 Rolling Deformation Zone

a-Section and Narrow Strip Rolling; b-Plate Rolling

The chemical composition of the metal and rolls, rolling temperature, and lubrication conditions affect the width spread through their influence on the coefficient of friction, and are determined by the shape of the deformation zone. In actual rolling of high-precision such capacitor material metals, the effects of these factors are even more pronounced.

The front and back tension during rolling is a factor that reduces the width expands. In some cases, the use of tension may result in a negative width expand, which reduces the width of the rolled piece.

1.3 Calculation of Width Expands

There are many formulas for calculating width expands. Here, we only introduce a semi-empirical formula that is widely used in flat roll rolling of non-ferrous metals:

                                                                                            (2)

where C — constant; for aluminum, C = 0.45 at around 400℃.

This formula considers key factors such as the influence of deformation zone length and processing rate on width spread, but does not consider the influence of workpiece width. Therefore, this formula is not applicable to rolling conditions where the workpiece width is equal to or less than its thickness.

1.4 Practical Significance of Width Spread

During flat roll rolling, due to the influence of width and uneven deformation, secondary tensile stress is generated at the edge of the workpiece, and secondary compressive stress is generated in the middle of the workpiece (see Figure 4). The generation of secondary tensile stress is the root cause of edge cracking, increasing geometric scrap loss. The greater the width spread, the greater the scrap loss. To reduce the losses caused by width spread, the following measures can be taken:

Figure 4 Secondary stress generated during flat roll rolling

(1) Reduce the coefficient of friction;

(2) Reduce the billet thickness;

(3) Appropriately distribute the reduction and rolling passes. These measures have been applied in XUANSN’s production practice.

2 Calculation of Rolling Force

2.1 Yield Strength

The yield strength of aluminum and aluminum alloys at which plastic deformation reaches 0.2% is called σ0.2. σ0.2 is an important parameter for calculating the rolling force of aluminum and aluminum alloys, and is particularly crucial in the processing of capacitor material metals where precise control of deformation characteristics is required. Yield strength is related to deformation temperature, deformation rate, and degree of deformation, as shown in Figures 5 and 6. For cold rolling, the main influencing factor is the degree of deformation.

Figure 5: Influence of Deformation Temperature, Deformation Rate, and Degree of Deformation on the Yield Strength of Pure Aluminum a-150℃; b-250℃; c-350℃; d-450℃; e-550℃

Figure 6 Relationship between yield strength and degree of deformation (t=20℃)

1-5A06；2-5A05；3-6A02；4-2A12；5-2A11；6-2A02；7-3A21；8-1035

In uniaxial rolling, the yield strength of the material is the deformation resistance, also known as the forced flow stress of the metal. In the rolling of sheet and strip, since the metal particles are in a triaxial compressive stress state, its forced flow stress is different from the yield limit in uniaxial tension, denoted by K:

                                                                                                 (3)

2.2 Rolling force

Rolling force is a crucial parameter in the design and control of rolling processes and equipment. It serves as an important basis for calculating the strength and elastic deformation of rolls and other mill components, verifying or determining motor power, establishing reduction regimes, achieving automatic control of plate thickness and shape, maximizing equipment potential, and improving labor productivity. Accurate calculation of rolling force is particularly critical when processing materials with high requirements for thickness accuracy and plate shape, such as capacitor material metals.

Rolling force refers to the vertical component of the resultant force exerted by the workpiece on the rolls. As mentioned earlier, during rolling, the metal exerts two forces on the rolls: one is the unit frictional force T tangential to the contact surface; the other is the resultant force N of the unit pressure perpendicular to the contact surface. The rolling force is the sum of the projections of these two forces perpendicular to the rolling direction, P_H (see Figure 7).

Figure 7. Stress analysis diagram under simple rolling conditions

The basic formula for calculating rolling force is:

                                                                                                            (4)

Where F = bl, , then:

                                                                                              (5)

Calculation steps for rolling force:

2.2.1 Calculate the forced flow stress K of the metal:

1) Calculate the average deformation rate

                                                                                                       (6)

• Calculate the degree of deformation

                                                                                                              (7)

3) Based on the obtained ū and ε values, look up the corresponding material values in Figures 5 and 6. σ_0.2.

4) Calculate the corresponding K value according to the formula K=1.15σ_0.2.

2.2.2 Calculate the average unit rolling force P

When calculating the cold rolling force, three factors need to be considered: the degree of work hardening of the material caused by the previous rolling passes; the elastic flattening of the rolls during rolling; and the effect of tension during rolling. For thin materials such as capacitor material metals, these factors have a particularly significant impact on the rolling force and need to be considered in detail.

1) Work hardening caused by processing rate. Since the cold rolling temperature is below the recrystallization temperature of the material, the work hardening generated in each pass cannot be eliminated. Therefore, when calculating the K value, the deformation should include the sum of the deformation of the previous passes, that is:

                                                                                          (8)

                                                                                            (9)

According to ε0 and ε1, the corresponding yield strength σ(0.2)0 and σ(0.2) can be obtained from the figure. 1. Calculation:

                                                                                               (10)

                                                                                               (11)

                                                                                                       (12)

2) The effect of cold rolling elastic flattening on rolling force. During cold rolling, due to the work hardening of the material, the roll produces elastic flattening. The projected length of the contact arc between the elastic flattening roll and the strip is extended from L to L’. Therefore, L’ should be considered when calculating the rolling force. L’ can be obtained with the help of Figure 8.

Figure 8 Calculation of the contact arc length after flattening using the graphical method

(For steel rolls, α=1.06×10^-4R; for cast iron rolls, α=2.06×10^-4R. If the straight line and the intermediate curve scale intersect at two points, the smaller value should be taken; if there is no intersection, it means that the processing rate used in this pass causes the roll flattening to be too large and cannot be rolled.)

Calculation steps of L’:

① Calculate;

② Calculate;

③ Obtain the value of from Figure 8, and calculate the value of L’ from .

3) The effect of cold rolling tension on rolling force. During cold rolling, tension rolling is used, which reduces the average unit rolling force. Since both front and rear tensions affect the rolling force, the average value of front and rear tensions should be considered when calculating cold rolling tension. When rolling thin plates or foils with high requirements for thickness uniformity and plate shape, such as capacitor material metals, tension control is particularly important for stabilizing the rolling force.

Generally, the magnitude of the front and rear tensions used can be determined according to the following empirical formulas depending on the thickness of the rolled piece:

                                                         (13)

                                                      (14)

After considering the three influencing factors for calculating cold rolling force, with the help of Figure 9 Find the pressure multiplier P_MF value, and then calculate the average unit pressure P using the following formula:

                                                                                              (15)

Finally, the cold rolling force is:

Figure 9: Relationship curve between pressure multiplier P_MF and

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                                                          (1)

The deformation condition in the width direction is:

                                                      (2)

Figure 1 Coordinate grid

When an object undergoes uniform deformation, the deformation is not only uniform in the height direction, but also uniform in the width direction and longitudinal direction. Uniform deformation must simultaneously meet the conditions specified in Equations 5-7 and 5-8. In fact, in the plastic deformation of metals, whether it is rolling deformation, forging deformation, extrusion deformation, drawing deformation, etc., the deformation is uneven, which will bring many problems after deformation. In most cases, our work is to study the law of uneven deformation of metals during plastic deformation, improve the conditions of plastic deformation, make it as uniform as possible, reduce the adverse effects of uneven deformation, and thus improve the stability and consistency of the performance of XUANSN capacitors.

1 Main causes of uneven deformation

The main causes of uneven deformation include:

(1) External friction on the contact surface. Due to the action of external friction, the metal flow near the contact surface is difficult, such as causing the cylindrical forging to become a drum shape (see Figure 2). In this case, the deformed metal can be roughly divided into three areas: Zone I represents the difficult deformation area caused by external friction; Zone II represents the easy deformation area at an angle of about 45° to the external force; Zone III represents the free deformation area with a medium degree of deformation. Due to uneven deformation, secondary tensile stress is generated in Zones I and III. However, Zone I is mainly in a triaxial compressive stress state, so the secondary tensile stress in Zone I generally has no obvious effect. In Zone III, the secondary tensile stress changes the stress state, causing tensile stress in the circumferential (tangential) direction. Consequently, longitudinal cracks may form on the side surfaces during upsetting. This uneven deformation caused by external friction can affect the material’s density and structural stability to a certain extent, potentially impacting capacitor performance.

Figure 2: Effect of Friction on Deformation and Stress Distribution During Upsetting

Due to the influence of external friction, the stress distribution on the contact surface is also uneven. During rolling, the friction mechanism of thick rolled pieces is similar to that of forging (see Figure 3). Zone I is the difficult deformation zone and has a conical shape. The metal in this difficult deformation zone does not undergo plastic deformation. Furthermore, the metal at the base of this cone does not slide relative to the roll surface, thus being called the contact zone. Zone II is the plastic deformation zone. This plastic deformation zone extends beyond the geometric deformation zone l. Compressive deformation of the metal progresses gradually from the surface of the rolled piece toward the center. Zones III and IV, due to the presence of rigid ends at both ends, experience longitudinal compression of the rolled piece, increasing in height. This uneven distribution of stress and deformation inside the metal not only affects the forming accuracy, but may also change the microstructure of the material, thereby potentially affecting the stability and consistency of the capacitor performance of XUANSN.

Figure 3 Schematic diagram of deformation during rolling

According to the relationship between the uneven distribution of metal flow velocity during rolling and the rigid ends of the rolled piece at the outlet and inlet, the deformation zone can be divided into seven zones (see Figure 4).

Figure 4 Schematic diagram of deformation zones of rolled piece

1 – rear rigid end zone; 2 – deformation occurrence zone; 3 – rear sliding zone; 4 – bonding zone;

5 – front sliding zone; 6 – deformation reduction zone; 7 – front rigid end zone

As can be seen from the figure, only on a vertical section in the bonding zone is the distribution of metal flow velocity in the height direction equal, and this section is called the neutral plane. Except for the neutral plane and the front and rear rigid end zones, the flow velocity of each layer of metal along the height of the rolled piece is uneven, which inevitably causes uneven deformation inside the metal.

(2)The shape of the deformed object and tool. During the rolling process, a rectangular billet is rolled using a convex roller (see Figure 5). The deformation degree of the edge part a of the rolled piece is small, while the deformation degree of the middle part b is large. Because the rolled piece itself is a whole and its longitudinal extension tends to be consistent, the middle part will apply secondary tensile stress to the edge to reduce its extension. This stress difference also exists in the manufacturing of high-precision electronic components. If the stress distribution of the metal substrate is uneven, it will have an adverse effect on the conductivity and mechanical reliability of the capacitor performance.

Figure 5: The situation of a rectangular billet rolled by a convex roller

(3)The properties of the object are uneven. The chemical composition, structure and temperature distribution of the deformed object are inconsistent, which all affect the uneven distribution of deformation and stress. Taking the uneven temperature distribution as an example, the deformation resistance of the high-temperature part is small, and the deformation resistance of the low-temperature part is large. Under the same external force, the deformation stress generated by these two parts is different, thereby generating secondary stress. In addition, the uneven temperature distribution inside the object will cause uneven expansion, thereby causing secondary thermal stresses that are mutually balanced inside the object. As a result of the superposition of these two secondary stresses, larger secondary stresses may be generated in certain areas of the deformed object, which may cause the material with lower plasticity to break.

2. Possible consequences of uneven deformation and methods of reduction

In order to maintain its integrity, an object that produces uneven deformation generates secondary stresses. These secondary stresses are pairs of internal forces that are mutually balanced in the deformed object. According to the distribution of uneven deformation in the deformed object, secondary stresses can be divided into three types: secondary stresses that are mutually balanced in several large areas (several large parts) or at the macro level of the object are called the first type of secondary stress; secondary stresses that are mutually balanced in individual areas of the deformed object (between several grains) or at the micro level are called the second type of secondary stress; secondary stresses that are mutually balanced between the parts within a grain or at the atomic level are called the third type of secondary stress.

The secondary stresses in the deformed object will have the following effects on the plastic deformation process:

(1)Increase deformation resistance;

(2) Reduce plasticity;

(3) Presence of residual stress, which reduces product quality;

(4) Increase the technical complexity coefficient during the operation process.

 Methods to reduce the impact of uneven deformation include:

(1) Rationally design the shape of processing tools, such as designing hot rolling rollers to a reasonable concavity and cold rolling and rolling to a reasonable convexity, to reduce the degree of uneven deformation of the metal, thereby improving the stability of the capacitor performance of XUANSN.

(2) Minimize the harmful effects of external friction, such as improving the surface finish of the tool and selecting a suitable lubricant.

(3) Select appropriate process parameters, such as deformation temperature, deformation speed, etc.

(4) Uniformly deform the structure and composition of the metal, such as high-temperature homogenization annealing of the ingot and intermediate annealing of the rolled piece.

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During the operation of the DC system, if the AC filter or shunt capacitor tripping and the stage III directly trips due to a fault, the operator should carry out on-site disposal in an orderly manner according to the following points:

(1) After the faulty filter or shunt capacitor tripping in the background of the monitoring interface, check the operation status of other spare filter groups. The duty officer should immediately arrange personnel to go to the protection relay room to check the unbalanced current of the replacement filter (or shunt capacitor) put into operation, and go to the site to check whether the primary equipment of the replacement filter (or shunt capacitor) is operating well, and notify the dispatch duty personnel of the on-site situation according to the dispatch relationship. Generally, when the DC system is running at full load, the converter station will have 1~2 groups of spare (here, the reactive dark spare capacity provided by the system is not considered for the time being) filters (capacitors). Regardless of whether it is running at full load or not, if the filter or shunt capacitor tripping, the operator must apply to the dispatcher to transfer the primary equipment of the filter (shunt capacitor) to the maintenance state for maintenance.

(2) Filter or shunt capacitor maintenance operation, in turn, the filter or shunt capacitor is switched to cold standby (open the isolating switch between the circuit breaker and busbar, and disconnect the operating power of the isolating switch), switched to maintenance (close the grounding switch, usually to reduce the operation, only close the grounding switch between the circuit breaker and the filter, and disconnect the power supply of the grounding switch motor).

(3) After the maintenance operation is completed, the operator needs to complete the safety measures for the maintenance of the filter or shunt capacitor, which mainly include: ① Use an insulating rod with a grounding wire at one end, wear insulating gloves, and discharge the capacitors one by one; ② Set up a three-phase 35kV grounding wire on the low-voltage side of the high-voltage capacitor tower; ③ Set up a fence at the interval leading to the adjacent live filter or shunt capacitor, and hang a sign with the value “Stop, high voltage danger!”; ④ At the entrance of the fence door of the filter to be repaired, hang a sign with the value “Enter and exit from here!” and “Work here!”. After the safety measures are completed, the operating work permit personnel will lead the maintenance work leader to the site to confirm the safety measures and explain the energized status of the surrounding equipment. Both parties will sign and confirm on the work ticket, and the work will be permitted to start.

2.Maintenance and treatment of equipment after tripping

2.1 Preparation before maintenance

Prepare the relevant tools, work vehicles, and materials before maintenance in accordance with the requirements of the “Standardized Operation Guide for Fault Replacement and Adjustment of Capacitors in XX Converter Station” in Appendix B, and place them on site in a unified and standardized manner. According to the relevant action information of the protection device, the fault phase of the capacitor can be confirmed, and the lifting platform can be slowly moved to the side of the capacitor of the fault phase (it is advisable to measure the capacitance value of the capacitor in a more comfortable posture for the staff), and the working platform is reliably grounded with a grounding wire. Finally, the working platform is supported to ensure that the platform is level. At this point, the preparation work before maintenance is completed.

2.2 Maintenance process

(1) Appearance inspection. When dealing with capacitor failure, you should first stand on the work lifting platform and conduct a visual inspection of all capacitors in the phase to be repaired layer by layer to confirm whether there is oil leakage, casing cracks, or severe bulging capacitors. If the above situation occurs, then conduct a focused capacitance test on the above capacitors. If it is confirmed to be a faulty capacitor, replace it. Usually, this method can only preliminarily find some capacitors with obvious abnormalities. It cannot be assumed that all faulty capacitors can be found by only this inspection. Under normal circumstances, oil leakage capacitors, casing cracks, or severe bulging capacitors found by visual inspection only account for 10% to 20% of all faulty capacitors. Most faulty capacitors still need to rely on capacitance value testing methods to be found.

(2) Capacitance value measurement. For the remaining capacitors that have no abnormalities in the appearance inspection, the capacitance value of the faulty phase capacitors can only be measured one by one. The faulty capacitor is determined by whether the capacitance value exceeds the standard and replaced; for any converter station, the faulty capacitors confirmed by this capacitance value measurement method generally account for 80% to 90% of the total number of faulty capacitors. It can be seen that in order to completely find all the faulty capacitors on the tower arm, it is necessary to measure and screen them one by one. To determine whether the capacitance value of the capacitor exceeds the standard, you should refer to the provisions of the change of the capacitance value of the capacitor exceeding the standard in Q/GDW 496-2010 or the manufacturer’s equipment manual, and make a comprehensive comparison with historical data to determine whether the capacitance value of the capacitor exceeds the standard.

In addition, during the inspection process, if the shunt capacitor tripping to found , special attention should be paid to the status of the capacitor. The shunt capacitor tripping may be caused by capacitor failure or unbalanced system load. Therefore, the parallel capacitor must be checked separately to ensure that no capacitor that may cause system failure is missed, and confirm whether it needs to be replaced based on the capacitance value measurement results.

However, in some special cases, such as the phase capacitor, the unbalanced current exceeds the standard and trips frequently (especially the early domestic capacitors often have similar situations), and each time after replacing a few capacitors, the capacitor fails again after a few days of operation. In response to such situations, in actual work, the capacitor capacitance value fault screening is usually carried out according to more stringent requirements than the specification requirements. For example, the specification stipulates that the initial value deviation of the capacitor is qualified within 5%~10%. In practice, the qualified capacitance value range is narrowed to within ±2% (basically, the fuse of the small capacitance element inside the capacitor cannot exceed 1, because too many fuses are blown, which increases the probability of capacitor failure after the next charging and commissioning). By raising the requirements for the capacitance value of qualified capacitors, the quality of maintenance is improved, ensuring that the equipment will not trip after long-term operation after maintenance. However, it should be noted that the shunt capacitor tripping may be affected by the uneven distribution of capacitor capacitance values. Therefore, when screening capacitors, special attention should be paid to these potential problems.

By raising the capacitance value standard of qualified capacitors, it is ensured that the equipment will not trip after long-term operation after maintenance. However, raising the capacitance value standard of qualified capacitors also increases the number of faulty capacitors to be replaced and the workload, so this is also contradictory. In the actual maintenance process, the capacitance value requirements of filter capacitors are generally followed, and the qualified range of the capacitance value of parallel capacitors can be appropriately expanded. In recent years, with the substantial improvement in the quality of domestic capacitors, the -5%~10% standard can basically guarantee the quality of equipment operation after maintenance.

We have described the test method of capacitor capacitance in the previous chapter. Here we introduce the measurement of capacitors on site. In actual field measurement, we face a series of practical problems: measuring capacitance in the complex electromagnetic environment of the converter station, with running filters or parallel capacitor equipment around; the capacitor is measured on the tower with as little or no wire removal as possible (the author’s on-site maintenance experience believes that less wire removal is more convenient than using a capacitor bridge test, and several comparisons have been made. I hope readers will experience it in maintenance practice); the maintenance platform is raised to the top of the capacitor tower and the induction electrode is strong. A series of unfavorable factors, how to better measure should follow the following principles:

1）Before measuring the capacitance value of the capacitor on the capacitor tower with a capacitance meter, remove the leads connecting the high and low voltage sides of the capacitor tower to other equipment according to the position shown in Figure 1, which cuts off the possibility of generating an induction loop and prevents the electromagnetic field of the DC converter station from generating an induced current in the closed loop, causing the capacitance value data to be unable to be measured. The author’s maintenance practice has proved that as long as the four primary wirings in Figure 1 are removed, the other series and parallel series connection lines do not need to be removed, and the capacitance meter can be used to measure directly. The value is accurate and there will be no electronic capacitance meter data jitter. In this case, the risk of shunt capacitor trippingis avoided. The actual position of the high and low voltage leads of the capacitor tower removed on site is shown in Figure 2.

Figure 1 Schematic diagram of the lead wire removal position

Figure 2 Actual position diagram of the lead wire removal
2) When using the above method to measure the capacitance value, the following issues should be noted: When removing the lead, there will be a strong induced electric “arcing” at the moment the lead is disconnected. Special attention should be paid to prevent arc injury. It is best to wear insulating gloves and goggles when disassembling and restoring the lead; In addition, the removal of the high-voltage side lead is actually equivalent to opening the maintenance safety grounding knife switch (from the perspective of safety supervision and inspection, this is not allowed. It can be protected by adding a temporary ground wire to protect the safety of the personnel performing capacitance testing on the top of the capacitor tower). When testing the capacitance value of the capacitor at the bottom layer, the temporary ground wire should also be removed. There is also induced electricity at the bottom layer, which is smaller than the top layer.
3) When the high-voltage and low-voltage leads are disconnected, the capacitance of the bridge arm of C₁, C₂, C₃, and C4 can also be measured with a capacitance meter.

Using the above method, the capacitance test of all capacitors on the capacitor tower can be successfully completed, and the specific location and number of the faulty capacitors can be obtained. It should be noted that after the faulty capacitor is determined, before the capacitor needs to be replaced, the disconnected leads on the high-voltage and low-voltage sides must be restored (or the high voltage still uses a temporary grounding wire). Otherwise, due to the excessive induction (the maintenance equipment has no maintenance safety measures for grounding), the next step of replacement cannot be performed.

Infrared temperature measurement. During daily operation (before the capacitor trips), infrared temperature measurement of capacitor equipment is also a common method to monitor whether there are potential faults in the running capacitor. This requires us to establish a complete equipment infrared temperature measurement information data ledger, strengthen operation and maintenance management, and control unplanned shutdowns. Through the infrared thermometer, abnormal conditions such as overheating of the capacitor body and loose joints can be detected, and equipment hidden dangers can be discovered early to facilitate the arrangement of planned maintenance and prevent equipment defects from turning into shunt capacitor tripping. DC converter stations will be assessed on the unplanned outage indicators of equipment to avoid outages caused by equipment failure.

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The main operating failure of dry-type air-core reactors is insulation breakdown between coil turns. The causes of failure include damp coils or insulation weaknesses, local discharge arc burnout, local overheating insulation breakdown, overvoltage, short circuit caused by small animals, and deformation and damage of coils due to mechanical stress generated by short-circuit current. Similar to common capacitor failures, reactor failures are also closely related to insulation problems. Especially under high voltage conditions, common capacitor failures are similar to insulation breakdown and dielectric degradation of reactors.

1.1 Deterioration of insulation material surface properties

(1) Causes of surface property degradation. This type of defect manifests itself in cracking, powdering, reduced surface properties, and liquid flow of insulation materials at high temperatures. The causes are improper material selection and improper formulation and curing process. The external insulation of dry-type air-core reactors is basically made of epoxy resin with good processability and cured at room or medium temperature. The disadvantage is that the curing speed is closely related to the curing agent, accelerator, and surrounding environment. The same formula and process can produce resin materials with very different performances due to differences in the activity of the curing agent and accelerator, the ambient temperature, and the humidity. Once the curing is not good, there will be a large number of low-molecular genes and incompletely cross-linked molecular chain bonds in the resin material. Under the action of water, light and other substances, they are easily hydrolyzed, re-reacted, and combined, which will lead to cracks and powdering on the insulation surface of the reactor, and the surface performance will be reduced.

Some reactors, especially those cured at room temperature, have defects such as local overheating or poor welds. These resins that seem to have been cured well will flow and re-cure under the action of high temperature (>80℃) [Lingbao Converter Station of Northwest-Central China Network had oily droplets at the bottom of the operating reactor (PLC reactor, operating temperature is about 85℃, as shown in the infrared temperature measurement spectrum in Figure 1) in 2006, when it was put into operation. In fact, it is resin liquid]. Generally speaking, this curing process is permanent and irreversible. Before curing, the resin flows and forms cavitation inside the insulation, which can easily cause inter-turn insulation damage.

Figure 1 PLC reactor infrared temperature measurement spectrum

Most manufacturers have limited means to control the quality of epoxy resin, and it is even more difficult to adjust the formula according to different batches of epoxy resin. Therefore, the materials and formulas are different, which makes it difficult for the epoxy resin to reach the optimal curing state and leave hidden dangers. The poor compatibility of the topcoat with the substrate will also accelerate surface degradation. Similar to common capacitor failures, due to inconsistent materials or unstable quality, performance degradation and shortened life may also occur.

(2) Treatment measures. Cracking, powdering, and surface performance degradation are all superficial degradation phenomena. Once discovered, they should be treated as soon as possible to avoid development and aggravation to form irreversible degradation. The treatment measures are simple. Use sandpaper to polish and remove deteriorated surface materials such as cracks and powdering, and then clean them carefully. It is best to use anhydrous solvents (such as anhydrous ethanol) for cleaning, and then apply paint or coatings with excellent weather resistance and good compatibility with the base material.

For the problem of resin flow caused by overheating, the only way is to strengthen monitoring, pay special attention to the appearance of small round granular resin materials (i.e. “sweating points”) at the overheating point, and contact the manufacturer as soon as possible to avoid the expansion of the fault.

1.2 Leakage Magnetic Field

(1) Causes of Leakage Magnetic Field. There is a grounding grid in the axial position of the reactor, and there are equipment, fences, frames, etc. in the radial position. All of these may cause serious leakage magnetic problems due to the closed loop formed by the metal body, which is difficult to completely solve on site. Generally speaking, there is only a large ferromagnetic material in the magnetic field range, and there is no closed loop. If there is a closed loop, such as a grounding grid, a frame, a metal fence, etc., the leakage magnetic field will induce a circulating current of hundreds of amperes. This will not only increase the loss, but also cause serious problems due to the coupling of the reverse magnetic field established by it with part of the winding of the reactor. If there is a closed loop in the radial position, the reactor winding will overheat or partially overheat, just like the short circuit of the secondary side of the transformer. If there is a closed loop in the axial position, the reactor current will increase and the potential distribution will change. Therefore, the leakage magnetic problem cannot be simply considered as heat generation or increased loss.

(2) Treatment measures. As long as the closed metal loop is eliminated, such as staying away from metal frames and not using metal fences, this problem can generally be solved. The more difficult thing is to avoid closed loops in the grounding grid and cement components. Before installation, check whether the installation point is in a closed grounding grid or a cement component containing a metal closed loop. Similar to the grounding problem in common capacitor failures, a full inspection before installation can effectively avoid potential risks.

1.3 Discharge marks

(1) Causes of discharge marks on the surface of the reactor. There are mainly two types of discharge marks on the surface of the reactor and discharge marks on the insulating support bar. There are certain differences in their generation and harm. The epoxy resin external insulation of the reactor is a hydrophilic material. In rainy and humid weather, a water film is easily formed on the surface, resulting in an increase in surface leakage current. If the material is damp or unevenly contaminated, local dry strips will be generated and the electric field will be concentrated, causing small arcs, which will then destroy the local surface characteristics and gradually develop into a more stable discharge channel. If the material has a low level of tracking resistance, carbonized shallow traces will appear on the insulation surface, causing the electric field to distort. The front end of the trace is more likely to form dry areas and spark discharges, causing a vicious cycle.

On the surface, the discharge trace is superficial insulation damage and does not seem to pose much harm, but in fact it is not. Its harm does not lie in the extent of the damage to the insulation, but because the insulation performance of the discharge trace is far lower than the normal insulation performance (2 to 4 orders of magnitude lower). It not only makes the surface prone to flashover, but also easily causes the potential distribution of the winding to be inconsistent with the surface potential distribution, causing the radial insulation that is basically not subject to voltage to bear a certain voltage, and making the winding prone to inter-turn insulation breakdown.

(2) Treatment measures. For the electric field distribution structure of a general reactor, if the resin material is well cured, its tracking resistance level can reach 1A 2.5 level.

For reactors that have been put into operation or have discharge traces, the leakage current and local current density under humid conditions should be reduced so that the heat generated cannot form a small arc. The specific measures are:

1) Apply hydrophobic paint to increase the resistivity of the reactor surface under humid conditions.

2) Set up several high-resistance bands, which can not only reduce the leakage current, but also cut the leakage current path into several sections to avoid current concentration and prevent arc formation.

3) To avoid leakage current concentration near the electrode and increase the density, set a current-sharing metal shielding ring with the same potential as the electrode and in close contact with the reactor surface to avoid air gap bubble discharge.

Through the above on-site analysis, it can be seen that the operation and maintenance personnel should strengthen the cleaning and maintenance of the reactor during the infrastructure installation or maintenance, and improve the level of the reactor surface resistance to leakage current. Even if the deterioration phenomenon is found, the compatibility of the materials should be paid attention to when using the paint for repair; metal closed loops should be avoided as much as possible around the reactor, and special attention should be paid to the ground grid and cement components that are easily overlooked; similar to the common capacitor failures, the leakage current increases due to the decrease in surface resistance, and the reactor also needs to take corresponding protective measures; even if the discharge traces of dry reactors are found, the leakage current can be reduced, isolated, and divided, the coil height can be increased, the creepage distance can be increased, and hydrophobic paint can be sprayed.

2.Causes and analysis of common capacitor failures

The most common fault of resistors in AC and DC filter devices (no resistors in parallel capacitor devices) is abnormal heating of the body. Defects can be found by infrared temperature measurement of the equipment. Generally, the horizontal comparison method is used. If the temperature difference of the same part of the equipment between different phases (if the resistor has an outer cover, as shown in Figure 2, the temperature of the outer cover can be measured) reaches more than 10°C, it should be monitored more closely, and if it exceeds 20°C, it should be regarded as a serious defect. Since the internal situation of the resistor cannot be directly observed, when the temperature exceeds 20℃, a power outage should be arranged as soon as possible under the condition of sufficient reactive power reserve.

3.Common faults and cause analysis of current transformers

3.1 Overheating of the iron core of the oil-filled current transformer

Current transformers in operation often show signs of abnormal heating of the iron core by infrared temperature measurement (generally lateral comparison temperature measurement, 5~10℃ higher than other normal phase temperatures). Cause analysis: Overheating of the iron core of the current transformer may be caused by long-term overload or open circuit of the secondary circuit, which causes magnetic saturation of the iron core. If the above abnormal phenomenon is found, first observe carefully and judge the cause of the overheating of the iron core through instrument indications. If it is caused by overload, reduce the load to below the rated value and observe its operation. If it is caused by the secondary circuit, stop running immediately (or reduce the load to a minimum), and take necessary safety measures during the processing to prevent electric shock. If discharge is caused by insulation damage, it should be replaced. Similar to the insulation problem in common capacitor failures, overheating of current transformers is often related to insulation damage or excessive load.

Figure 2 Resistors with metal covers

3.2 Abnormal sound during operation of oil-filled current transformers

Current transformers in operation will cause abnormal sound when overloaded, open circuited in the secondary circuit, discharge due to insulation damage, or suspended potential discharge due to poor contact of the end screen. Local corona caused by uneven application of semiconductor paint and loose bolts clamping the core will also cause loud noises. If abnormal sound occurs, the cause of the abnormal sound should be correctly determined and the correct treatment method should be taken in time. Careful observation should be made and the cause of the abnormal sound should be determined by instrument indications. If it is caused by overload, the load should be reduced to below the rated value and its operation should be observed. If it is caused by the secondary circuit, the operation should be stopped immediately (or the load should be reduced to a minimum), and necessary safety measures should be taken during the handling process to prevent electric shock. If the discharge is caused by insulation damage, it should be replaced. If the suspended potential discharge is caused by poor contact of the end screen, the current transformer needs to be powered off and the end screen contact connection should be tightened.

3.3 The oil level of the oil-filled current transformer is low

Generally, the operation and maintenance personnel will consider the oil level to be normal as long as they see the oil level scale. When the oil level of the current transformer is low, they should first observe whether there is oil leakage on the ground and around the transformer. Secondly, use an infrared thermometer to detect whether the current transformer body and iron core with low oil level are overheated (if there is no power outage plan for maintenance in a short time, the infrared temperature measurement cycle should be shortened and follow-up testing should be strengthened). Finally, check whether the secondary circuit current of related protection devices and instruments is normal. If any or several of the above inspection items are abnormal, the filter should be replaced in time to repair the transformer. Similar to the low oil level and overheating problems in common capacitor failures, timely detection and maintenance are the key to avoiding equipment damage.

3.4 Silicone rubber cracking of silicone rubber transformers

As shown in Figure 3, 35kV silicone rubber current transformers are often used in domestic converter station filters and shunt capacitor devices in the early stage. This type of transformer is prone to cracking defects of primary lead silicone rubber during long-term operation. Since the filter field equipment has a fence during operation, the cracking position of the primary lead cannot be found through inspection. If it is not found, it is easy to reduce the primary insulation capacity and cause protection misoperation. Therefore, the annual routine maintenance of the converter station should focus on checking the silicone rubber current transformers that have cracks and cracking trends. Generally, when this problem occurs, the repair method is adopted when the insulation is good after the test verification. After the treatment is completed, the test verification is carried out again. It is qualified only when it meets the test requirements. Similar to the insulation damage in common capacitor failures, regular inspection and repair are effective means to prevent faults.

Figure 3 Silicone rubber cracking at the primary lead outlet

4.Analysis of common capacitor failuresof lightning arrester

As a common overvoltage protection device in both DC converter stations and ordinary AC substations, this section mainly analyzes the defects that are more likely to occur in lightning arresters in AC and DC filters of DC converter stations.

Due to the low voltage level of the lightning arrester in the AC filter, it is usually not easy to have problems. The most common failure of the lightning arrester body is that the insulation of the lightning arrester is damp, resulting in insulation damage of the lightning arrester. In this case, the lightning arrester can only be replaced.

Due to its particularity, the DC filter lightning arrester may cause insulation breakdown of the lightning arrester and short circuit to the ground in certain operating modes or certain fault conditions of certain DC systems, and the lightning arrester needs to be replaced as a whole. Similar to the insulation damage problem in common capacitor failures, the insulation of the lightning arrester is also one of the common failures. Timely detection and replacement are the key to ensuring the safe operation of the equipment.

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In DC converter stations, shunt capacitors, as one of the key equipment, play an important role in the stable operation of regional power grid interconnection and high-power power transmission. DC converter stations usually shoulder the important tasks of regional power grid interconnection and high-power power transmission. The safe and stable operation of a single station often determines the safe and stable operation of the large power grid system, and the safe and stable operation of each device becomes the cornerstone of the safe and stable operation of the converter station. China, as the country with the largest number of DC converter stations in operation and the richest operating experience in the world, has accumulated a lot of converter station operation and maintenance experience. Years of operating experience of DC converter stations have shown that the tracking and control of operation and maintenance personnel during the equipment installation process can effectively reduce the number of defects and hidden dangers after the equipment is put into operation to a considerable extent, and ensure the safe and stable operation of the equipment for a considerable period of time.

The equipment of ordinary AC substations is all included in DC converter stations. At the same time, converter stations also have equipment that AC substations do not have (converter valves, DC field equipment, water cooling system equipment, AC and DC filters and shunt capacitors equipment, etc.). The technical content and structural complexity of these equipment are much more complex than those of ordinary AC substation equipment, which determines the complexity of its equipment installation, construction and commissioning process. Tracking the equipment installation process can directly integrate the various experiences accumulated in the daily operation and maintenance of the converter station into the equipment installation process, eliminate incorrect, unreasonable, and non-standard installation methods and installation problems found on site in the bud, and ensure the health level of the equipment after commissioning.

The complexity and importance of DC equipment also determine that it cannot only focus on result acceptance, surface acceptance, and functional acceptance like ordinary substations. The operation and maintenance personnel of the DC converter station should track and accept the entire process from the equipment civil foundation, equipment factory test, equipment entry, unpacking, installation, on-site handover test, and commissioning. In particular, it is necessary to strengthen the tracking and acceptance of hidden projects to eliminate various hidden dangers in the infrastructure stage to the greatest extent. The construction of DC converter stations is a heavy task and time-sensitive. It is particularly necessary for converter station operation and maintenance personnel to strengthen the tracking of equipment installation. Equipment operation and maintenance personnel are more likely to discover and solve problems with a sense of responsibility as the owner, which further guarantees the quality of equipment installation. At the same time, the installation tracking of DC converter station equipment can in turn cultivate DC converter station operation and maintenance professionals, so that tracking personnel can better understand and master all kinds of equipment knowledge.

Tracking of equipment installation process is crucial for DC converter station operation and maintenance and talent training. At present, the State Grid Corporation system converter station has initially formed a relatively complete method system for equipment installation tracking. Through the preparation of tracking, acceptance work instructions, equipment infrastructure installation and commissioning daily reports and other operational documents and summaries, the operation and maintenance personnel are guided to learn equipment installation.

2.AC and DC filter and shunt capacitors device installation tracking content

The installation tracking content of AC and DC filters and shunt capacitors devices mainly has 5 stages (of course, this is also the stage process of most equipment tracking): ① Tracking of civil construction foundation; ② Tracking after the equipment arrives; ③ Tracking of equipment installation process; ④ Tracking of equipment commissioning and testing; ⑤ Acceptance after the equipment construction is completed. The following five aspects are used to describe the installation tracking content and precautions of AC and DC filters and shunt capacitors.

2.1 Tracking of civil engineering construction

The tracking of civil engineering construction is usually considered a “chicken rib” project in the eyes of many substation and converter station operation and maintenance personnel. The tracking and acceptance are not taken seriously, which leads to many problems. For example, the ground treatment is not up to standard, resulting in the collapse and settlement of the ground around the equipment foundation after 1 to 2 years of operation, which seriously threatens the safe and stable operation of the equipment. A domestic converter station once had a hidden fault of the TA secondary cable pipe and TA junction box seriously separated due to the collapse of the foundation around the AC filter high-voltage TA (current transformer), as shown in Figure 1 and Figure 2. The operator had to apply for the suspension of the filter group for disposal to prevent the serious consequences of the mother differential action caused by the failure of the filter TA circuit. This situation is particularly aimed at the foundation with collapsible loess structure in some areas of the northwest. If it is not handled properly during the infrastructure stage or later maintenance, it is easy to cause this problem.

Figure 1 Schematic diagram of the junction box of the TA pipe sinking

Figure 2 Schematic diagram of the bottom of the pipe sinking

Of course, to solve this problem, on the one hand, we require the construction unit to work hard on the foundation treatment during the infrastructure construction phase to keep the soil around the foundation stable (3/7 of the gray soil can be compacted to a certain thickness or the opinions of the relevant design parties can be sought) to prevent collapse and sinking; on the other hand, the method shown in Figure 3 can be used on the upper and lower parts of the pipe, the upper pipe is welded to the fixed position at the bottom of the TA to prevent the pipe from sinking, and the lower part uses a plate with an “L”-shaped cross-section to support the pipe to prevent sinking. In general, during the infrastructure stage, the TA manufacturer or the construction party is required to weld the upper pipe to the fixed position at the bottom of the TA. If this method is used, the empty pipe is still pulled down, and the bottom support method can only be further adopted to prevent sinking. Ensuring the safe operation of key equipment such as shunt capacitors on a stable foundation is the guarantee for the overall safe and stable operation of the converter station.

Figure 3 Schematic diagram of TA pipe penetration sinking treatment

In addition, if the treatment around the equipment foundation is not up to standard, it may also cause the equipment to tilt, causing the insulator to break under lateral tension and causing equipment accidents.

Tracking of civil foundation construction is often the first step in equipment on-site tracking. Based on experience, the tracking is mainly focused on the following aspects.

(1) Tracking of civil construction around the equipment foundation. Whether the backfill soil around the equipment concrete foundation is carried out in accordance with the process required by the civil construction drawings, such as: the compaction thickness of the lime soil (lime and clay mixed in a certain proportion), the thickness of the gravel layer, the thickness of the concrete surface layer, whether there are horizontal or tilt requirements on the ground, etc. All these require our tracking personnel to be familiar with the drawings first, and then go to the site to track in a targeted manner. In this way, we can learn something and become familiar with the site. As shown in Figure 4, the backfill soil treatment around the concrete foundation is not up to standard, resulting in ground settlement, which in turn threatens the safe and stable operation of equipment including shunt capacitors.

Figure 4 Settlement around equipment foundation

(2) Tracking of equipment foundation civil construction. The so-called equipment foundation usually refers to the concrete structure on which the equipment body is installed. The concrete structure is embedded to a certain depth below the ground to ensure that the equipment above the ground is firm and stable. This part of the tracking should check whether the construction party has carried out the construction of the structure according to the requirements of the drawings, and whether the shape of the structure, the construction depth, and the maintenance of the concrete after construction (especially in the cold winter environment) meet the requirements. Otherwise, it will seriously affect the quality of the concrete.

(3) Grounding grid construction. In civil construction, the grounding grid construction should pay special attention to the process acceptance. After the construction unit completes the laying and welding of the grounding grid, due to the next step of construction, it is generally necessary to backfill the earth quickly. Before backfilling the earth, the construction party will generally notify the supervision and operation unit tracking personnel to take photos for acceptance. This acceptance process is usually called hidden project acceptance in engineering. Since the grounding grid will be used as a safe grounding, protective grounding, signal grounding, etc. for various equipment in the future, and also as a discharge channel for various fault currents, it also plays an important role in the operation of the entire substation. The grounding grid acceptance should pay attention to whether the material, shape, etc. used in the grounding grid meet the requirements of the regulations (design drawings), and whether the welding process meets the welding quality requirements. After the tracking personnel take photos and keep them, they should be filed and organized in time, and archived as the basic data for substation construction in the future.

(4) Cable trench construction and filter field drainage facility construction. On-site tracking personnel track whether the cable trench construction meets the design requirements. In addition, they should also pay attention to whether the trench firewall layout meets the standard requirements (familiarize yourself with the standards before tracking), whether there are drainage holes in the cable trench, etc. If the cable trench is not drained smoothly, it is very likely to cause a large amount of water in the trench to be unable to be discharged after raining as shown in Figure 5. In severe cases, it may cause cable damage, DC grounding, and protection malfunction. The filter field is usually located on a large area of ​​hardened concrete ground. It should be noted that the drainage facilities on the ground are constructed according to the requirements and whether the concrete ground has any inclination requirements. If it rains during the construction process, this is the best opportunity to check whether the drainage of the large area of ​​concrete construction ground is smooth and whether the drainage wells and rainwater wells are functioning normally. At this time, personnel should be organized to inspect the large area of ​​concrete ground, drainage wells, and rainwater wells, and urge the construction unit to deal with serious problems in a timely manner. During the inspection of underground drainage wells, attention should be paid to whether the well walls and underground parts are well constructed. Otherwise, long-term operation may cause the well walls to collapse, soil to enter the well, and the drainage pipes to be blocked (usually the construction quality of this part is poor and it is not easy to attract the attention of all parties), as shown in Figures 6 and 7.

Figure 5 Water accumulation in the cable trench

Figure 6 Drainage well with poor wall construction technology

Figure 7 Drainage well with blockage at the bottom of the well

(5) Methods for solving problems during tracking. During the civil engineering tracking process, if the construction party fails to construct according to the design drawings or does not meet national standards, design specifications (such as the installation standards of shunt capacitors) and relevant enterprise standards, or if more serious hidden dangers and defects are found, the tracking personnel should take photos and record them, and issue a project contact form to the supervisor, construction party or design institute, requiring rectification within a time limit. The use of the project contact form is applicable to all tracking occasions, and the communication records of these contact forms with infrastructure units and equipment manufacturers should be archived as engineering infrastructure materials.

2.2 Tracking after the arrival of the filter equipment

After the filter equipment arrives and before unpacking, the supervisor and the operation and maintenance unit will generally be notified to come to the site for witnessing, and there will usually be personnel from the equipment manufacturer on site. At this time, the following points should be noted:

(1) Check whether the external packaging of various filter equipment (shunt capacitors, resistors, reactors, mutual inductors, lightning arresters, etc.) is damaged. Pay attention to whether the equipment with damaged external packaging is intact and conduct a careful inspection. Minor damage that may cause long-term operation hazards of the equipment (not considered by the construction party or manufacturer to constitute damage) should be boldly raised and photographed for preservation. If necessary, an engineering construction contact form should be issued, or all parties should be organized to discuss until a clear reply is given.

(2) Pay attention to the collection of equipment certificates, factory manuals, test reports, etc., or the construction unit should collect them and hand them over as data later, but the tracking personnel should keep a record of the data to ensure the integrity of the future data handover.

(3) Pay attention to the collection and collation of basic equipment information to prepare for the compilation of substation operation procedures, maintenance and test procedures, and equipment ledgers. The data collected by photography or recording mainly include: ① Nameplates of capacitors, reactors, resistors, lightning arresters, and transformers, which should correspond to the equipment (there are usually multiple types of capacitors, reactors, resistors, transformers, lightning arresters, etc. in a group of filters); ② Schematic diagram of capacitor tower grouping: Usually, before the capacitor leaves the factory, the capacitor tower bridge arm has been balanced according to its actual capacitance value, so the position of each capacitor on the tower has been fixed, and the construction party only needs to install and wire according to the grouping diagram. While paying attention to collecting the above equipment parameters, special reminders should also be paid to collecting information on insulators in the filter field, mainly including model, design creepage distance, etc., to accumulate data for future equipment anti-pollution flashover related work.

(4) After the capacitor equipment arrives at the site, in order to ensure product quality, batch sampling tests should generally be carried out, and at least one capacitor unit of each type in each batch should be carried out. A batch refers to all products that are oiled at the same time. When the number of a certain capacitor in a batch of products exceeds 50 units, the sampling rate should not be less than 2%. If any capacitor fails any test, the construction unit has the right to reject all capacitors in the batch. During this process, the operation and maintenance personnel should supervise the construction unit to carry out this work to prevent capacitors with batch problems from being put on the tower, which will leave long-term hidden dangers for the operation of the equipment.

2.3 Tracking of filter equipment installation process

Installation of capacitor tower: During the installation of capacitor tower, the construction unit should be supervised from time to time to install according to the equipment number of the capacitor equipment when it leaves the factory. It is usually hoisted layer by layer. After the hoisting is completed, the upper layer of supporting insulators is installed (or the upper layer of insulators is directly installed on the capacitor layer to be hoisted, and hoisted together, as shown in Figure 8).

Figure 8 Hoisting operation of capacitor tower layer

During the installation, the construction unit should also be reminded to pay attention to the protection of finished products. The capacitor porcelain bushing should not be stressed to avoid hidden oil leakage defects, which cannot be exposed in a short time. After commissioning, the capacitor will cause oil leakage under the action of thermal effect. At the same time, the supporting insulators should be prevented from being stepped on and damaged by tools. It is strictly forbidden for personnel to pass tools by “throwing” up and down during construction. When connecting the wires between capacitors, tighten the connections according to the torque requirements (currently, the more common capacitor joint connection uses M16 nuts, as shown in Figure 9, and the tightening torque is generally 20NM) to prevent the joints from loosening and heating or over-tightening, which may cause oil leakage after the joints are put into operation (equipment manufacturers will configure torque wrenches with appropriate torques when leaving the factory, and usually configure special tools and instruments with the equipment, which are proposed by the operation and maintenance unit during the review stage of the equipment bidding and procurement technical specifications, or some manufacturers configure them specifically for users).

Figure 9 Connection diagram of capacitor joint wire clamps, screws and nuts
Installation of other equipment: mainly including the installation of resistors, reactors, lightning arresters, and TAs in the filter field. In addition to urging the construction unit to pay attention to the protection of finished products during the installation process, the following points should also be noted: ① The bolts and nuts used to fix the resistors and reactors should be non-ferromagnetic stainless steel bolts and nuts (hot-dip galvanized materials can be used for wire clamp connections); ② If the TA in the filter field is an oil-filled TA, special attention should be paid to the good protection of the end screen and the reliable connection. Timely restoration should be paid after the test; ③ All kinds of wire connections should prevent the wires from causing stress on the equipment; ④ There should be drainage holes at the bottom of some crimping hardware connections. Figures 10 to 13 are the on-site installation photos of the type reactor, resistor, oil-filled TA, and lightning arrester.

Figure 10 On-site installation photo of dry-type reactor

Figure 11 On-site installation photo of resistor

Figure 12 On-site installation photo of oil-filled TA

Figure 13 On-site installation photo of lightning arrester in filter field

2.4 On-site commissioning and test tracking of filter equipment

On-site commissioning and test tracking of filter equipment are generally rarely used after the equipment is put into operation (unless tuning test is required after replacing the inductor component), but as on-site operation and maintenance tracking personnel, they should be familiar with the most basic commissioning knowledge, including commissioning purpose and commissioning methods.Correct parameter setting and verification of shunt capacitors during commissioning are also key steps to ensure the normal operation of the filter system.

A set of AC and DC filters must go through a series of calculations, verifications, and commissioning from design to before and after delivery to play its filtering role in the DC transmission system. The first two phases are generally in the design and manufacturing stages, and to play a more ideal role, they must go through on-site commissioning. Debugging of the filter: First, ensure the safe operation of the system. After debugging, the tuning indicators and other indicators meet the design requirements to avoid the input of the filter causing the resonance surface between the system and the filter to amplify the incoming wave current. In addition, the overvoltage generated when the filter is put into operation should be limited to a limited range so as not to damage other equipment. Second, ensure the safe operation of the filter itself. After the filter is debugged and put into operation, the harmonic current flowing into the filter will not cause the capacitor, inductor, and resistor of the filter to be overloaded, and the overvoltage and overcurrent when the filter is switched on will not cause damage to the components of the filter body. These measures ensure that the filter and its shunt capacitors operate stably and reliably in the system.

Inspection before debugging (Tongtang said cold debugging): After all the filter equipment is installed, a detailed installation inspection is performed on all the equipment to ensure that the wiring of each device is correct. The general principle of debugging is to change the reactance value by adjusting the distance between the upper and lower coils of the reactor so that it resonates with the capacitor at the debugging frequency, thus completing the debugging before the filter is put into operation.

Debugging method: There are three methods for debugging filters: amplitude-frequency characteristic method, discharge oscillation method, and phase-frequency characteristic method. The first two methods have large errors, and the phase-frequency characteristic method is more commonly used in practice. Input the resistor voltage and the total voltage of the filter into the two channels of the same filter respectively, and compare the phase angles to determine whether the filter is resonant. Usually, the two voltages are input into different axes (coaxial input has large errors and is not conducive to accurate tuning). When observing the changes in the two phases, the graph changes from an ellipse to a straight line, and the resonance can be determined. This is easy to observe and has high accuracy.

The main process of debugging using the phase-frequency characteristic method:

(1) Connect the wires according to the measurement requirements.

(2) Place a thermometer on site for temperature measurement.

(3) Calculate the tuning frequency under the current situation, whether the tuning frequency of the filter is positive or negative, and what the deviation value is (positive and negative can be determined based on the system impedance).

(4) Turn on the power supply and adjust the oscilloscope for observation.

(5) Adjust the display of the frequency meter and the power supply frequency of the signal generator (which should be the tuning frequency).

(6) Adjust the input voltage of the signal generator so that the oscilloscope graph is easy to observe and not distorted, which is convenient for improving accuracy.

(7) Adjust the distance between the upper and lower coils of the filter reactor (actually change the mutual inductance of the reactor) and change its reactance value until the oscilloscope graph changes from an ellipse to a straight line.

However, the actual change in the inductance value of the reactor is not a continuous adjustment (that is, the distance between the upper and lower coils is graded), so the oscilloscope graph can only be the most approximate straight line during the actual debugging process.

The above method can be used for cold tuning of high-pass filters and double-tuned filters, so this method is more practical in engineering. After the filter is debugged by this method, the filter can meet the design requirements and be put into system operation safely and reliably.

Before debugging and testing the filter, the tracking personnel need to be familiar with the debugging method in advance, master the test steps during the debugging process, and focus on learning debugging wiring and instrument operation during the debugging period.

2.5 Acceptance of filter equipment after construction

(1) The following general requirements should be noted for the acceptance of filters and shunt capacitors equipment (including DC filters):

1) First, the construction unit should complete the construction, organize the personnel of the unit to conduct self-acceptance, keep the self-acceptance records, and submit to the supervision unit that the conditions for acceptance are met after the self-acceptance is passed.

2) After receiving the notice from the construction unit that the self-acceptance is completed and requesting it to organize the acceptance, the supervision unit will start to organize the acceptance. The acceptance must be organized by the on-site supervision unit, and the construction unit, operation and maintenance unit personnel, design unit and even manufacturer personnel will be organized to participate.

3) List the problems found during the acceptance one by one, preferably with photos, and provide timely feedback for rectification.

4) If there are many problems of the same type or obvious construction quality problems, the acceptance can be stopped. Due to many problems and low-level construction quality problems, the operation and maintenance unit may consider that the conditions for acceptance are not met and require the construction unit to rectify them as soon as possible before re-accepting according to the acceptance process.

5) After the acceptance is completed, the signing procedures of all parties concerned should be completed at the same time. The operation and maintenance unit can completely lock the fence door of the filter equipment at this time, and any personnel are strictly prohibited from entering the equipment area without authorization.

(2) Acceptance content:

1) Appearance inspection.

There should be no residue on the filter and shunt capacitors device (capacitor tower, resistor inside and outside, reactor inside and outside, transformer, lightning arrester, etc.), and they should be clean. The capacitor tower and other primary wiring should be correct, and the wiring connection terminals should not be loose (the nuts of the capacitor connector M16 should be checked according to the torque of 20NM). The grounding point of the equipment should be firm and clearly marked. The insulators of each equipment, the porcelain bushings of capacitors, current transformers, and lightning arresters should be clean, without cracks or damage, and there should be no signs of oil leakage at all joints and welds of capacitors. The nuts, flat washers and spring washers for connecting the lead wires of the capacitor bushing should be complete. The capacitor shell should not be bulging, deformed, rusted, etc.

The capacitor frame should be kept in a horizontal position, the support insulators should be vertical to the ground, firmly fixed, and the paint should be intact. The capacitor should be installed with its nameplate facing the side of the channel and numbered in sequence. The connection wires of the capacitor terminals should meet the design requirements, the wiring should be symmetrical, neat and beautiful, and the busbars and branch lines should be marked with phase colors. The shell of the capacitor insulated from the ground on each layer of the capacitor tower should be connected to a fixed potential.

Current transformer: The porcelain sleeve has no cracks or damage, the junction box is well sealed, the secondary lead is not stressed at the pipe penetration, and the plugging is good; the end screen is reliably grounded; the oil level of the oil-filled TA is indicated at 1/3 to 3/4. For silicone rubber transformers, the main attention should be paid to the good sealing of the junction box, and the silicone rubber of the primary lead should be well sealed and crack-free.

Appearance inspection of the reactor: The insulation of the pillars and coils should be free of damage and cracks: the coil is not deformed; the pillar insulators and their accessories are complete. All exposed parts of the reactor should have a good anti-corrosion layer and meet the painting requirements of outdoor anti-corrosion electrical products. The inside of the coil is clean and free of debris, and the reactor air duct should be clean and free of any debris. In addition, special attention should be paid to the fact that the fixing bolts (nuts) on the reactor body and the bottom bracket should be stainless steel (non-magnetic) parts, and galvanized materials should not be used to prevent heat during operation.

All the above-mentioned filter (or shunt capacitors) equipment connection bolts tightening in the fence (except for capacitors connected with wire clamps or materials that clearly cannot be checked according to the standard torque) must be checked with a torque wrench with the corresponding torque to prevent the equipment joints from heating up after commissioning. The tightening torque of the bolts refers to the equipment connection bolt control torque table in Appendix A. It should be pointed out that the bolts tightened in accordance with Appendix A generally refer to the bolts that use the galvanizing process (this process method is used to manufacture bolts in most cases in substations) and the strength grade is clearly marked on the bolts. Bolts of other materials can only be implemented as a reference. For example: there are a considerable number of stainless steel bolts (such as A2-70 material) in the filter field, and in general, the tightening torque of stainless steel bolts of the same diameter commonly used in substations is implemented with reference to the tightening torque of 4.6-grade galvanized bolts.

2) Equipment acceptance test in the device. Before the filter and shunt capacitors device are put into operation, the construction unit shall conduct acceptance tests on all equipment in the filter and shunt capacitors in accordance with GB50150-2006 “Electrical Equipment Installation Engineering Electrical Equipment Acceptance Test Standard”. The acceptance test must be qualified, and the test data will be archived and used as the basis for various evaluations and maintenance of the equipment in the future (because when the test score of the equipment status quantity test is performed, the acceptance test data is generally used as the initial data of the equipment). In order to ensure the capacitance balance between the capacitor tower bridge arms of the DC converter station filter and shunt capacitors, an unbalanced current test is generally required to ensure that the unbalance meets the requirements.

3) Acceptance of relay protection devices. The filter and shunt capacitors device will be equipped with large group filter protection and small group filter protection, and the DC filter will be equipped with a DC filter protection device. Before the conditions for commissioning are met, the protection device must also be accepted at the same time. The various indication checks of the protection device, the set value check of the protection device, and the protection transmission record check of the construction unit should all be completed and correct. The specific contents of other acceptance of filter and shunt capacitors protection devices are the same as those of various protection devices in ordinary AC substations. They can be implemented by referring to relevant regulations. This book will not elaborate on them.

4) Materials to be submitted for equipment acceptance. When accepting filter and shunt capacitors devices, the construction unit shall submit the following materials and documents: various equipment product manuals, factory test records, qualification certificates and installation drawings provided by the manufacturer; on-site adjustment test records of the construction unit; various technical documents, meeting minutes, notice rectification orders, etc. that occurred during the construction process; design data files, design change notices, etc.

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1.1 Principle of Aluminum Electrolytic Capacitor-And their Basic Structure

Two parallel dielectric materials that are close to and insulated from each other can be combined to store a certain amount of charge and electrical energy. The electronic components that can hold charge and are prepared using the principle of aluminum electrolytic capacitor are called capacitors. Capacitors are one of the electronic components used in large quantities in electronic equipment. They are widely used in electronic circuits such as isolated DC circuits, coupling circuits, bypass circuits, filter circuits, tuning circuits, energy conversion, and circuit control. With the development of the electronics industry, electrical products are constantly developing towards high performance, miniaturization, and integration. Capacitors are essential components in modern electrical products. They are large in size and cannot be directly implanted in large-scale integrated circuits. Therefore, the miniaturization of large-capacity capacitors, that is, high specific capacitance, has become one of the key links in the development of high-tech electrical products.

Electrolytic capacitors with positive and negative polarity can be made using a certain dielectric material with a certain thickness. Figure 1.1 schematically shows the structure of a flat plate capacitor formed by connecting a cathode dielectric material (-) with a thickness of d_c and an anode dielectric material (+) with a thickness of d_a in series.
The capacitance formula of a flat plate capacitor isWherein, C represents capacitance, ε_o and ε_r represent the vacuum dielectric constant and the relative dielectric constant of the dielectric material, respectively, and S and d represent the opposing area of ​​the dielectric material and the thickness of each dielectric material, respectively.Figure 1-1 Side view of electrolytic capacitor composed of cathode dielectric material and anode dielectric material
(such as aluminum oxide film with thickness of d_c and d_arespectively)

The dielectric material used for electrolytic capacitors should have the highest possible relative dielectric constant; dielectric materials for capacitors with excellent performance should also have the following advantages: high dielectric strength (operating voltage per unit thickness) so that it will not be broken down under high voltage; low loss angle to reduce working energy consumption and temperature rise. On the other hand, the relevant materials should also be cheap so that they can be widely and massively used. The relative dielectric constant of alumina is relatively high and insensitive to frequency, which is 9 to 10. The dielectric loss of alumina at 1MHz (tanδ, the tangent value of the loss angle δ) is less than 4×10^-4, and the intrinsic dielectric loss is about 10^-5. Ordinary alumina with a certain porosity can have a dielectric strength higher than 12kV/mm, and the dielectric strength of industrially prepared alumina can reach 30~40kV/mm or higher. On the other hand, alumina is cheap and the process for making aluminum capacitors is relatively mature, so aluminum capacitors have become the most widely used capacitor components.

The above is the principle of aluminum electrolytic capacitor-and the basic structure explanation

1.2 The Relationship Between the Capacitance parameters of Cathode And Anode And The Capacitor Aluminum foil

It can be seen from Figure 1.1 that if the working voltage of the capacitor as a whole is U, the voltages of cathode aluminum oxide film and anode aluminum oxide film are U_c and U_a respectively, and there are:

The cathode aluminum oxide film and anode aluminum oxide film with capacitance characteristics actually form a series relationship with each other. According to the series circuit principle of capacitors and formula (1.1), the cathode aluminum oxide film capacitor C_c and the anodic aluminum oxide film capacitor C_ahave the following relationship with the total capacitance C and voltage:On the cathode shown in Figure 1.1, there is usually only a very thin layer of aluminum oxide film. From formula (1.1), it can be seen that when d_c is very small, the cathode aluminum oxide film capacitance C_cwill be very high, which makes the value of the first term on the right side of formula (1.3) close to 0; therefore, the capacitance of the capacitor shown in Figure 1.1 mainly depends on the anodized aluminum oxide film capacitance C_{a o}. On the other hand, by comparing formula (1.2) and formula (1.4), it can be found that the working voltage U is borne by the cathode aluminum oxide film and the anodized aluminum oxide film respectively, and the voltages U_c and U_a borne by the two are inversely proportional to their capacitances C_c and C_a respectively. It can be seen that when the cathode aluminum oxide capacitance C_c is very high. The actual working voltage borne by the cathode aluminum oxide film is very low, and the working voltage of the capacitor is mainly borne by the anodized aluminum oxide.

The cathode and anode can be separated by electrolytic paper made by immersing a certain electrolyte, and then the cathode aluminum oxide film, electrolytic paper, and anodized aluminum oxide film parallel to each other are rolled into a cylindrical shape to make a capacitor element. The electrolyte is usually required to have low AC dielectric loss and high DC resistance. Figure 1.2 shows the winding schematic diagram of the aluminum electrolytic capacitor. After the cathode aluminum oxide film, electrolytic paper, and anodized aluminum oxide film are wound, they are placed in a metal shell, and then the cathode aluminum oxide film and the anodized aluminum oxide film are led out with conductive leads, and a usable aluminum electrolytic capacitor product is made.

Figure 1.2 Schematic diagram of aluminum electrolytic capacitor and its winding (hatched area: electrolytic paper; dotted line: metal shell)

The structural characteristics of aluminum electrolytic capacitors determine that the anodized aluminum film should have the largest possible capacitance and the highest possible withstand voltage; the sandwiched electrolytic paper can replace the cathode aluminum film and become the cathode facing the anodized aluminum film, so in fact it is mainly the anodized aluminum film that bears the dielectric function of the aluminum electrolytic capacitor. It can be seen that the production level of the anodized film determines the technical quality level of the aluminum electrolytic capacitor.

At present, it is still difficult for people to directly and large-scale use aluminum oxide to produce aluminum electrolytic capacitors in a very economical way. Usually, a metal aluminum foil of a certain thickness is first produced by forming processing, which is called a plain foil: then the surface of the aluminum foil is subjected to a specific controlled oxidation treatment to obtain a large area of ​​a certain thickness, continuous, and voltage-resistant aluminum oxide film for the production of aluminum electrolytic capacitors. The anodized aluminum film and the cathode aluminum oxide film shown in Figure 1.2 are actually aluminum oxide films attached to the metal aluminum foil. After the metal aluminum foil is oxidized, the unoxidized aluminum foil substrate plays the role of carrying the aluminum oxide film and the conductor.

The above is the principle of aluminum electrolytic capacitor-The relationship between the capacitance parameters of the cathode and anode and the explanation of capacitor aluminum foil

1.3 Principle of Aluminum Electrolytic Capacitor-Principle of High Specific Capacitance of Capacitor Aluminum Foil

From the capacitance formula of formula (1.1), it can be seen that when the dielectric material is determined to be aluminum oxide, ε_o and ε_r are both constants. If you want to increase the capacitance of the capacitor, you need to increase the confrontation area S of the aluminum oxide film shown in Figure 1.1 or reduce the thickness d of the aluminum oxide film. When designing a capacitor, the thickness d of the aluminum oxide film cannot be reduced arbitrarily, because the aluminum oxide film needs to have a certain dielectric strength, that is, the ability to withstand a certain voltage without being broken down. When the working voltage of the electrolytic capacitor is determined, the lower limit of the oxide film thickness is determined. At this time, only by increasing the area of ​​the dielectric film can a high capacitance be obtained. However, simply increasing the area can increase the capacitance, but it will also increase the volume of the capacitor, so it does not meet the requirements of the miniaturization of electrical products.

Through exploration and research, people have found that with the help of special electrochemical corrosion technology, a large number of tunnels of a certain size or densely distributed small pits can be corroded on the surface of aluminum foil, so that the surface area of ​​the aluminum oxide film that can be generated on the same aluminum foil area is greatly increased; thereby, the specific capacitance of the aluminum electrolytic capacitor, that is, the capacitance per unit area of ​​aluminum foil, can be significantly increased. As mentioned above, it is necessary to generate an aluminum oxide film on the surface of the aluminum foil to be used as a dielectric material; the thickness of the oxide film is closely related to the working voltage. The higher the voltage, the greater the thickness of the oxide film required. Therefore, according to the different working voltages of the capacitor, the size of the tunnels or holes etched on the surface of the aluminum foil is also different. High specific capacitance electrolytic capacitor aluminum foil can be divided into three categories: cathode aluminum foil, high voltage anode aluminum and low voltage anode aluminum foil. “Figure 1.3 shows a schematic diagram of the side structure of the surface of the high specific capacitance electrolytic capacitor aluminum foil after corrosion. Making tunnels or holes on the aluminum foil can greatly increase the surface area of ​​the aluminum foil without increasing or even reducing the weight of the aluminum foil, which provides a prerequisite for significantly increasing the area of ​​the aluminum oxide dielectric film. The cathode aluminum foil has the lowest working voltage and the highest required specific capacitance, so the holes on the surface of the cathode aluminum foil are the most dense. The high voltage anode aluminum foil has the highest working voltage, and requires a relatively coarse corrosion tunnel to accommodate a thicker aluminum oxide film. The working voltage of the low voltage anode aluminum foil is between the cathode aluminum foil and the high voltage anode aluminum, and requires slightly coarse and sparse corrosion holes.

Figure 1-3 Schematic diagram of the side structure of the aluminum foil corrosion of high specific capacitance electrolytic capacitors

1-uncorroded aluminum foil,   2-cathode aluminum foil and fine corrosion pits,  3-high voltage anode aluminum foil and coarse corrosion tunnels,

4-low voltage anode aluminum foil and slightly coarse corrosion pits; hatched area: electrolyte)

After a corrosion tunnel or hole pit is produced on the surface of the aluminum foil by means of a specific corrosion technology, the tunnel wall or hole pit wall needs to be oxidized, which is called chemical formation, to form an aluminum oxide dielectric film. The aluminum foil after corrosion and chemical formation is called corrosion foil and chemical formation foil respectively. The working voltage of cathode aluminum foil is very low. Anode aluminum foil with a working voltage below 150V can usually be called low-voltage anode aluminum foil!”. With the increase of the withstand voltage value, the size of the corrosion hole pit will gradually increase, so as to make a thicker aluminum oxide film, and then make an aluminum oxide film with higher withstand voltage under the condition that the oxide film does not block the hole pit. After the working voltage is determined, the thickness d of the aluminum oxide film is also determined. Figure 1.4 shows a schematic diagram expressing this relationship. When making a high-voltage anode aluminum foil with a withstand voltage value of more than 350V, the size of the required corrosion hole pit will be significantly increased. At this time, the conventional low-voltage aluminum corrosion method is not enough to achieve uniformly distributed large-sized corrosion holes, so it is necessary to use tunnel corrosion technology for high-voltage anode aluminum foil to obtain a thick straight tunnel structure perpendicular to the aluminum foil surface.

Figure 1.4 Schematic diagram of the corrosion structure of aluminum foil (above), the oxide film inside it (below) and the relationship between withstand voltage

(Left: thin oxide film aluminum foil with low withstand voltage value; middle: thick oxide film aluminum foil with high withstand voltage value;
Right: oxide film aluminum foil with high voltage resistance and thick tunnel wall. Dark area: aluminum oxide film; shadow area: aluminum substrate)

As can be seen from Figure 1.4, as the withstand voltage increases and the size of the corrosion pits or tunnels increases, not only the density of the pits and tunnels on the surface of the aluminum foil decreases, but also the increase in the total surface area of ​​the aluminum foil, that is, the increase in the total area of ​​the corrosion pit wall, will also decrease. This relationship between the working voltage and the corrosion pits leads to a downward trend in the maximum specific capacitance that can be achieved by the aluminum foil as the withstand voltage of the aluminum foil increases. Figure 1.5 schematically shows the approximate relationship between the withstand voltage of the anode aluminum foil and the maximum specific capacitance that can be achieved. Of course, under the condition of the same withstand voltage

Figure 1-5 Schematic diagram of the relationship between the withstand voltage and the maximum specific capacitance of the anode aluminum foil

The specific capacitance of aluminum foil can be further improved by adjusting the internal structure and corrosion process of aluminum foil. The specific capacitance of cathode aluminum foil is usually above 500ufcm². The specific capacitance of low-voltage anode aluminum is 10-100μf/cm ²
‘, and the specific capacitance of high-voltage anode aluminum foil is generally less than 1uf/cm ²

The dielectric strength of industrial aluminum oxide reported so far can reach above 50kV/mm[31. However, the dielectric strength of aluminum oxide is also closely related to the thickness of the aluminum oxide film; the lower the thickness of the aluminum oxide film, the higher the dielectric strength. Figure 1.6 shows the observed relationship between the dielectric strength and breakdown voltage of aluminum oxide film. As the thickness of the aluminum oxide film decreases, its breakdown voltage (aluminum oxide film thickness x dielectric strength) also decreases; but it can be inferred that when the thickness of the aluminum oxide film is reduced to a level below micrometers, its dielectric strength will reach hundreds of kilovolts per millimeter; therefore, the thickness of the oxide film on the surface of the aluminum foil of the electrolytic capacitor usually does not exceed 1um.Figure 1.6 Schematic diagram of the influence of the thickness of polycrystalline aluminum oxide film on dielectric strength (thick line) and breakdown voltage (thin line) (50Hz)

The above is the Principle of Aluminum Electrolytic Capacitor-Explanation of the principle of high specific capacitance of capacitor aluminum foil

2 Brief Description of the Basic Processing Steps of Capacitor Aluminum Foil

2.1 Principle of Aluminum Electrolytic Capacitor-Basic Production Process of aluminum Electrolytic Capacitors

Usually, a large amount of high-purity aluminum is required to manufacture aluminum for electrolytic capacitors, which refers to the anode aluminum foil that occupies the main body of aluminum in electrolytic capacitors. From the initial alumina raw material to the final production of chemical foil that can be wound into aluminum electrolytic capacitors, the relevant production process mainly includes three stages: high-purity aluminum production, light foil production, and corrosion chemical foil production. The basic process of high-purity aluminum production includes: dissolution, decomposition and calcination of bauxite to obtain alumina raw materials, electrolysis to produce primary aluminum (>99% A1), and three-layer liquid method or segregation method to produce high-purity aluminum (>99.99% AI). In the electrolytic capacitor aluminum foil production industry, primary aluminum is also collectively referred to as ordinary aluminum, and high-purity aluminum is also called refined aluminum. The basic process of light foil production includes: dissolution, batching and semi-continuous casting milling of high-purity aluminum, heating and hot rolling, cold rolling, foil rolling, cleaning, annealing, etc. The basic process of etching foil production includes: corrosion pretreatment and multi-stage corrosion, corrosion post-treatment and drying, formation pre-treatment and multi-stage formation, intermediate treatment, final formation, cleaning and drying, etc. The brief process of electrolytic capacitor aluminum foil production is shown in Figure 1.7. Figure 1-7 Schematic diagram of the simple process of electrolytic capacitor aluminum foil production

The above is the Principle of Aluminum Electrolytic Capacitor-The basic production process of aluminum electrolytic capacitors is explained

2.2 Traditional Production Method of high-Purity Aluminum

High-purity aluminum is a necessary raw material for the production of anode aluminum foil. Traditionally, it is produced by the three-layer liquid electrolytic refining method invented in 1901; Figure 1.8 shows a schematic diagram of the electrolytic cell of the three-layer liquid electrolytic refining method. The upper and lower parts of the electrolytic cell carry the electric field, the bottom is the anode, and the top is the cathode. When the electrolytic cell is working, the interior is composed of three layers of liquid. Raw aluminum and weighting agents are added from the left inlet of the electrolytic cell, usually the weighting agent is electrolytic copper. After the two are dissolved and mixed, they enter the bottom layer of the electrolytic cell to form molten high-copper raw aluminum at the anode end, which is called anode aluminum alloy, and its copper content can be 30% to 45%. The addition of different copper contents makes the density of the anode aluminum alloy liquid reach 3.0 to 3.5 g/cm ²
at high temperature, and its melting point is reduced to 550 to 590 ℃. The upper layer of the anode aluminum alloy liquid is the electrolyte layer, which is mainly composed of vapors such as NaF, CaF₂, AlF₃, BaF₂, or chlorides such as NaCl, BaCI₂, or fluorine chlorides [3], and its density is 2.5-2.7 g/cm³. Above the electrolyte layer is the molten high-purity aluminum liquid layer, which has a density of about 2.3 g/cm ³ at high temperature and is at the cathode end. It can be seen that the density of each of the three layers of liquid in the electrolytic cell determines that they can maintain a stable stratified distribution state during the high-purity aluminum production process.

Under high temperature and electric field, the Al atoms in the anode aluminum alloy liquid at the bottom of the electrolytic cell will lose electrons and show electrochemical dissolution, that is, the reaction A1 (liquid) -3e^––AI³⁺ will occur. At the same time, under the action of the electric field, AI ³⁺ will pass through the electrolyte layer and enter the high-purity aluminum liquid layer, and obtain electrons at the cathode and be reduced to Al atoms, that is, the reaction AI³⁺ +3e^– -→Al (liquid).

In addition to a large number of copper atoms, the anode aluminum alloy also contains many other impurity atoms of the original aluminum. Among them, Cu, Si, Fe and other atoms have higher electrode potentials than A1 atoms, so they will not undergo electrochemical dissolution.

Figure 1.8 Schematic diagram of the electrolytic cell of the three-layer liquid electrolytic refining method
(0-aluminum ingot and electrolytic copper inlet, 1-electrolytic cell shell, 2-refractory bricks, 3-magnesia brick lining,
4-anode electrode, 5-anode carbon cell bottom, 6-copper-containing molten anode aluminum,
7-fluoride, chloride or fluoride chloride electrolyte, 8-molten high-purity aluminum liquid, 9-graphite cathode electrode)

The electrochemical dissolution behavior is described above and it always remains in the anode aluminum alloy liquid. In addition, the electrode potential of atoms such as Na, Ca, and Mg is lower than that of Al atoms. Although it is easy to form Na⁺, Ca²⁺, and Mg²⁺ and enter the electrolyte layer, the lower electrode potential is not conducive to their release of electrons in the high-purity aluminum liquid layer, so they will remain in the electrolyte layer. The three-layer liquid electrolytic refining method utilizes the above differences in electrochemical properties between Al atoms and other atoms, thereby realizing the production of high-purity aluminum.

Al atoms in the anode aluminum alloy liquid continuously pass through the electrolyte layer, reducing the aluminum content in the alloy; the aluminum content in the anode aluminum alloy liquid can be supplemented by continuously adding raw aluminum ingots from the population, so that its composition is kept within the set reasonable range. At the same time, high-purity aluminum liquid is continuously absorbed from the high-purity aluminum liquid layer and cast into high-purity aluminum ingots. In this way, the production process from raw aluminum to high-purity aluminum can be continuously implemented.

In the above production process of high-purity aluminum, the huge difference in copper content between the anode aluminum alloy liquid and the high-purity aluminum liquid will cause 0.35-0.40V concentration polarization and voltage drop in the electrolyte layer. All Al atoms in the anode aluminum alloy liquid must overcome the concentration polarization voltage under the action of the external electric field, pass through the electrolyte layer and transform into high-purity aluminum. Therefore, the production of high-purity aluminum by the three-layer liquid electrolytic refining method is a high-energy consumption process.

The above is the principle of aluminum electrolytic capacitor -The traditional production method of high-purity aluminum is explained.

2.3 Processing of light foil

The processing of electrolytic capacitor aluminum foil mainly involves the production process from high-purity aluminum to corrosion processing. Figure 1.9 shows a schematic diagram of some major processes in the processing of foil. In the semi-continuous casting production process, the high-purity aluminum ingot is first dissolved, and then the appropriate ingredients and micro-components are adjusted according to the composition design. Finally, during the pouring, a 200-250 mm thick semi-continuous casting slab is made by the continuous condensation process in the finisher [Figure 19 (a)]. The production of electrolytic capacitor aluminum foil has very strict requirements on its purity. The semi-continuous casting production process is easy to cause different degrees of contamination on the surface of the slab. Therefore, before rolling processing, it is necessary to use a double-sided milling machine to remove a certain thickness of the billet on the upper and lower rolling surfaces of the semi-continuous casting slab. The uniformity of the trace alloying elements inside the aluminum foil of electrolytic capacitors will also have an important impact on the product performance. Therefore, before hot rolling [Figure 1.9 (b)], it is necessary not only to perform conventional hot rolling heating on the semi-continuous casting slab, but also to specially design the heating process so as to simultaneously implement homogenization annealing to promote the uniform distribution of trace elements inside the slab. Hot rolling usually processes the semi-continuous casting slab into a hot-rolled plate with a thickness of 6 to 8 mm. The hot rolling and cold rolling of the aluminum foil of electrolytic capacitors [Figure 1.9 (c)] are not only to obtain the thickness of the final aluminum foil, but also to play an important role in adjusting the internal structure of the aluminum foil, which is obviously different from ordinary aluminum processing. Cold rolling includes conventional cold rolling and foil rolling. The thickness of aluminum foil after conventional cold rolling is usually less than 1 mm; according to the requirements of different product varieties and specifications, the final thickness of the aluminum foil after foil rolling is mostly in the range of 0.015 to 0.12 mm. Most anode aluminum foils need to be used in a non-work hardened state, so annealing is required after cold rolling. At present, vacuum annealing technology is mostly used in China to treat aluminum foil [Figure 1.9 (d)] to prevent excessive oxidation of the aluminum foil surface. During the cold rolling process, a large amount of rolling oil and lubricants come into contact with the aluminum foil surface and cause surface stains. After cold rolling, many foreign substances such as aluminum chips and dust will also adhere to the surface. These defects may react to a certain extent on the surface of the aluminum foil during high-temperature annealing, and have an adverse effect on subsequent corrosion and chemical processing. Therefore, the cold-rolled aluminum foil needs to be cleaned before annealing!

Figure 1.9 Schematic diagram of the main processes of light foil processing:

(a) Semi-continuous casting; (b) Hot rolling; (c) Cold rolling and foil rolling;

(d) Vacuum annealing (1-casting ladle, 2-high-purity aluminum liquid, 3-crystallizer, 4-semi-continuous casting slab, 5-cold-rolled aluminum foil roll)

The above is the principle of aluminum electrolytic capacitor-Explanation of the processing of light foil

2.4 Corrosion And Chemical Processing of Aluminum Foil

Corrosion and chemical processing of aluminum foil are the final key processes for obtaining aluminum foil for high specific capacitance electrolytic capacitors. This processing stage is completed under the electrochemical conditions of appropriate electric field, temperature and corrosive medium.

Figure 1.10 shows the schematic diagram of the main process of aluminum foil corrosion process [5-6]. In the pretreatment stage, alkaline washing is often used to remove oil stains, natural oxide film, etc. on the surface of aluminum foil to provide a suitable surface state for the corrosion process. Then the aluminum foil is subjected to corrosion pore treatment, that is, the starting points of corrosion pores are uniformly distributed on the flat surface of the aluminum foil by electrochemical means. The lower the pressure resistance requirement of the aluminum foil, the higher the required pore density. After pores are formed, the generated corrosion pores need to be properly expanded according to the pressure resistance requirement of the aluminum foil, including the expansion of the corrosion pore diameter and the extension of the pore depth, so as to further increase the surface area of ​​the aluminum foil. After appropriate post-treatment, the corroded foil is obtained.

Figure 1.10 Schematic diagram of the main processes of aluminum corrosion process

(1-unwinding, 2-pretreatment, 3-cleaning, 4-drying, 5-primary pore formation, 6-secondary pore formation, 7-electrolytic pore expansion, 8-post-treatment, 9-foil collection)

Figure 1.11 shows the main process diagram of the high-voltage anode corrosion foil formation process. In the pretreatment stage, it is necessary to remove surface impurities, surface residues, etc., and a certain surface electrochemical reaction occurs to provide a suitable surface state for the formation process. Subsequently, the corrosion foil is subjected to multi-stage formation treatment, and the formation voltage of the latter stage is usually higher than that of the previous stage; the thickness and withstand voltage level of the oxide film after formation are related to the working voltage of the capacitor. Appropriate intermediate treatment needs to be added during the formation process; sometimes appropriate high-temperature treatment needs to be added to adjust the crystal structure and density of the formed oxide film. After proper post-treatment, the formed foil that can be used for winding capacitors is obtained.

Figure 1-11 Schematic diagram of the main process of high-voltage anode corrosion foil formation process

(1-unwinding, 2-pretreatment, 3-cleaning, 4-drying, 5-primary formation, 6-secondary formation, 7-tertiary formation, 8-intermediate treatment, 9-high temperature treatment, 10-end formation, 11-post-treatment, 12-foil collection)

For different product varieties and specifications of electrolytic capacitors, the process and process links of related electrochemical treatment will be very different. Figures 1.10 and 1.11 are just schematic descriptions of some of the process steps that may be involved in the relevant process. For the specific process, please pay attention to the information on Xuansn Electronics’ related website.

Summary:

The above is the principle of aluminum electrolytic capacitor -An explanation of the corrosion and chemical processing of aluminum foil

For more information about capacitors, please click:https://xuansncapacitor.com/

The post Principle of Aluminum Electrolytic Capacitor appeared first on Dongguan Xuanxuan Electrolytic Technology Co,.ltd.

1 Working status of electrolytic capacitors-ripple current generated by half-bridge LLC resonant converter

For a single-phase AC input LLC resonant converter, a power factor correction unit needs to be configured. At this time, the ripple current involved in the DC bus capacitor of the LLC resonant converter is the 100Hz ripple current component and switching frequency ripple current component generated by the power factor correction circuit, and the switching frequency ripple current generated by the LLC resonant power converter. Portion.

When the LLC resonant converter has the maximum output power, it draws the largest current from the DC bus, including both DC and AC components. The AC component must be absorbed by the DC bus capacitor connected in parallel to the DC bus. The main waveforms of the half-bridge LLC are shown in Figure 11-1. Channel 2 in the figure is the current that the LLC converter draws from the DC bus.Figure 1-1 Main waveforms of half-bridge LLC converter

Next, analyze the relationship between the current flowing through the DC bus and the DC bus capacitor, and determine the effective value of the current flowing through the DC bus capacitor corresponding to the unit output power.

1.1 Working status of electrolytic capacitors—DC bus capacitor ripple current analysis when LLC resonance duration duty cycle is 0.2

Since the current waveform consists of divided sine waves and straight lines, for the convenience of analysis, the channel 2 waveform in Figure 1-1 can be divided into moments of each time segment, and the current in the LLC resonance state segment is assumed to be DC.

The current waveform that the half-bridge LLC resonant converter draws from the DC bus is shown in Figure 1-2.

According to the waveform diagram, when the half-bridge LLC resonance duration is 20% of the switching cycle, the current leads by 18°, and the average current is

The corresponding effective current value is

Figure 1-2 DC bus current waveform

The current flowing through the DC bus capacitor is

In the formula, IBUSav and IBUSrms are the average value of the DC bus current and the effective value of the DC bus current respectively.

Usually, the mains rectifier part of the half-bridge LLC resonant converter is a power factor correction method. If the DC bus voltage is 380V, the ripple current corresponding to the unit output power is 3.68mA. If the power factor corrected output voltage is 400V, the ripple current corresponding to unit output power is 3.5mA.

1.2 Analysis of DC bus capacitor ripple current when LLC resonance duration duty cycle is 0.25

When the half-bridge LLC resonance duration is 25% of the switching cycle, the current leads by 18°, and the average current is

The corresponding effective current value is

The current flowing through the DC-Link capacitor is

Obviously, the increase in the LLC resonance duty cycle will cause the ripple current flowing into the DC bus capacitor to increase. By only increasing the LLC resonance time duty cycle by 0.05, the ripple current increases by about 9%.

Usually, the mains rectifier part of the LLC half-bridge resonant converter is a power factor correction method. If the DC bus voltage is 380V, the ripple current corresponding to the unit output power is 5.73mA/W. If the power factor corrected output voltage is 400V, the ripple current corresponding to unit output power is 5.44mA/W.

Working status of electrolytic capacitors -The above is the analysis of the ripple current generated by the half-bridge LLC resonant converter.

2 Working status of electrolytic capacitors-ripple current generated by LLC full-bridge resonant converter

In each switching cycle of the half-bridge LLC converter, there will be half a cycle that is independent of the DC bus, and the resonant capacitor provides output power and resonant energy. Therefore, in the current that the half-bridge LLC resonant converter draws from the DC bus, the AC component will be very large. In order to reduce this DC component, the full-bridge LLC resonant converter circuit topology can be used. The waveform of channel 2 in Figure 1-3 is the current waveform obtained from the DC bus by the full-bridge LLC resonant converter.

As can be seen from Figure 1-3, compared with the half-bridge LLC resonant converter, the current waveform that the full-bridge LLC resonant converter draws from the DC bus does not have the negative half-cycle power of the half-bridge LLC resonant converter, which is provided by the resonant capacitor. The phenomenon becomes that both the positive half cycle and the negative half cycle are provided with electric energy by the DC bus.

According to the waveform diagram, it can be seen that under the same current amplitude condition, the average current obtained by the full-bridge LLC resonant converter from the DC bus doubles, that is, the output power doubles, and the effective current value becomes √2​ times. Under the corresponding unit output power condition, the effective value of the AC current component flowing through the DC-Link capacitor is greatly reduced compared to the half-bridge LLC resonant converter.

Figure 1-3 Main waveforms of full-bridge LLC resonant converter

Under the conditions of a duty cycle of 0.75 and the same output power, the corresponding current of the 380V DC bus is 4.46mA/W, and the corresponding current of the 400V DC bus is 4.23mA/W. Under unit output power conditions, the effective value of the current flowing through the DC-Link capacitor in the full-bridge LLC resonant converter is 78% of the effective value of the current flowing through the DC-Link capacitor in the half-bridge LCC resonant converter.

2.1 Working status of electrolytic capacitors—DC bus capacitor ripple current distribution when LLC resonance duration duty cycle is 0.2

When the LLC resonance time is 20% of the switching cycle, the current leads by 18°, and the average current is

The corresponding effective current value is

The current flowing through the DC bus capacitor is

Usually, the mains rectifier part of the LLC full-bridge resonant converter is a power factor correction method. If the DC bus voltage is 380V, the ripple current corresponding to the unit output power is 1.84mA/W. If the power factor corrected output voltage is 400V, the ripple current corresponding to unit output power is 1.75mA/W.

Obviously, under unit output power conditions, the ripple current flowing through the DC bus capacitor of the full-bridge LLC resonant converter is 1/2 of the DC bus capacitor of the half-bridge LLC resonant converter.

2.2 Analysis of DC bus capacitor ripple current when LLC resonance duration duty cycle is 0.25

When the half-bridge LLC resonance duration is 25% of the switching cycle, the current leads by 18°, and the average current is

The corresponding effective current value is

The current flowing through the DC bus capacitor is

Obviously, the increase in the LLC resonance duty cycle will cause the ripple current flowing into the DC bus capacitor. The increase only increases the LLC resonance time duty cycle by 0.05, and the ripple current increases by 38.2%.
Usually, the mains rectification part of the LLC full-bridge resonant converter is a power factor correction method. If the corresponding rectification DC bus voltage is 380V, the corresponding ripple current per unit output power is 2.55mA/W. If the power factor corrected output voltage is 400V, the ripple current corresponding to unit output power is 2.42mA/W.

2.3 Ripple current generated by quasi-fully resonant bridge converter

The optimal working state of the LLC resonant converter is the quasi-full resonant operating mode, which allows the converter to provide power to the output with the largest possible duty cycle while ensuring the zero-voltage turn-on condition of the switching tube. In this mode, the efficiency of the LLC resonant converter is relatively highest. The main waveforms are shown in Figure 1-4.

As can be seen from Figure 1-4, the current waveform flowing through the DC bus capacitor is close to the “absolute value” sine wave, and the sine wave analysis result can be approximated as the actual operating current.Figure 1-4 Main waveforms of quasi-full resonant full-bridge converter

The current required by the quasi-full resonant converter from the rectifier and filter circuit is

The relationship between the current peak value, the output power Po and the DC bus voltage Uo is

In the formula, eta is the converter efficiency.
The corresponding effective current value is

It can be seen from the circuit principle that the ratio of the effective value to the average value of the absolute sine function is 1.11. The effective value of the current flowing through the corresponding DC bus capacitor is

If the DC input voltage of the converter is 400V, the effective value of the DC bus capacitor ripple current per unit output power is 1.2mA/W; if the DC input voltage is 380V, the effective value of the DC bus capacitor ripple current per unit output power is 1.26mA. /W.

If it is a half-bridge quasi-full resonant converter, the DC input voltage is 400V, the effective value of the DC bus capacitor ripple current per unit output power is 2.4mA/W; if the DC input voltage is 380V, the DC bus capacitor ripple current per unit output power The effective current value is 2.52mA/W.

2.4 Summary

To sum up, the closer the LC resonance period of the bridge LLC resonant converter is to 1 in the entire switching cycle, the smaller the effective value of the current flowing through the DC bus capacitor per unit output power is. Therefore, unless there are special circumstances, the smaller the proportion of the LLC resonance mode in the switching cycle, the better, as long as the switching tube can be maintained at zero voltage.

In the full-bridge LLC resonant converter, the effective value of the current flowing through the DC bus capacitor per unit output power is 1/2 of that of the half-bridge LLC resonant converter.

Working status of electrolytic capacitors -The above is the analysis of the ripple current generated by the LLC full-bridge resonant converter.

3 Working status of electrolytic capacitors—Ripple current of output capacitor of single LLC converter

LLC resonant converter During LLC resonance, the converter does not provide power to the output end, and the output filter capacitor releases (discharges) the stored energy to provide power to the load.

Therefore, there will be no LLC resonant current part in the output ripple current analysis of the LLC resonant converter, replaced by zero current.

3.1 Working status of electrolytic capacitors—Analysis of output capacitor ripple current when LLC resonance duration duty cycle is 0

When the LLC resonance duration duty cycle is 0, the output current waveform of the full-bridge LLC converter is shown in Figure 1-5. Channel 4 in the figure is the output rectifier current waveform.

As can be seen from Figure 1-5, at the highest operating frequency of the full-bridge LLC converter, the LLC resonance process only needs to maintain the zero-voltage turn-on of the switching tube and the reverse recovery process of the output rectifier. Therefore, the LLC resonance process can be designed very carefully. short, making the LLC resonant converter operate close to the full resonant operating mode. The corresponding output rectified current closely approximates an absolute sine wave. Therefore, absolute value sine wave current analysis is used.

The relationship between the average current and the peak current is

Figure 1-5 Full-bridge LLC converter output current waveform when LLC resonance duration duty cycle is 0

The relationship between the current effective value and the current peak value is

Deduct the effective value of the rectifier output current to obtain the current flowing through the output rectifier filter capacitor as

3.2 Analysis of output capacitor ripple current when LLC resonance duration duty cycle is 0.2

The output current waveform of LLC half-bridge resonant converter is shown in Figure 1-6.
​                                                                                                             Figure 1-6 Output current waveform of LLC half-bridge resonant converter

In order to simplify the analysis, it can be considered that the combination of the diode current waveform sine wave and the dead zone produces an error within the allowable range of numerical analysis. The effective value of the output current is

When the dead zone is 20% of the entire cycle, equation (1-20) becomes

The corresponding average output current is

Similarly, when the dead zone is 20% of the entire cycle, equation (1-22) becomes

The current flowing through the output rectifier filter capacitor is
​

3.3 Analysis of output capacitor ripple current when LLC resonance duration duty cycle is 0.25

For the LLC resonant converter in constant voltage/constant current mode, the output voltage range can reach 75% to 100%, so the LLC resonance duration duty cycle will reach 0.25.

When the dead zone is 25% of the entire cycle, the effective value of the output current of the corresponding output rectifier is

The corresponding average output current is

The current flowing through the output rectifier filter capacitor is

Obviously, the shorter the LLC resonance duration, the smaller the effective value of the current flowing into the output rectifier filter capacitor relative to the average output current; under the condition that the average output current is the same, the longer the LLC resonance duration, the smaller the effective value of the current flowing into the output rectifier filter capacitor. The larger the effective current value.

Working status of electrolytic capacitors—the above is the analysis of the ripple current of the output capacitor of a single LLC converter

Summarize:

Working status of electrolytic capacitors—This article mainly talks about the ripple current of the output capacitors of half-bridge LLC, full-bridge LLC and single-channel LLC converters.For more information on electrolytic capacitors, please click:https://solidcapacitor.com

The post Working Status of Electrolytic Capacitors LLC Resonance Type appeared first on Dongguan Xuanxuan Electrolytic Technology Co,.ltd.

1.1 Self-healing of electrolytic capacitors-Reasons for repairing aluminum oxide film

During the manufacturing process of electrolytic capacitors, damage to the aluminum oxide film cannot be avoided. Due to the existence of this damage, the liquid aluminum electrolytic capacitor that has just been impregnated with electrolyte and assembled or sealed has a voltage-resistant, aluminum oxide film positive electrode foil and electrolytic The liquid is in a conductive state, so the damaged aluminum oxide film must be repaired. The following is an explanation of the self-healing of electrolytic capacitors – repairing the aluminum oxide film.

How to repair damaged aluminum oxide film? For liquid aluminum electrolytic capacitors, repair the damage to the aluminum oxide film of the positive electrode foil to restore the repaired aluminum oxide film to the required voltage resistance.

The aluminum oxide film repair process requires “material”, which is aluminum, and the positive electrode foil itself is aluminum. In addition to aluminum, an “oxidant”, water or hydroxide, is also required to oxidize the positive electrode foil. The positive electrode foil repair process is to use boric acid aqueous solution to obtain an aluminum oxide film with a thickness that meets the requirements through electrolysis, that is, to repair the oxide film damaged during the nailing, impregnation, and assembly processes. In addition to applying voltage, this repair process also requires the electrolytic capacitor core to be impregnated with electrolyte.

Due to the special chemical properties of water, water is not allowed to exist in the electrolyte of electrolytic capacitors. Even water in the “aqueous” electrolyte can only exist in the form of hydrates, and only in low-voltage electrolytic capacitors. For high-voltage electrolytic capacitors, the electrolyte is required to be “anhydrous”. Therefore, assembled or sealed electrolytic capacitors cannot use water to repair the aluminum oxide film.

At this time, it is necessary to use the oxygen in the hydroxide radicals to oxidize the aluminum and reconstruct the oxide film where the oxide film is damaged. A branch of ethylene glycol, the solvent of the electrolyte, has two hydroxyl groups, which can be used to oxidize aluminum and reconstruct the aluminum oxide film.

The above is an analysis of the reasons for the self-healing of electrolytic capacitors to repair the aluminum oxide film.

1.2 Self-healing of electrolytic capacitors – aging at room temperature

Room temperature aging is to gradually apply DC aging voltage to the assembled or sealed electrolytic capacitors under normal temperature conditions, and finally reach the set aging voltage value.

The normal temperature aging voltage is generally 1.15 times the rated voltage. For example, the normal temperature aging voltage of an electrolytic capacitor with a rated voltage of 400V is 1.15 times that of 400V, that is, 460V.

The main processes that cause damage to the aluminum oxide film include cutting of the positive electrode foil, riveting and flattening during the core nailing and rolling process, and from slitting to nailing and rolling. Part of the alumina film may be damaged due to mechanical force. The dielectric properties of the positive electrode guide pin or guide bar without an aluminum oxide film are basically the same as those of the aluminum oxide film. Therefore, the positive electrode guide pin or guide bar must also obtain an aluminum oxide film with sufficient voltage resistance during the aging process. This is The area of the aluminum oxide film that needs to be repaired during the aging process and the gas production and heat generation during the aging process are increased. In electrolytic capacitors of certain specifications, the heat or gas produced by the aluminum oxide film produced on the guide pins or guide strips during the aging process will exceed the amount of heat or gas produced by the damaged oxide film during the repair and manufacturing process. , causing the electrolytic capacitor to dry out during the aging process or the internal pressure is too high to produce a convex bottom, resulting in defective products.

The normal temperature aging process is to obtain the required oxide film through anode oxidation on the surface of the above-mentioned damaged aluminum oxide film or the positive electrode guide pin or guide bar without aluminum oxide film.

It should be noted that the aging process is an exothermic reaction process and will release a large amount of heat. If this heat cannot be dissipated, it will cause the electrolytic capacitor to overheat, resulting in convex bottoms or dry packs. In order to prevent the electrolytic capacitor from overheating or drying out during the aging process at room temperature, the aging current needs to be limited to ensure that the aging heat and the heat dissipation of the electrolytic capacitor are balanced at a suitable temperature.

In addition to generating heat, the aging process is also an oxidation-reduction reaction that takes away oxygen from the water in the electrolyte, causing the aluminum on the surface of the aluminum foil to be oxidized. The remaining hydrogen will form hydrogen gas, so the aging process will produce gas. Since liquid aluminum electrolytic capacitors are relatively sealed components, excessive gas production in the electrolytic capacitor shell will cause the internal pressure of the electrolytic capacitor to be too high, eventually causing a convex bottom phenomenon and resulting in scrap.

Aging current that is too small will increase the aging time and also produce undesirable aluminum hydroxide or hydrated aluminum oxide. Water and aluminum oxide will increase leakage current, which is undesirable. It can be seen from this why the “cracking” process is needed in the process of making high-pressure foil, which is to let the formed aluminum foil pass through an area of about 400°C to crack the hydrated alumina into gamma alumina. The aging process of electrolytic capacitors cannot reach a high temperature of 400°C, so the leakage current characteristics of the repaired oxide film are not as good as the leakage current characteristics of the oxide film of the chemical foil.

After room temperature aging, high temperature aging is also required.

The above is an analysis of the self-healing of electrolytic capacitors – aging at room temperature.

1.3 Self-healing of electrolytic capacitors-High temperature aging

Although room temperature aging can solve the leakage current problem at room temperature, as the operating temperature increases, the withstand voltage of the alumina film repaired by room temperature aging decreases, and the leakage current also increases, making it unable to adapt to the requirements of high operating temperatures. It must undergo high-temperature aging so that the aluminum oxide film at the aging repair site can meet the pressure requirements at high temperature and the leakage current drops to the desired value.

High-temperature aging temperature: The maximum ambient temperature is 85°C. The high-temperature aging temperature of electrolytic capacitors is 85°C. The maximum ambient temperature is 105°C. The high-temperature aging temperature of electrolytic capacitors is 105°C. The maximum ambient temperature is 125°C. The high-temperature aging temperature of electrolytic capacitors is 105°C. 125℃, the maximum ambient temperature is 140℃. The high-temperature aging temperature of electrolytic capacitors is 140℃, and the maximum ambient temperature is 150℃. The high-temperature aging temperature of electrolytic capacitors is 150℃. The higher the temperature, the more difficult high-temperature aging is.

If an electrolytic capacitor with a maximum ambient temperature of 105°C is aged at an ambient temperature of 85°C, the actual withstand voltage at an operating temperature of 105°C will not reach the aging voltage value, or even the rated voltage value. If it is 125℃, 145℃ or even 150℃, the high temperature withstand voltage loss will be more.

Since heat and gas will also be generated during high-temperature aging, a suitable aging current is required to avoid waste products such as convex bottoms and dry bags caused by overheating during high-temperature aging.

High-temperature aging requires relatively sufficient time to reduce the leakage current to a leakage current value that meets quality requirements. Generally speaking, the longer the aging time, the smaller the leakage current value. However, long aging time will reduce productivity and increase energy consumption, mainly because the power consumption of high-temperature aging heating cannot be ignored.

High temperature aging voltage: Taking 400V/105℃ electrolytic capacitors as an example, the high temperature aging voltage is 410~430V. The higher the high temperature aging voltage, the better the quality of the electrolytic capacitor.

The above is the self-healing of electrolytic capacitors – high temperature aging analysis

1.4 Serious consequences of leakage current

Because the higher the quality of the aluminum oxide film of the positive electrode foil, the leakage current is lower, even almost zero! However, although the positive electrode foil is made of high-purity aluminum, there are also some impurities that will affect the formation and performance of the aluminum oxide film. As long as the aluminum oxide film is defective, leakage current will occur.

In addition to the purity of the positive electrode foil affecting the quality of the aluminum oxide film, the aging process also affects the quality of the aluminum oxide film. The better the aluminum oxide film is repaired during the aging process, the lower the leakage current will be.

In fact, the aging process of electrolytic capacitors is a compromise between performance and cost.

In terms of materials, the quality of the formation foil affects the final leakage current of the electrolytic capacitor. Under the same formation voltage conditions, the better the quality of the formation foil, the better the quality of the electrolytic capacitor made under the same manufacturing process conditions, the leakage current will be relatively higher. Small.

In terms of manufacturing process, the smaller the damage and burrs caused by cutting the positive electrode foil, under the same manufacturing process conditions, the better the quality of the final electrolytic capacitor, which means the smaller the leakage current. Therefore, cutting is a very important process in the manufacturing process of electrolytic capacitors.

The nailing and rolling process will also cause damage to the formed foil. The smaller the damage caused by this process, the smaller the final leakage current of the electrolytic capacitor.

What will happen if the leakage current exceeds the standard?

First of all, excessive leakage current of electrolytic capacitors (especially high-voltage electrolytic capacitors) may cause serious heating of the electrolytic capacitors. In most cases, the leakage current of electrolytic capacitors can be considered to be repairing the aluminum oxide film. Therefore, the greater the leakage current, the more gas will be produced. Coupled with excessive heat, the electrolytic capacitor may suffer early failure with a convex bottom.

Due to excessive leakage current, electrolytic capacitors generate more heat than normal electrolytic capacitors, which will cause the electrolyte in the electrolytic capacitor to lose too quickly, leading to dry packaging during the life cycle, loss of capacitance and early failure.

The above is the self-healing of electrolytic capacitors – analysis of the consequences of serious leakage current

1.5 Reasons for the increase in leakage current caused by storage

The storage process of electrolytic capacitors will increase the leakage current. In the early days after electrolytic capacitors are made, the leakage current increases rapidly. After some high-voltage electrolytic capacitors are stored at room temperature for half a year, the leakage current will increase to 3 times the initial value. The longer the storage time, the leakage current will tend to increase steadily.

The main reasons for the increase in leakage current of electrolytic capacitors during storage are:

The storage process of electrolytic capacitors will increase the leakage current. In the early days after electrolytic capacitors are made, the leakage current increases rapidly. After some high-voltage electrolytic capacitors are stored at room temperature for half a year, the leakage current will increase to 3 times the initial value. The longer the storage time, the leakage current will tend to increase steadily. .

The main reasons for the increase in leakage current of electrolytic capacitors during storage are:

1. The microbattery effect caused by impurities in the positive electrode foil (especially copper) destroys the aluminum oxide film.

2. Chloride ions in electrolytic capacitors have the strongest ability to destroy the aluminum oxide film. Therefore, the requirements for chloride ions in the aluminum foil, shell, electrolyte, and rubber plugs of electrolytic capacitors are extremely strict. Once chloride ions are present, the leakage current of the electrolytic capacitor will will increase and life span will be shortened. Therefore, electrolytic capacitors are extremely sensitive to the presence of chloride ions.

3. In order to enhance the aluminum oxide film repair ability of aluminum electrolytic capacitors, phosphoric acid and other substances need to be added to the electrolyte. Phosphoric acid is a medium-strength acid that corrodes the aluminum oxide film under certain conditions, and this corrosion will increase leakage current. Some people believe that under long-term high temperature, the corrosive effect of phosphoric acid on the aluminum oxide film will cause the aluminum oxide film to become thinner and reduce the pressure resistance. Therefore, the formation voltage of the positive electrode foil of a high-voltage long-life electrolytic capacitor is higher than the formation voltage of the formation foil of a 2000-hour life electrolytic capacitor.

High temperature storage without DC bias will cause electrolytic capacitor leakage current to increase. In the electrolytic capacitor data sheet, the storage time at high temperature without DC bias is only 1000h, while at high temperature with rated DC bias voltage, the electrolytic capacitor can be stored at least 3000h, or even more than 20000h. It can be seen that high-temperature storage without DC bias causes great damage to electrolytic capacitors. In the actual storage process of electrolytic capacitors, the storage temperature is generally required to be no higher than 35°C.

The above is an analysis of the reasons for the increase in leakage current caused by the self-healing of electrolytic capacitors due to storage.

1.6 Problems with electrolytic capacitors placed beyond their expiry date

Qualified electrolytic capacitors produced will have the problem of increased leakage current after being left for a long time, so there are requirements for the storage time of electrolytic capacitors. The life of electrolytic capacitors that have exceeded the storage time cannot be guaranteed, and even if they are aged and repaired again, they will not be able to obtain the expected life at the factory.

Electrolytic capacitors placed directly beyond the expiry date can only be used in experimental circuits and cannot be sold because the lifespan cannot be guaranteed. After aging repair, it can be used in applications that are not sensitive to longevity. Electrolytic capacitors that have been stored for an extended period may have a short life in high temperature environments, so they must not be used in high temperature environments!

Is it true that after normal temperature aging and high temperature aging, the service life of electrolytic capacitors can be guaranteed by placing them for an extended period? The conclusion is that it cannot be determined, strictly speaking it cannot! The reason is that the oxide film medium is corroded by the electrolyte in an environment without repair function (without power), or other metal impurities in the electrolytic capacitor and the electrolyte form a primary battery and corrode the aluminum oxide film. This is the difference between aluminum oxide film in dry sealing environment.

In short, electrolytic capacitors placed overdue cannot be sold as qualified products, and electrolytic capacitors placed overdue by customers cannot be assembled into products, otherwise they may cause early failure. Even if the products made by customers are stored beyond the expiration date, the life of the electrolytic capacitors will be reduced.

Repair of overdue electrolytic capacitors: Different from the aging process in the production process of electrolytic capacitors, repair of overdue electrolytic capacitors is often a time-consuming process that requires the slow application of DC voltage. For example, apply to 1/3 of the rated voltage, remove the DC power supply after 3 to 4 hours, and then test the remaining voltage of the electrolytic capacitor after leaving it for half a day or one day.

If the remaining voltage is more than 90% of the initial voltage, when applying DC voltage for repair again, you can continue to slowly increase the voltage within the leakage current range given in the data sheet, maintain it for half a day or even a day, and then continue to increase the repair voltage until it gradually rises to the rated voltage. The voltage value is maintained for 24h or even 72h. After repair, remove the DC power supply and leave it for 24 hours. If the remaining voltage is not less than 90% of the initial voltage, the repair can be considered complete. If the residual voltage is relatively low, less than 2/3 of the initial voltage, the electrolytic capacitor has no repair value, or is used in a state where it can explode to 90% of the initial voltage value.

Why do overdue electrolytic capacitors require such a complex and lengthy repair? The fundamental reason is that the oxide film of an electrolytic capacitor that has been stored for an extended period of time, especially the part of the oxide film that has been repaired due to aging, is seriously damaged during the extended storage period. Rapid repair may cause the electrolytic capacitor to have a convex bottom or close to a convex bottom.

The above is the self-healing of electrolytic capacitors – analysis of the problems of electrolytic capacitors that have been placed for an extended period of time.

1.7 Application process of aluminum oxide film repair

The self-healing properties of electrolytic capacitors are only reflected in the ability of electrolytic capacitors to “repair” defects in electrolytic capacitors. This repair is at the expense of certain substances and the performance of electrolytic capacitors. Usually, the self-healing properties of electrolytic capacitors only repair local minor “weaknesses” of electrolytic capacitors. For example, aluminum electrolytic capacitors use applied voltage to anode oxidize tiny defects in the anode oxide film to repair the defective parts, but this process will consume part of the electrolyte; while tantalum electrolytic capacitors and polymer electrolytic capacitors are conductive channels that make the “weak” parts Reduce or block these conductive channels. This actually reduces the effective electrode area of both capacitors. If the “weakness” is extremely small, the impact on capacitance is negligible.

There is nothing that can be done about major defects such as high leakage current and breakdown. Therefore, the self-healing characteristics of electrolytic capacitors are the same as those of metallized capacitors. It only removes the tiny “weaknesses” in the capacitor and cannot repair the breakdown capacitor.

The above is the self-healing of electrolytic capacitors – analysis of aluminum oxide film repair during the application process

2 Self-healing of electrolytic capacitors – self-healing characteristics of solid aluminum electrolytic capacitors

2.1 Solid aluminum electrolytic capacitors do not have the ability to repair the aluminum oxide film

Unlike liquid electrolytic capacitors, the failure mode of solid aluminum electrolytic capacitors is short circuit, which is unfamiliar to electronic engineers who have adapted to the open circuit failure of liquid electrolytic capacitors.

After the solid aluminum electrolytic capacitor is short-circuited, a huge current will flow, causing the capacitor to overheat, decomposing the polymer conductive polymer into non-conductive substances and opening the circuit, that is, the short circuit is followed by the open circuit. If the open-circuit process has not yet occurred, the solid aluminum electrolytic capacitor will generate high heat due to the extreme short-circuit current, the polymer will vaporize, and high pressure will be generated inside the capacitor, causing it to explode. The solid aluminum electrolytic capacitor will have no open-circuit process and corresponding phenomena.

It is undeniable that solid aluminum electrolytic capacitors cannot avoid defects in the aluminum oxide film. How to turn these defects into non-conductive properties? This is the self-healing ability of solid aluminum electrolytic capacitors.

Different from liquid aluminum electrolytic capacitors, solid aluminum electrolytic capacitors did not consider the function of repairing the aluminum oxide film in the polymer conductive polymer at the beginning of the design, or it was impossible to achieve this function at all. This makes solid aluminum electrolytic capacitors in principle It does not have the ability to repair the aluminum oxide film.

Due to the above reasons, solid aluminum electrolytic capacitors will not allow the aluminum oxide film to be damaged, or high molecular conductive polymers will not be allowed in the damaged aluminum oxide film. In actual solid aluminum electrolytic capacitors, the polymer conductive polymer is in close contact with the aluminum oxide film (including defects).

How to deal with polymer conductive polymers at defects in aluminum oxide films?

The above is the self-healing of electrolytic capacitors——Analysis on the repairability of solid aluminum electrolytic capacitors without aluminum oxide film

2.2 The necessity of aging solid aluminum electrolytic capacitors

Many product data sheets or catalogs (samples) of electrolytic capacitors always say that electrolytic capacitors have “self-healing” capabilities. So what is the “self-healing” capability of solid aluminum electrolytic capacitors?

Self-healing of electrolytic capacitors: During operation or storage of electrolytic capacitors, parts of the positive oxide film may be damaged or defective for some reason, resulting in some weaknesses that increase the leakage current of the capacitor. However, since the electrolytic capacitor uses the electrolyte as the negative electrode, under the action of an external voltage, the electrolyte solution can release oxygen and re-form the oxide film where the oxide film is damaged, which plays a self-repair role and restores the working ability of the liquid electrolytic capacitor. This kind of This phenomenon is called self-healing of electrolytic capacitors.

The self-healing principle of solid aluminum electrolytic capacitors is the exact opposite process, which requires burning and decomposing the polymer conductive polymer at the defects of the aluminum oxide film.

The self-healing principle of solid aluminum electrolytic capacitors is shown in Figure 1-1.

The simplest way to decompose polymer conductive polymers is high-temperature aging. This is the necessity of the aging process of solid aluminum electrolytic capacitors.

Figure 1-1 Self-healing principle of solid aluminum electrolytic capacitors

The aging process of solid aluminum electrolytic capacitors can be simply understood as: placing the solid aluminum electrolytic capacitors in the highest operating temperature environment, and applying a DC aging voltage with current limiting function to the rated voltage. Leakage current occurs at defects in the aluminum oxide film, and the temperature reaches the decomposition temperature of the polymer conductive polymer, causing the polymer conductive polymer there to no longer conduct electricity. Therefore, the aging process is a process in which the component decomposes the high molecular conductive polymer at the defects of the aluminum oxide film.

It should be noted that this aging process must be a small current and long-term process, otherwise the defective solid aluminum electrolytic capacitor will be short-circuited and explode if it is directly powered on.

The above is the self-healing of electrolytic capacitors – analysis of the necessity of aging of solid aluminum electrolytic capacitors

3 Self-healing of electrolytic capacitors – self-healing characteristics of tantalum electrolytic capacitors

Once a solid tantalum electrolytic capacitor with manganese dioxide as the negative electrode is made, defects in the oxide film cannot be repaired. In other words, defective tantalum electrolytic capacitors will be scrapped if no measures are taken.

The failure mode of a tantalum electrolytic capacitor with manganese dioxide as the negative electrode is a short circuit, rather than an open circuit like a liquid aluminum electrolytic capacitor, so you will see an explosion when the tantalum electrolytic capacitor fails.

Next, we will introduce the repair of tantalum electrolytic capacitors with manganese dioxide as the negative electrode with slight defects.

For solid electrolytes, defects (such as cracks, other metal impurities such as nickel) may occur in the tantalum pentoxide dielectric layer, which will increase conductivity or leakage current. The defects are shown in Figure 1-2, cracks and cracks in the tantalum pentoxide film. nickel.

The self-healing effect mainly involves flowing a large current through the defects of the tantalum pentoxide film to generate high heat, causing the manganese dioxide used as the negative electrode to decompose into high-resistance manganese trioxide and “block” the defects. It should be noted that the large current mentioned here is definitely not the current generated by a voltage source that directly applies the rated voltage and the current is not limited, otherwise it will cause an explosion. Self-healing should be an aging process under appropriate current value conditions. Manufacturing imperfections are eliminated through this aging process. Defects created during use can be “burned away” during use.

The self-healing effect of the tantalum polymer electrolytic capacitor is that a large current flows through the defects of the tantalum pentoxide film to generate high heat, causing the conductive polymer at the defect to evaporate. Then there is no negative electrode at the defect of the tantalum pentoxide film, thus The conductive path at the defect is cut off. Its self-healing principle is shown in Figure 1-3.

The above is the self-healing of electrolytic capacitors – analysis of the self-healing characteristics of tantalum electrolytic capacitors

Figure 1-2 Self-healing principle of tantalum electrolytic capacitors

Figure 1-3 Self-healing principle of tantalum polymer electrolytic capacitor

Summarize:

The self-healing of electrolytic capacitors – mainly talks about the repair of the aluminum oxide film of liquid aluminum electrolytic capacitors, the self-healing of solid aluminum electrolytic capacitors, and the self-healing characteristics of tantalum electrolytic capacitors. If you want to know more about electrolytic capacitors, please click:https://capacitorsfilm.com

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