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1.1 Leakage current and its characterization parameters

When a constant DC voltage is applied across the capacitor, its charging current starts to be very large and gradually decreases over time. However, the decline will not continue indefinitely, but will reach a more stable state after reaching a certain final value. This final value is called the leakage current, or simply the leakage current IL. During testing, the leakage current of electrolytic capacitors is generally related to the following factors:

(1) The level of applied DC voltage;
(2) The capacitance of the capacitor itself;
(3) The length of time the voltage is applied (applying voltage for a longer time can improve the quality of the oxide film);
(4)The environment in which the capacitor is located;
(5) The type of base metal of the capacitor and the product manufacturing process.

In order to facilitate the comparison of the leakage current of various types of electrolytic capacitors under different voltages and capacitances, the leakage current constant is defined
as follows:

K=I/CU                                            (7-77)

The leakage current constant unit is generally uA/(V·uF). The value of K can be derived from the parallel equivalent circuit

K=U/R·1/CU=1/RL                          (7-78)

Among them, R is the leakage resistance of the capacitor. In resistor-capacitor parallel circuits, RC is generally called the time constant. Therefore, from equation (7-78), we can know that the leakage current constant is the reciprocal of the capacitor time constant. For aluminum electrolytic capacitors, it can be deduced based on the properties of its oxide layer

R=Pd/A                                           (7-79)

In the formula: P is the resistivity of the oxide layer; d is the thickness of the oxide layer; A is the effective surface area of the oxide film. and

C=εoεrA/d                                       (7-80)

but

K=1/εoεrPv (7-81)

Generally speaking, for the aluminum oxide layer, its resistivity is about 10^13~10^14Ω·cm, and its dielectric constant is about 10. Substituting it can be obtained

K≈0.1~0.01[uA(V·uF)]                                  (7 -82)

The upper limit generally corresponds to the K value specified for ordinary products, and the lower limit is approximate to the K value of long-life products.

From the above analysis, it can be seen that the leakage current constant of aluminum electrolytic capacitors is closely related to the insulation quality of its anodized film. The oxide film medium of the actual device is not a perfect ultra-thin film, and there are more or less various tiny flaws, holes, gaps and other defects on its surface. The leakage current of the product consists of impurity ion current and electron current passing through these defects. For liquid electrolyte type capacitors, since the oxide film is always in the working electrolyte, the contact between the electrolyte and the oxide film is very good and can be repaired at any time, so the leakage current constant is extremely small, as low as 2X10^-3uA/(uF· V). For solid electrolyte type capacitors, there is more or less poor contact between the oxide film and the solid electrolyte. In addition, the solid electrolyte has almost no ability to repair the oxide film, so the leakage current constant is large, up to more than 0.2uA/(uF· V).

1.2 Effect of material purity on leakage current

Material purity has a strong impact on leakage current. Impurities will make the generated oxide film susceptible to electrolyte erosion, so the purity of raw materials should be improved as much as possible during the manufacturing process. When the purity of the anode foil is increased from 99.9% to over 99.99%, the leakage current of the manufactured capacitor can be reduced by an order of magnitude. In addition, chloride ions have a great destructive effect on the oxide film, so the purity of the electrolyte also has a great impact on the leakage current. On the basis of using 99.999% high-purity aluminum foil, ultrapure water with a resistivity of 18.2 Ma·cm, extremely low chloride ion content in chemicals, and strict requirements for purification in the process operation room, extremely low leakage current parameters can be obtained. The leakage current of aluminum electrolytic capacitors at room temperature can be as low as K-0.001uA/(uF·V) or even K=0.0001uA/(uF·V), which is comparable to tantalum capacitors and can work reliably below 55°C. 30 years.

1.3 Effect of temperature on leakage current

Under constant voltage, if the temperature gradually increases from room temperature, the leakage current will increase rapidly in the form of an exponential function. Generally, at the positive limit temperature (85°C), the leakage current of aluminum electrolytic capacitors is 2.5 to 3 times that of room temperature. For example, the leakage current coefficient of CD26/27 aluminum electrolytic capacitor is 0.03 uA/(uF·V) at 20℃, and its leakage current coefficient is 0.075 to 0.15 at the extreme temperature of 85℃. From an essential analysis, no matter what form of electrolytic capacitor, the leakage current has a common reason as the temperature rises, that is, the migration ability of impurity ions in the oxide film of the capacitor will sharply increase as the temperature rises.
Of course, each type of capacitor also has its own special reasons, and the degree of dependence is also different. For example, when using ammonium borate ethylene glycol/water system as the working electrolyte, at high temperatures, the oxide film is hydrated and changes part of its structural form, resulting in a sharp increase in leakage current. However, N,N-dimethylformamide is used as the The working electrolyte of a solvent will not have the destructive effect of hydration of the oxide film; in addition, if there are chloride ions inside the electrolyte or aluminum foil, the corrosion of the base metal and oxide film will increase at high temperatures, which will also lead to an increase in current, but this type of Unlike the increase in migration ability, the process will not change immediately as the temperature rises, but will take time to gradually have an impact.
In addition, since aluminum and tantalum oxide films also have a slow relaxation ion polarization form, at high temperatures, the interstitial ions in their structures can also participate in conduction and contribute to an increase in leakage current.

1.4 Effect of applied voltage size and duration

Leakage current increases with increasing applied voltage. It rises slowly when the voltage is lower than the rated voltage, and rises sharply when it slightly exceeds the rated voltage.

The reason why the leakage current increases with voltage can be understood as follows: when the applied voltage increases and the film thickness remains unchanged, the electric field intensity in the film will increase as the voltage increases. The relationship between electron current and field strength in the oxide film conforms to the following hyperbolic sine function relationship:

i= Asinh(BE)                                     (7-83)

In the formula: A and B are relevant constants, and the value of A has an exponential relationship with temperature. Therefore the leakage current increases.

In addition, when the field strength reaches a certain value, the impurity ions that are weakly bound in the crystal lattice may break away from their bonds, thus increasing the number of participating conductive particles and increasing the leakage current.

In addition, the leakage current generally decreases slowly as the voltage application time increases, and finally reaches a steady state. There are three explanations for this phenomenon.

(1) Electrolytic capacitors have slow polarization, especially foil-type structures using liner paper. Due to their large capacitance, they show a large absorption current during the charging process, so it takes a long time to complete. .
(2) The oxide film of the electrolytic capacitor may be chemically or electrochemically damaged due to the presence of impurities, causing flaws or defects on the surface. Therefore, when voltage is applied, the electrolyte will repair part of the damaged film layer, causing the leakage current to decrease. The evidence is that for capacitors with a short interruption time, the relationship between the leakage current and the application time is not very obvious after the voltage is continued to be applied.
(3) During the use of electrolytic capacitors, after a period of time, the micropores and gaps in the film will be filled with oxygen, blocking the path between the electrolyte and the anode, so the leakage current is reduced. But when the voltage is removed, the oxygen in the holes is released. Therefore, the next time the voltage is continued to be applied, oxygen needs to be regenerated to plug the hole conductive path.

In order to measure the leakage current of aluminum electrolytic capacitors, it is generally specified that the measured current after 5 minutes is smaller than the actual leakage current. However, under production conditions, considering the decline of production efficiency and leakage current, the initial leakage current drops quickly, so enterprises can set a leakage current value of 30 seconds during production.

1.5 Effect of storage period and storage conditions

The longer the storage period of the aluminum electrolytic capacitor, the greater the possibility of the oxide film being damaged due to the role of the impurity metal ions and the aluminum foil forming a micro-battery and the hydration erosion of the aluminum oxide film by the working electrolyte with more water. The membrane may have a dissolving and damaging effect. This phenomenon is called disempowerment. Therefore, after a long period of storage, if the rated voltage is immediately connected, the leakage current will be very large, and it will decrease very slowly within a normal period of time, and the oxide film will not have time to repair. At the same time, the capacitor generates a lot of heat, and more parts of the oxide film are broken down, which may cause permanent damage. Therefore, generally for electrolytic capacitors that have been stored for a long time, the operating voltage should be gradually increased to the rated voltage so that the oxide film can be repaired in advance and normal operation can be restored.

In short, the size of the leakage current is the most direct indicator of the performance of the electrolytic capacitor and the suitability of the process and the degree of civilized production. Impurities in any raw materials and pollution during operation (dust, sweat, etc.) may lead to uneven oxidation growth. And there are defects on the film, and the main part of the steady-state leakage current, the electron current, is carried out through the defects and the edge of the foil.

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The Capacitor Symbol in Circuit Diagrams

The capacitor symbol, with its distinctive appearance, stands out among the myriad of other symbols in circuit diagrams. It consists of two parallel lines separated by a gap, akin to the metal plates found inside a capacitor. These plates, when charged, store electrical energy temporarily, allowing capacitors to perform a wide range of functions in electronic circuits.· +

The history of the capacitor symbol dates back to the early days of electrical engineering, where inventors and engineers sought a visual representation that would convey the capacitor’s core properties without ambiguity. Over time, international organizations like the International Electrotechnical Commission (IEC) and the American National Standards Institute (ANSI) worked to standardize this symbol, ensuring consistency across circuit diagrams worldwide.

Understanding the Capacitor Symbol

The simple layout of the capacitor symbol holds valuable information about its function and characteristics. The two parallel lines represent the conductive plates of a capacitor, while the space between them symbolizes the insulating material, also known as the dielectric. It is this dielectric that allows the capacitor to store electric charge, as it resists the flow of charge between the plates.

Moreover, the placement and alignment of the lines in the capacitor symbol are intentional. The vertical alignment indicates that the capacitor is non-polarized, meaning it can be connected in either direction within a circuit. However, specialized capacitors, such as electrolytic capacitors, employ a unique symbol to denote polarity, preventing potential damage from incorrect connections.

Symbols for Different Types of Capacitors in Circuits

The circuit symbols of capacitors can be classified based on various factors, such as capacitor type, capacitance, polarity, and specific applications. Here’s a classification of capacitor circuit symbols:

1.Circuit symbol for  non-polarized capacitors

The circuit symbol for a non-polarized capacitor is typically represented by two parallel lines with equal length, and no arrow or polarity markings. This symbol indicates that the capacitor is not polarity-sensitive, meaning it can be connected in either direction within the circuit.These capacitors include various types, such as ceramic capacitors, film capacitors (polyester, polypropylene, etc.), non-polarized tantalum capacitors, non-polarized aluminum electrolytic capacitors, supercapacitors, and certain variable capacitors used in tuning applications. They are widely used in electronic circuits for their versatility and flexibility in orientation. Always check the datasheet or product specifications to confirm the polarity characteristics of a capacitor and select the appropriate type based on the application requirements and voltage considerations for proper functioning and reliability.The following are common non-polarized capacitor symbols：

2.Circuit symbol for  polarized capacitors

Polarized capacitors have specific positive (+) and negative (-) terminals, making them sensitive to polarity in a circuit. There are two main types: electrolytic capacitors (often made of aluminum or tantalum) and tantalum capacitors. They offer high capacitance in compact sizes and are commonly used in power supply circuits and portable electronics. Proper orientation is crucial, and connecting them in reverse can cause serious issues, so it’s essential to follow polarity markings and consult datasheets for voltage ratings to ensure safe and reliable operation.

3.Circuit symbol for Variable capacitors

Variable capacitors are adjustable capacitors that allow manual or electronic changes to their capacitance. They are commonly used in radio-frequency circuits for tuning and resonance adjustments. Trimmer capacitors are a type of variable capacitor used for precise calibration. They come in air or ceramic dielectric types, and their capacitance can be adjusted mechanically or electronically. Variable capacitors are essential for frequency control in electronic devices and are widely used in radios, oscillators, filters, and tuning systems.

How to distinguish the positive and negative poles of electrolytic capacitors?

Distinguishing the positive and negative poles of electrolytic capacitors is essential to ensure proper and safe operation in a circuit. Electrolytic capacitors are polarized, meaning they have specific positive (+) and negative (-) terminals, and connecting them in reverse can cause catastrophic failures. Here’s how to identify the positive and negative poles of electrolytic capacitors:

Look for Markings: Most electrolytic capacitors have clear markings indicating their polarity. The negative terminal is usually denoted by a stripe or arrow running down the side of the capacitor, indicating the negative (-) lead. Sometimes the positive terminal may have a “+” symbol, but the stripe is a more common indicator for the negative side.

Longer Lead: Electrolytic capacitors often have one lead longer than the other. The longer lead typically corresponds to the positive (+) terminal, while the shorter lead is the negative (-) terminal.

Case Shape: In some cases, the shape of the capacitor case can provide a clue about polarity. The negative terminal of some aluminum electrolytic capacitors may have a slightly flat or indented shape compared to the positive terminal, which may have a rounded shape.

Datasheet: If the capacitor is still in its packaging or you have access to its datasheet, it will provide clear information on the polarity.

Remember to always double-check the markings and confirm the polarity before connecting the capacitor to your circuit. Connecting the capacitor in reverse can lead to overheating, leakage, or even explosion in extreme cases. Be cautious and ensure the correct polarity is observed.

How to use a multimeter to measure the positive and negative electrodes of capacitors

Measuring the positive and negative poles of a capacitor using a multimeter is a straightforward process. However, it’s essential to handle capacitors with care, especially if they might still hold a charge. Follow these steps to measure the polarity of a capacitor using a digital multimeter:

Note: Before proceeding, make sure the capacitor is fully discharged and disconnected from any power source.

Step 1: Set the Multimeter to Capacitance Mode

1. Turn on your digital multimeter (DMM).
2. Select the capacitance mode (usually denoted by “F” for Farads) on the dial or function setting of your multimeter.

Step 2: Discharge the Capacitor (if needed)

1. To ensure your safety, discharge the capacitor if it’s still holding a charge. You can do this by connecting a resistor across its terminals. Alternatively, you can use a low-voltage power supply (around 5V) to gradually discharge the capacitor.

Step 3: Identify the Terminals

1. Look at the capacitor to find the markings or labels that indicate the positive and negative terminals. Common symbols include “+” or “POS” for the positive terminal and “-” or “NEG” for the negative terminal.
2. If there are no markings on the capacitor, look for the longer leg or the one with a stripe or indentation; this typically indicates the negative (-) terminal. The other leg would be the positive (+) terminal.

Step 4: Place the Multimeter Leads

1. Take the black (common) lead of the multimeter and connect it to the negative (-) terminal of the capacitor.
2. Take the red (positive) lead of the multimeter and connect it to the positive (+) terminal of the capacitor.

Step 5: Read the Display

1. Once the multimeter leads are correctly connected, the display should show the capacitance value, which indicates the polarity of the capacitor. If the reading is positive, then the red lead is on the positive terminal and the black lead on the negative terminal.

Step 6: Reverse the Leads (Optional)

1. If the multimeter displays a negative capacitance value, it means you’ve swapped the leads. Simply reverse the leads: connect the red lead to the negative terminal and the black lead to the positive terminal.
2. The multimeter should now display the positive capacitance value, indicating the correct polarity.

Step 7: Verify the Polarity

1. After identifying the positive and negative terminals, double-check that they are indeed correct based on the capacitor’s markings or other labeling methods.

Remember, always be cautious when working with capacitors, especially when they might still be charged. If you’re unsure about any step or if you’re dealing with high-voltage capacitors, consider seeking assistance from a qualified professional.

Conclusion

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1 Harmonic components of the converter station

1.1 Overview

The converter valve cyclically conducts and connects the AC and DC ends of the converter circuit according to a certain conduction sequence. When the converter operates in the rectification mode or the inverter mode, it will absorb capacitive reactive power from the AC system. Harmonic voltages and currents are generated on the AC and DC sides, resulting in power frequency sine wave distortion of the AC side voltage and current, and the DC side voltage and current are not smooth, but with ripples. The harmonic order is the ratio of the frequency of the harmonic to the fundamental frequency of the AC grid. Under ideal conditions, the harmonics generated by the commutation are called characteristic harmonics, and the harmonics generated by the converter are all related to the basic pulsating unit 6 pulsating the commutating unit.

The principle of the 6-pulse converter is explained below. The wiring diagram of the principle of the 6-pulse converter unit is shown in Figure 1-1. The 6-pulse converter unit consists of a converter transformer, a 6-pulse converter, and the corresponding AC filter, DC filter and control and protection equipment.

The 6-pulse converter generates 6K±1 and 6K characteristic harmonics on the AC side and the DC side respectively (K is a positive integer). Therefore, a 6K±1-order AC filter needs to be configured on the AC side, and a 6K-order DC filter is usually required for overhead lines on the DC side. 6-pulse is the basic and the smallest unit that can constitute a converter. Usually, most DC systems use 12-pulse, that is, two basic 6-pulse converter units are connected in series on the DC side, as shown in Figure 1-2 Principle wiring as shown in the figure.

The 12-pulse commutation unit is composed of two 6-pulse commutation units with a voltage phase difference of 30 on the AC side connected in parallel on the AC side (in series on the DC side). The 12-pulse converter generates characteristic harmonics of 12 K ± 1 and 12 K times on the AC side and the DC side, respectively. Therefore, only 12 K ± 1 and 12 K filters are required on the AC side and the DC side, respectively, thereby simplifying the filter device, reducing the floor space, and reducing the cost of the converter station. This is the main reason for choosing the 12-pulse commutator unit as the basic commutator unit. From this, it can be seen that the relationship between the characteristic harmonic order and the pulsation number is shown in Table 1-1.

In the actual operation of the DC system, the operating conditions cannot be ideal, and the asymmetry of various parameters and controls during the operation of the converter and the saturation of the transformer in the system will generate non-characteristic harmonics. Non-characteristic harmonics are all other harmonics except characteristic harmonics. For example, the HP3 filter installed on the AC side of many converter stations is used to filter out the 3rd non-characteristic harmonics, and the 6/42nd order double-tuned filter installed on the DC side of some converter stations is used to filter out the DC side non-characteristic harmonics.

There are two methods to reduce the harmonics generated during the commutation process of the converter, including increasing the pulsation number of the converter and installing a filter. The larger the pulsation number of the converter, the higher the number of characteristic harmonic currents, and the smaller the effective value of the harmonic currents. In general, the rms value of the n-th harmonic current is equal to 1/n of the rms value of the fundamental current. However, when the converter unit reaches more than 12 pulsations, the structure of the converter transformer is complicated, the manufacturing is difficult, and the economy is poor. Therefore, 12 pulsations is the best solution. When the 12-pulse converter is determined, in order to eliminate the characteristic and non-characteristic harmonics generated by the 12-pulse, it is necessary to install filters to limit the harmonics.

1.2 AC side characteristic harmonics

(1) Characteristic harmonics on the AC side of the converter transformer. When the commutation angle u of the converter is not counted, the waveform of the line current on the valve side of the converter transformer (that is, the AC side of the converter device) is a series of equal time intervals, and positive and negative rectangular pulse waves appear alternately.

The expression of the converter transformer valve side line current is

It can be seen from formula (1-1) that when u=0, the valve side line current of the three-phase 6-pulse converter transformer contains only Kp±1 harmonics except the fundamental current, and the amplitude of the fundamental current is .The effective value of the fundamental wave current is the amplitude divided by the square root of 2 and the magnitude is .The effective value of the nth harmonic current is 1/n of the effective value of the fundamental wave current.

(2) The AC side line current of the converter transformer. When the connection mode of the converter transformer is Yy or Dd, and the transformation ratio is 1:1, the AC side line current is the same as the valve side current, and the expanded Fourier series is the same; when the converter transformer is replaced with Yd, the transformation ratio is 1: when the Fourier series expression of the AC side current is

(3) The AC side of the 12-pulse converter transformer is current, and the double-bridge 12-pulse converter unit is composed of two groups of 6-pulse converter units. One single-phase three-winding converter transformer can be installed, and two single-phase double-winding converter transformers can be installed. Winding converter transformer. When the winding connections are Yy and Yd respectively, and the transformation ratio is 1:1 and 1:, the total current expression of the double-bridge 12-pulse AC side is expressed as

The effects of the commutation angle u and the delay angle a on the resonant current are as follows:

1) When the commutation angle u increases, the resonant current decreases. The higher the harmonic order, the faster the harmonic current decreases.

2) Within a certain range, the decreasing speed of the resonant current also increases with the increase of the u angle.

3) When the harmonic current is around u=360 degrees/n, the harmonic current drops to the minimum value, and then increases slightly.

4) If U is a fixed value, the variation of each harmonic current with different angle a is small.

5) In any case, the effective value of harmonic current will not exceed 0.78Id/n.

1.3 DC side characteristic harmonics

The DC side characteristic harmonic calculation is usually based on the DC voltage curve, using the Fourier series to expand, to obtain the DC component and the sine and cosine components of each harmonic, so as to obtain the harmonic current of each order; The equivalent circuit corresponding to the harmonic, the harmonic current is obtained from the harmonic voltage and impedance.

(1) The expression of the harmonic voltage on the DC side of the converter is:

In the formula: Ud(a, o) is the DC component of the harmonic voltage; Udn(a, o) is the n-th harmonic voltage; a is the delay angle; it is the no-load DC voltage when a=0.

(2) The expression of the harmonic current on the DC side is:

where: is the load impedance of the converter; R is the load resistance of the converter; , is the inductance of the smoothing reactor; L is the internal inductance of the converter.

1.4 Non-characteristic harmonics of converter station

The operating conditions of the DC transmission system cannot be ideal in practice, and these non-ideal factors lead to the generation of non-characteristic harmonics in the system.

(1) Causes of non-characteristic harmonics on the AC side of the converter station. There is ripple current in DC current; ripple voltage exists in AC voltage; AC fundamental voltage is not necessarily strictly symmetrical, that is, there is negative sequence voltage; there is phase-to-phase difference in converter transformer impedance; Yy group 6 pulsating converters and Yd group 6 There may be differences in the firing angle of the pulsating converters; the commutation voltages of the Yy group converters and the Yd group converters are different due to the different transformation ratios of the converter transformers; the firing pulses are difficult to be completely equidistant.

(2) Causes of non-characteristic harmonics on the DC side of the converter station. There are various waves in the AC voltage; the leakage reactance and transformation ratio of the Yy group converters and the Yd group converters are not equal; any operating parameters of the two-pole converters that constitute a converter station are not equal; the three-phase leakage reactance of the converter transformer unbalanced.

1.5 Other harmonic sources on the AC side of the converter station

For the AC filter of the converter station, in addition to the characteristic harmonics generated by the converter and the non-characteristic harmonics generated by different equipment parameters and operating parameters of the converter station, there are also background harmonics of the AC system. and harmonics from saturation of converter transformers or other transformers.

(1) Background harmonics. With its own AC power system network and various user terminals (such as traction stations, electrified railway loads, various industrial motor loads, miniaturized rectifier load terminals, etc.), the background harmonics are brought to the AC network by these loads. Another important factor that generates background harmonics is the low-order harmonics caused by the saturation of the AC system transformer. This needs to be solved by selecting the rated tap position of the transformer and optimizing the operating voltage level of the AC system. The background harmonics of the system are very harmful to the power grid, especially for the converter stations with a large number of AC filters. The main reason is that these background harmonics of the power system have a tendency to flow to the nearby converter stations. Magnets absorb these harmonics, so a reasonable determination of these background harmonics has a big impact on the economical operation of the filter.

(2) Harmonics generated by transformer saturation. The transformers referred to here include both ordinary power transformers and converter transformers for DC converter stations. The saturation of converter transformers and ordinary power transformers will generate harmonics, which also belong to the category of background harmonics. There are three situations that cause converter transformer saturation in the converter station: (1) During the voltage recovery process after the transformer is put in and the short-circuit fault is removed, the transient saturation generated by the converter transformer produces transient stress on the filter, and this stress affects the filter. The component ratings of the filter do not generally play a role and are therefore usually not taken into account in filter calculations. ② The voltage of the AC busbar increases. For a properly designed flow system, the tap of the converter transformer always increases with the increase of the AC voltage, and there will be no obvious saturation phenomenon. ③ The DC component stored in the converter transformer may run in a supersaturated state for a long time, and the resulting harmonic current may be a long-term burden on the AC filter.

2 Types of filters

AC and DC filters include conventional passive AC and DC filters and active AC and DC filters. In addition to the above two kinds of AC filters, there are continuously adjustable AC filters. That is to say, there are 3 types of AC filters: passive, active, and continuously adjustable, and there are 2 types of DC filters: passive and active.

Passive filters are composed of resistors, capacitors and inductors, and have low impedance within the specified range of 1 or 2 harmonic frequencies or under the high-pass frequency band, so that most of the harmonic current generated by the converter flows into the filter. From reducing the harmonics injected into the AC system to meet the requirements of reducing harmonic content. There are three types of AC filters widely used in DC converter stations: single-tuned filter, double-tuned filter and two-order high-pass filter. At present, new DC converter stations tend to use double-tuned high-pass filter and multi-tuned high-pass filter. .

Active filters have various types such as magnetic flux compensation, harmonic injection and DC ripple injection. By detecting the current and voltage in the line, the corresponding harmonic current is analyzed and calculated, and then the same harmonic current is generated. , Compensation current in opposite directions, the two cancel each other to achieve the purpose of eliminating harmonics. It can dynamically control harmonics and reactive power, and is not affected by frequency and is not prone to resonance, but its main circuit generally uses fully-controlled power electronic devices, while the current technical level of fully-controlled devices. The capacity of the active filter is small and the operating frequency is not high, so the compensation capacity of the active filter is limited, which is a practical technology to be further developed.

Continuously adjustable AC filter is a filter type that continuously adjusts the inductance of the reactor by using the control signal on the basis of the passive filter, so that it is always in an ideal tuning state. At present, passive AC and DC filters are basically used in domestic DC transmission projects. Passive filter technology has been very mature in design, manufacture, debugging, installation and operation. Active AC and DC filters and continuously adjustable AC filters are only used in individual DC transmission projects. This chapter only describes passive AC and DC filters.

2.1 Classification of passive filters

The passive filter in the DC transmission system refers to the LC passive filter. LC passive filters can be divided into tuned filters and damped filters. Tuned filters for converter stations include single-tuned filters, double-tuned filters, and triple-tuned filters; damping filters include first-order high-pass filters (not commonly used), second-order high-pass filters (basically not used at present), third-order high-pass filters High-pass filter, C-type damping filter (derived from a third-order filter). The easiest way to determine the filter order is to judge according to the number of nonlinear components (capacitors and inductors are nonlinear components).

2.2 Impedance characteristics of passive filter

A single-tuned filter is a filter circuit composed of components such as resistor R, inductor L, and capacitor C in series. It has the smallest impedance at a certain low-order harmonic (or near the low-order harmonic) frequency. device. For the low-order filter frequency, there is one filter branch, as shown in Figure 1-3, where the frequency when the second-order circuit resonates is   

The double-tuned filter has very low impedance to two low-order frequencies at the same time, which can absorb (filter out) two characteristic harmonics at the same time. It is actually equivalent to two single-tuned filters, and has two RLC branches in parallel, as shown in Figure 1-4.

The above two types are very commonly used in DC converter stations, such as converter station HP3, HP11/13, HP24/26 filters, which are typical single- and double-tuned filters.

The three-tuned filter has very low impedance to 3 kinds of low-frequency harmonics at the same time, which can absorb (filter out) 3 kinds of characteristic harmonics at the same time. It is actually equivalent to three single-tuned filters, and has three RLC parallel branches, as shown in Figure 1-5. Three-tuned tuners have also been used in some domestic converter stations, but they are not as common as single-tuned and double-tuned applications.

The damping filter is realized by connecting a damping resistor in parallel with the filter reactor. Compared with the single-tuned filter, there is an additional parallel resistor on the physical element. Functionally, the damping filter has a wide frequency range near the resonance frequency. The filter impedances are all at low values and are not sensitive to detuning because their resonant frequency is a frequency band. In the higher frequency range, the filter impedance approaches the limit determined by the damping resistor, as shown in Figure 1-6.

The damping filter can simplify the filter branch, and its purpose is to facilitate switching when the operating mode changes and the required filter capacity is different. Since the rated parameters of the HVDC system are of the same order of magnitude as the short-circuit level of the system, the possibility of low-order harmonic resonance between the system and the filter capacitors is increased. As for whether the series or parallel resonance occurs, it depends on the low-order harmonics. The source is in the AC system or in the converter station, and the damping filter can better solve this problem.

Advantages of damping filters, filter performance and load are insensitive to temperature, system frequency offset and allowable deviation of components; a wide range of harmonic frequencies can be filtered out, reducing grouping by harmonic order The investment cost of tuning also reduces the corresponding floor space; it reduces the workload of on-site debugging and maintenance, and basically no on-site tuning work is required; it can also absorb and filter a considerable part of non-characteristic harmonics; For station reactive power control, this filter grouping is less, not only economical investment, but also easy to control.

Tuning the filter in practical applications avoids causing detuning of the filter. Detuning means that the low-impedance circuit path of the original design corresponding frequency changes (does not match the original frequency). There are many factors that cause the detuning, mainly including the frequency deviation of the AC capacitor; the deviation of the component data from the specified value due to the temperature change; the fuse of the internal capacitor element of the capacitor is blown, which causes the filter capacitance value to change. To avoid detuning of the tuning circuit due to the above reasons, a low-value series resistor is usually added to the tuning filter to widen the tuning range.

In addition to the categories mentioned above, filters can also be divided into series filters, parallel filters and series-parallel filters (combination of the two) according to the series-parallel connection with the system. A series filter is a series circuit with high impedance at harmonic frequencies, which prevents entry from the converter into the grid or DC line. A parallel filter is a parallel circuit with very low impedance at harmonic frequencies, allowing harmonics to flow into the filter without entering the grid or DC line. Compared with the series filter, the parallel filter has many advantages. For example, the series filter must pass all the current of the main circuit and must be insulated with full voltage to the ground; while one end of the parallel filter can be grounded, and the current passing through it is only The harmonic currents filtered out by it and a much smaller fundamental current than in the main circuit, the insulation requirements are also relatively low. Parallel filters are generally much cheaper than series filters with the same effect, and parallel filters are also widely used in converter stations.

3. Configuration principles and differences of AC and DC filters

3.1 AC filter configuration principle

In addition to filtering out the harmonic current generated by the converter, the AC filter configured in the converter station also provides the required part of the fundamental wave reactive power to the converter. Generally, there are the following configuration principles; in the converter stations that have been operating in my country, the rated voltage level of the filter is the same as the voltage level of the busbar on the AC side of the converter; the corresponding single-tuned, double-tuned or triple-tuned filters are reasonably configured, but The types should not be too many; in the case of meeting the requirements of the DC converter station to filter out harmonics, the filters are grouped with a small number of cores. When the filter cannot meet the reactive power consumption of the converter, use parallel capacitor groups as much as possible to supplement it; when all filters are put into operation, they should meet the performance requirements of various harsh operating modes such as continuous overload of the DC system; any one When the group filter is out of operation, it should meet the requirements of the operating conditions of the system; when the DC converter station is running with unlocked power, the input capacity of the filter should be the smallest.

3.2 DC filter configuration principle

As mentioned earlier, theoretically, a 12-pulse converter only generates 12K harmonic voltages on the DC side. However, in actual operation, due to various asymmetric factors, the converter will generate non-characteristic harmonics on the DC side. Different DC converter stations have different configurations of non-characteristic harmonic filters. The filtering of harmonics is the same at each converter station. Through practical engineering experience, some non-characteristic harmonics with lower order generated by the stray capacitance of the converter transformer have larger amplitudes, and a larger wave feeder capacity is required to filter them out. The DC filter configuration should fully consider the amplitude of each harmonic and its proportion in the equivalent interference current (that is, the coupling coefficient and weighting coefficient of each harmonic current when calculating its equivalent interference current). In addition, the two DC poles of the same converter station have correspondence, so the two poles should be equipped with the same DC filter.

The DC filter configuration scheme commonly used in domestic conventional large-scale DC transmission systems is as follows: install one or two parallel double-tuned filters <or three-tuned filters between the DC high-voltage busbar and the neutral busbar of each pole of the converter station. 〉Used to filter out the characteristic harmonics of the DC side, and configure different DC filters to filter out the non-characteristic harmonics according to different DC engineering designs; for the characteristic harmonic filters of the DC side, the center tuning frequency should be higher for the harmonic amplitude. The characteristic harmonics of the 12-pulse converter, and the high-order harmonics that have a greater impact on the equivalent interference current; for the non-characteristic harmonics with low order, a neutral bus can be connected between the low-voltage side of the 12-pulse converter and the ground. Bus impulse capacitors, so that there is no need to specially install low-order harmonic filters to save investment.

3.3 Difference between DC filter and AC filter

The DC filter is usually connected as a parallel filter between the pole bus and the neutral bus of the converter station. The primary circuit structure of the DC filter is similar to that of the AC filter, and there are also various circuit structures. The commonly used ones are with or without high-pass The characteristic single-tuned, double-tuned and three-tuned filters have a circuit structure similar to that of an AC filter. Although DC filters have many things in common with AC filters, there are also some differences:

(1) The AC filter provides power frequency reactive power to the converter station, so its reactive power capacity is usually designed to be larger than the reactive power setting capacity required by the filtering characteristics, while the DC filter does not need this requirement.

(2) For the AC filter (parallel capacitor), the voltage acting on the high-voltage capacitor can be considered to be evenly distributed on a plurality of capacitors connected in series. For the DC filter, the high-voltage capacitor acts to isolate the DC voltage and has a high tolerance. of the DC voltage. Due to the existence of the DC leakage resistance, if no measures are taken, the DC voltage will be unevenly distributed along the leakage resistance. Therefore, a voltage equalizing resistor must be installed in parallel inside the DC filter capacitor unit.

(3) The impedance range of the AC system connected in parallel with the AC filter is relatively large at a certain frequency. Under certain power grid conditions, such as switching of the AC line, local faults of the power grid, etc., the AC filter capacitor and the AC will be caused. Resonance between system inductances. Therefore, damping measures are required even in precisely tuned (band-pass tuned) AC filter circuits. The impedance of the DC side of the converter station is generally constant, thus allowing the use of an accurately tuned (band-pass tuned) DC filter. The determination of the circuit structure of the DC filter should be based on the equivalent interference current generated by the DC line. Since the amplitude of the characteristic harmonic current is the largest, the circuit structure of the DC filter should be consistent with these harmonics (that is, the harmonic order is 12). , 24, 36– harmonics) to match.

(4) The high-voltage capacitor is the most valuable component in the DC filter equipment, because it must be designed as a capacitor that can withstand a high DC voltage. The difference in the cost of the capacitor is also the difference between the AC filter and the DC filter. a significant difference. One of the main means to reduce the investment cost of DC filters is to design the filters as double-tuned or multi-tuned harmonic filter circuits with common high voltage capacitors. Sometimes, in order to filter out the non-characteristic harmonics on the DC side and avoid installing high-cost DC filters that withstand high DC voltages, a filter capacitor is usually installed between the neutral point of the converter station and the ground. The function of installing this capacitor is to provide a low-impedance channel for the harmonic current whose main component is the 3rd harmonic on the DC side. Furthermore, as mentioned above, due to the presence of stray capacitance to ground in the converter transformer winding, it provides a channel for DC harmonics, especially lower-order DC harmonic currents, so the neutral point should be determined for such harmonics. capacitor parameters. Generally speaking, the selection range of the capacitance value of the capacitor should be ten microfarads or several millifarads, and at the same time, parallel resonance with the inductance of the grounding line at the critical frequency should be avoided.

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Liege Airport in southern Belgium is the largest airport in terms of cargo throughput and one of the important air cargo hubs in Europe. Benefiting from the dividends brought by the construction of the “One Belt and One Road”, Liege Airport has continued to expand its business in recent years, with a good momentum of development, which has also brought close connections between Chinese and European small and medium enterprises.

In an interview with Belgian media, Liege Airport CEO Luke Paldon said that the rapid development of e-commerce is not only beneficial to the aviation industry, but also to other industries, not only to China, but also to the world.

Seeing the business opportunities brought about by the strong development of cross-border e-commerce, Yang Kaijing, who has been engaged in the catering industry at Liege Airport for many years, joined the logistics industry this year. Although his logistics company has just started, this year’s “Double Eleven” is still too busy. He said that online shopping has a bright future in Europe and the company will focus on helping European small and medium-sized enterprises export to China in the future.

In recent years, the construction of the “Air Silk Road” has yielded fruitful results. The network of freight routes between China and Europe has become denser and the structure of cargo sources has been optimized. This has helped more and more cross-border e-commerce companies to “fly” into a new world. Focusing on the new platform for aviation economic cooperation, China-EU cross-border e-commerce has worked closely to jointly pave the “road to prosperity in the air.”

Zhou Lihong, president of the EU-China Chamber of Commerce, believes that the “Air Silk Road” can achieve coordinated development through the rapid formation of point-to-point trade exchanges.

According to data from the Civil Aviation Administration of China, China has signed intergovernmental air transport agreements with 96 countries and regions along the “Belt and Road” to build an air bridge for the “Belt and Road” cooperation.

This year, during the sudden epidemic, the China-Europe “Aerial Silk Road” assumed the important task of “The Road to Life in the Air”. 300,000 masks arrived in Belgium by air, and more than 2.5 million yuan of anti-epidemic materials arrived in Luxembourg… China’s aid materials arrived in European countries smoothly and in time.

Since the outbreak of the epidemic, cross-border transportation capacity in many European countries has been limited, and Liege Airport has maintained efficient operation and has become a European anti-epidemic rescue center. The Liege hub of the “World Electronic Trade Platform” jointly established by Alibaba Group and the Belgian government effectively guarantees the logistics channels between China and Belgium, and enables efficient transportation of a large number of anti-epidemic materials, helping European countries severely affected by the epidemic to cope with the crisis. These routes and flights have become a vivid portrayal of the “Belt and Road” countries working together to fight the epidemic.

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Once there are any problems in the process of using safety capacitors, the first thing to do is to test the safety capacitors. This is very important. How to detect safety capacitors? What methods are there to detect?

The detection of safety capacitors needs to connect the red test lead of the multimeter to the negative pole and the black test lead to the positive pole. At the moment of contact, the pointer of the multimeter deflects to the right by a large degree. For the same electrical barrier, the greater the capacity, the greater the swing, and then gradually turn to the left until it stops at a certain position. The resistance at this time is the forward leakage resistance of the electrolytic capacitor, which is slightly larger than the reverse leakage resistance.

Practical experience shows that the leakage resistance of the capacitor should generally be more than several hundred kΩ, otherwise, it will not work normally. In the test, if there is no charging phenomenon in the forward and reverse directions, that is, the hand does not move, it means that the capacity has disappeared or the internal circuit is broken; if the measured resistance is small or zero, it means that the capacitor has a large leakage or has been broken down. Can no longer be used.

Through the above method to detect the safety capacitor, you can easily judge whether it is good or bad and whether it can still be used. If the test result shows that it has been broken down or there is a serious leakage phenomenon, it must be replaced in time without delay. For safety capacitors, once damaged, its performance is likely to be affected.

The importance of safety capacitor testing is very important. The first is the stability test of the label, and the second is the capacitor discharge test. If these tests for the safety capacitor are ignored, it is difficult to guarantee the quality and quality of the selected safety capacitor.

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High-voltage ceramic capacitors are epoxy resin encapsulated capacitors with dielectric ceramic as the core material. With the progress of the times and the development of science and technology, high-voltage ceramic capacitors mainly refer to capacitors with AC working voltage above 10KV, or ceramic capacitors with DC working voltage above 40KV.

Due to the stability and long-life characteristics of ceramic capacitors, as well as safety and reliability, high-voltage ceramic capacitors are getting more and more attention, and will replace film-type oil-immersed capacitors within a certain range. Therefore, the definition of high-voltage ceramic capacitors will follow the development of technology Deepen the connotation continuously with the application. Military DC High Voltage Ceramic Capacitors will also become a must.

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Recently, at a video meeting of senior officials of ASEAN and China, Japan and South Korea (10+3), Deputy Foreign Minister Luo Zhaohui stated that the 10+3 countries’ response to the new crown pneumonia epidemic and economic recovery are “double harvests” in East Asia in the first half of this year. One of the two major characteristics of the regional situation.

As friendly neighbors, China and ASEAN are each other’s important cooperative partners. The sudden outbreak of the epidemic has set a severe test question for countries all over the world. China and ASEAN countries have withstood the test. While doing their own epidemic prevention work, they have carried out active cooperation and jointly responded to the epidemic at the regional level, setting a model for international cooperation in the fight against the epidemic.

Currently, epidemic prevention and control has entered a stage of normalization. Both China and ASEAN countries are faced with the arduous task of preventing a rebound of the epidemic and restoring economic and social order. Cooperation will remain the main theme of bilateral relations. Especially in the fields of trade and investment, the cooperation between China and ASEAN is worth looking forward to.

According to the data on imports and exports of goods released by the General Administration of Customs of China in the first half of this year, the total value of China’s imports and exports to ASEAN reached 2.09 trillion yuan, a year-on-year increase of 5.6%, accounting for 14.7% of China’s total foreign trade, and ASEAN continued to maintain China’s largest trade Partner status.

In the face of challenges and opportunities, China and ASEAN countries have shown a strong willingness to cooperate and have acted. At present, the two sides have been closely cooperating on joint prevention and control and strengthening macro policy coordination. China and Singapore are the first to establish a “fast channel” for personnel exchanges, and work together to promote the smooth flow of the regional supply chain. The special meeting of China-ASEAN Ministers of Transportation to respond to the new crown pneumonia epidemic was held recently. The two sides agreed to strengthen cooperation and work together to ensure the smooth flow of the transportation and logistics system between China and ASEAN, which will add a strong impetus to the resumption of work and production.

The mountains and rivers are connected, and the friendship is connected. I believe that China and ASEAN countries will work hand in hand to overcome the impact of the epidemic, find new opportunities for development in the crisis, and deepen friendship due to this precious experience of overcoming difficulties, and become a strong driving force for buildingChina and ASEAN a strong momentum for cooperation.

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SMDcharacteristics electrolytic capacitors two: rated capacity can be very large, can easily achieve tens of thousands of μf or even a few f (but can not be compared with the electric double layer capacitance).

SMD electrolytic capacitor features three: the price has an overwhelming advantage over other types, because the electrolytic capacitors are composed of ordinary industrial materials, such as aluminum and so on. The equipment for manufacturing electrolytic capacitors is also ordinary industrial equipment, which can be mass-produced at a relatively low cost.

The positive and negative distinctions of chip electrolytic capacitors

1.one is a common chip electrolytic capacitor, which is a cylindrical square shape, and the end marked with “-” is positive; there is also a silver surface-mounted capacitor, which should be aluminum electrolysis. The top is round and the bottom is square, which is very common on the optical drive circuit board. This type of capacitor has a negative end marked with a “-“.

2.Appearance judgment

• The end marked with a different color from the surface of the capacitor is negative, and the other is positive

3.Multimeter resistance file judgment

• Connect the two test leads of the resistance file to the two ends of the capacitor, the resistance value will be displayed from small to large, and finally tend to infinity.
  Reverse the test lead and measure again, the resistance value will be displayed from small to large, and finally tend to infinity.
• When the resistance value increases faster, the positive test lead indicates negative.

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