Design Method of Active Filter for Compensating Reactive Power and Controlling Harmonics Simultaneously

Abstract

The present invention relates to a design method for an active filter that compensates for reactive power and controls harmonics simultaneously. The primary system of this active filter includes: a turn-off device commutator valve and a connection device; the turn-off device commutator valve is connected to the AC bus through the connection device, and the AC bus is respectively connected to the AC system and the harmonic source; the connection device includes a connection transformer and a connection passive filter connected in parallel with each other; the method includes the following steps: performing equivalent analysis on the active filter to obtain the equivalent circuit diagram of the active filter, and performing harmonic analysis and fundamental frequency analysis on the active filter based on this equivalent circuit diagram; designing the device components of the active filter based on the equivalent circuit diagram of the active filter and the results of harmonic analysis and fundamental frequency analysis. The present invention can be widely applied to the field of transmission system design.

Description

[0001] This application is a divisional application of the application with the application number 202011022716.3, the application date of September 25, 2020, and the invention creation name of "Active Filter for Simultaneously Compensating Reactive Power and Controlling Harmonics and Its Control and Design Methods". Technical Field

[0002] The present invention relates to a design method of an active filter for simultaneously compensating reactive power and controlling harmonics, belonging to the field of transmission system design. Background Art

[0003] When an LCC DC conversion project operates, it consumes a large amount of reactive power and generates a large amount of harmonics, mainly characteristic harmonics such as the 12th and 24th harmonics and non-characteristic harmonics such as the 3rd and 5th harmonics. Therefore, a large number of AC filters and shunt capacitors must be installed for reactive power compensation and harmonic compensation, resulting in the AC filter yard composed of AC filters and shunt capacitors accounting for nearly half of the land area of the LCC DC conversion station and consuming a large amount of investment.

[0004] AC filters belong to passive filtering devices, and their filtering effect is affected by the impedance of the AC system. Detailed design is required according to the conditions of the AC system during the design process of the conversion station. Currently, the construction and development of the AC system are relatively fast, and new technologies and new equipment are continuously connected to the AC system, changing the characteristics of the AC system and also changing the filtering effect of the AC filter. Moreover, with the construction of UHV DC projects, conversion stations are becoming more and more dense, and there is a possibility of resonance between the AC filter and the impedance of the AC power grid, which will amplify the harmonic sources already existing in the AC power grid, causing large-area over-standard of low-frequency harmonics in the AC power grid. And the over-standard of harmonics will affect the safe operation of equipment in the AC power grid, and the network loss caused by harmonic power flow will also waste a large amount of electric energy.

[0005] At the same time, AC filters and shunt capacitors are controlled by circuit breakers, and the response of mechanical equipment is slow, unable to adapt to rapid changes in power or the blocking of converters under faults, resulting in the inability of AC filters and shunt capacitors to be cut off in time, causing AC overvoltage. In addition, harmonic problems and reactive power compensation problems also exist in the AC power grid at the same time. There are a large number of harmonics in the AC transmission grid, mainly brought by the increasing number of non-linear loads at present, and the fluctuation of AC voltage is mainly controlled and stabilized by reactive power compensation equipment. Therefore, if a fast and continuously adjustable active reactive power and harmonic compensation device can be used to replace the passive compensation device of the traditional LCC conversion station, it will greatly improve the power response flexibility and filtering performance of the LCC conversion station, reduce the area of the AC filter yard, reduce the land occupation, and also bring benefits to the AC power grid.

[0006] At present, active filtering equipment has been applied on the DC side of converters in domestic DC projects, replacing DC filters. It has only been applied abroad on the AC side. This is because the harmonics generated by converters are mostly high-order characteristic harmonics such as the 12th and 24th harmonics, which are easy to pass through capacitive devices such as capacitors. Reactive power compensation equipment, in essence, compensates for fundamental frequency current, and fundamental frequency current easily flows through inductive devices such as inductors. Active filtering equipment often cannot simultaneously take into account fundamental frequency reactive power compensation and high-frequency filtering. Therefore, it cannot replace passive filters that simultaneously serve as reactive power compensation equipment and filtering equipment on the AC side. Summary of the Invention

[0007] In view of the above problems, the purpose of the present invention is to provide a design method for an active filter that simultaneously compensates for reactive power and controls harmonics, gives an effective topological structure of the active filter, and designs the main equipment parameters of the active filter.

[0008] To achieve the above purpose, the present invention adopts the following technical solutions:

[0009] A design method for an active filter that simultaneously compensates for reactive power and controls harmonics. The primary system of the active filter that simultaneously compensates for reactive power and controls harmonics includes: a turn-off device converter valve and a connection device; the turn-off device converter valve is connected to the AC bus through the connection device, and the AC bus is respectively connected to the AC system and the harmonic source; the turn-off device converter valve is composed of 3-phase converter bridge arms, and each converter bridge arm is composed of a DC support capacitor and a turn-off device. By controlling the opening and closing of the turn-off device, the output voltage of the turn-off device converter valve is controlled; the connection device includes a connection transformer and a connection passive filter connected in parallel. The connection transformer, as an inductive device, is used to pass fundamental frequency reactive current in the low-frequency band; the connection passive filter, as a capacitive device, is used to pass high-frequency harmonic current in the high-frequency band; it includes the following steps:

[0010] (1) Conduct equivalent analysis on the active filter to obtain the equivalent circuit diagram of the active filter, and conduct harmonic analysis and fundamental frequency analysis on the active filter based on this equivalent circuit diagram;

[0011] (2) Based on the equivalent circuit diagram of the active filter and the results of harmonic analysis and fundamental frequency analysis, design the equipment components of the active filter.

[0012] Further, in step (1), when conducting equivalent analysis on the active filter, the high-voltage side of the connection transformer is equivalently a parallel capacitor C p , the low-voltage side is equivalently a parallel inductor L p , the connection between the high-voltage side and the low-voltage side is equivalently a connection inductor L s , and it satisfies:

[0013] C p =(k - 1) / ω 2 L

[0014] L s = L / k

[0015] L p = L / k(k - 1)

[0016] Wherein, k is the turns ratio of the connecting transformer; L is the equivalent value of the short - circuit inductance of the connecting transformer on the high - voltage side; ω is the frequency.

[0017] Further, in the step (2), the method for designing the device components of the active filter based on the equivalent circuit diagram of the active filter and the results of harmonic analysis and fundamental - frequency analysis includes the following steps: (2.1) Based on the equivalent circuit diagram of the active filter, design the tuning frequency of the connected passive filter according to the harmonic orders to be filtered, and design the connecting devices of the active filter in combination with the requirements of harmonic filtering and reactive - power output; (2.2) Calculate the final voltage output and current output of the active filter according to the harmonic - compensation requirements and reactive - power compensation power requirements of the active - filter design requirements, and design the capacity of the turn - off device converter valve in the active filter accordingly.

[0018] Further, in the step (2.1), when designing each parameter in the connecting device, make the equivalent connecting inductance L of the connecting transformer s resonate at a frequency near the middle of the target harmonic order and the fundamental frequency with the connected passive filter.

[0019] Further, in the step (2.1), determine the specific parameters of the connected passive filter according to the system - allowed single - time switching of reactive power and the target filtering order, and determine the specific parameters of the connecting transformer according to the parallel - resonance order and reactive - power output capacity of the connected passive filter.

[0020] Further, in step (2.2), the method for designing the commutation valve capacity of the turn-off device of the active power filter includes the following steps: (2.2.1) According to the maximum harmonic compensation requirement of the active power filter design requirement, calculate the harmonic voltage output and harmonic current output required by the active power filter under this harmonic; (2.2.2) According to the maximum reactive power compensation power requirement of the active power filter design requirement, calculate the minimum fundamental voltage output and fundamental current output of the commutation valve of the active power filter; (2.2.3) According to the calculated minimum fundamental voltage output and fundamental current output of the commutation valve of the active power filter, and the calculated harmonic voltage output and harmonic current output, calculate the final voltage output and current output of the active power filter; (2.2.4) According to the final voltage output and current output of the active power filter, design the commutation valve capacity and component equipment of the active power filter.

[0021] Due to the above technical solutions adopted by the present invention, it has the following advantages: 1. The present invention adopts a grid connection method in which the turn-off device commutation valve of the active power filter is directly connected to the AC system through the parallel structure of the passive power filter and the transformer. The passive power filter is used as the harmonic branch for harmonic compensation, and the transformer branch is used as the fundamental frequency current branch for reactive power compensation, compensating reactive power and harmonics simultaneously with a smaller commutation valve capacity of the turn-off device; 2. The present invention gives the basic control strategy of the active power filter, which has engineering practicability; 3. The present invention gives the equivalent analysis circuit of the active power filter, which is convenient for analyzing the harmonics passing through the active power filter and for designing the passive power filter and the connecting transformer connected to the active power filter. Brief Description of the Drawings

[0022] Figure 1 is a schematic diagram of the topological structure of the active power filter designed by the present invention;

[0023] Figure 2 is a schematic diagram of the control strategy principle of the active power filter designed by the present invention;

[0024] Figure 3 is a schematic diagram of the voltage and current of the active power filter designed by the present invention;

[0025] Figure 4 is a vector diagram of the voltage and current of the active power filter designed by the present invention;

[0026] Figure 5 is the topology of the passive power filter HP57 for connecting the active power filter designed in the Jinhua Station of the present invention example. Detailed Embodiment

[0027] The present invention will be described in detail below with reference to the accompanying drawings and embodiments. However, it should be noted that the following drawings are provided only for a better understanding of the present invention, and the following description of the embodiments is merely illustrative and in no way limits the present invention and its use. The numerical expressions and values of the steps described in the embodiments also do not limit the scope of the present invention, and different values may be used in this embodiment and other embodiments.

[0028] For technologies and methods known to those of ordinary skill in the relevant art, detailed discussions may not be made, but in appropriate cases, these technical methods should be regarded as part of the specification.

[0029] It should be noted that once a reference numeral and letter are defined in a certain drawing or expression, no further discussion thereof is required in the subsequent drawings.

[0030] Embodiment 1

[0031] As Figure 1 shown, an active filter for simultaneously compensating reactive power and controlling harmonics provided by the present invention, its primary system includes a turn-off device converter valve and a connection device, and the converter valve is connected to the AC bus through this connection device, and this AC bus is respectively connected to the AC system and the harmonic source. Among them, the turn-off device converter valve is composed of 3-phase converter bridge arms, and each phase converter bridge arm is composed of a DC support capacitor and a turn-off device. By controlling the opening and closing of the turn-off device, the output voltage of the turn-off device converter valve is controlled. Since zero-sequence harmonics need to be filtered out, generally the 3-phase converter bridge arms are connected in a Y-connected neutral point grounded manner. The connection device includes a transformer and a passive filter connected in parallel. The transformer, as an inductive device, has a very low impedance in the low-frequency band and mainly passes the fundamental frequency reactive current; the passive filter, as a capacitive device, has a very low impedance in the high-frequency band and mainly passes the high-frequency harmonic current.

[0032] Embodiment 2

[0033] As Figure 2 shown, the present invention also provides a control method for an active filter for simultaneously compensating reactive power and controlling harmonics, including the following content:

[0034] (1) Measure the harmonic current of the harmonic source and the harmonic current on the low-voltage side of the passive filter connected in the active filter, and calculate the harmonic modulation voltage of the converter valve in the active filter based on the measured current data to perform harmonic control.

[0035] The active filter uses the harmonic current as the control variable, controlling the harmonic current flowing into the AC bus on the high-voltage side of the active filter to have the same amplitude and opposite phase as the harmonic current flowing out of the harmonic source, so that they cancel each other out. In the topology structure of the active filter designed in the present invention, the connecting transformer and the connecting passive filter as connecting devices are in a parallel structure. The low-voltage side current and the high-voltage side current of the connecting passive filter are the same, while the low-voltage side current of the connecting transformer is k times the high-voltage side current (k is the transformer turns ratio). Therefore, the control target of the high-voltage side current of the active filter cannot be directly converted into the control target of the harmonic current at the outlet of the converter valve of the active filter. Therefore, the present invention adopts the following method for harmonic control:

[0036] (1.1) Measure the harmonic current of the harmonic source and the harmonic current on the low-voltage side of the connecting passive filter in the active filter (which is equal to the harmonic current on the high-voltage side of the passive filter) to obtain the harmonic current i of the harmonic source s and the harmonic current i on the low-voltage side of the connecting passive filter F ;

[0037] (1.2) Subtract the harmonic current i of the harmonic source s and the harmonic current i on the low-voltage side of the connecting passive filter F to obtain the target harmonic current i on the high-voltage side of the connecting transformer T1 ;

[0038] (1.3) Multiply the target harmonic current i on the high-voltage side of the connecting transformer T1 by k to obtain the target harmonic current i on the low-voltage side of the connecting transformer T2 ;

[0039] (1.4) Add the target harmonic current i on the low-voltage side of the connecting transformer T2 and the harmonic current i on the low-voltage side of the connecting passive filter F to obtain the control target value i of the current at the outlet of the converter valve C ;

[0040] (1.5) Input the obtained control target value of the current at the outlet of the converter valve into the inner loop of the harmonic current control of the active filter, and output the corresponding harmonic modulation voltage of the converter valve through a certain controller;

[0041] (1.6) Repeat steps (1.1) to (1.5) until the output of the active filter reaches a stable state.

[0042] (2) Measure the fundamental current on the low-voltage sides of the connecting passive filter and the connecting transformer and the fundamental frequency voltage of the AC bus, and calculate the fundamental modulation voltage of the converter valve in the active filter based on the measured current and voltage data for reactive power compensation.

[0043] Since the transformer presents pure inductance at the fundamental frequency and the passive filter presents basically pure capacitance at the fundamental frequency, the two devices still present pure capacitance or pure inductance after being connected in parallel. Therefore, the connection structure of the active filter designed in the present invention basically does not present resistance at the fundamental frequency. If the commutation valve of the active filter generates a voltage in the same direction as the AC bus, reactive current can be injected into or absorbed from the AC bus. Specifically, the method for reactive power compensation includes the following steps:

[0044] (2.1) Measure the low-voltage side currents of the connecting transformer and the connecting passive filter respectively to obtain the low-voltage side currents of the connecting transformer and the connecting passive filter;

[0045] (2.2) Multiply the low-voltage side current of the connecting transformer by 1 / k times to obtain the high-voltage side current of the connecting transformer, and add it to the low-voltage side current of the connecting passive filter to obtain the total current on the high-voltage side of the active filter;

[0046] (2.3) Extract the fundamental wave from the total current on the high-voltage side of the active filter to obtain the fundamental wave component of the total current on the high-voltage side of the active filter;

[0047] (2.4) Measure the fundamental frequency voltage of the AC bus, and calculate the fundamental frequency voltage of the AC bus obtained and the fundamental wave component of the total current on the high-voltage side of the active filter obtained in step (2.3) to obtain the reactive power injected by the active filter into the AC bus;

[0048] (2.5) Subtract the reactive power injected by the active filter into the AC bus obtained in step (2.4) from the reactive power set value of the active filter. The obtained difference is calculated by a suitable controller to obtain the amplitude of the fundamental frequency modulation voltage of the commutation valve of the active filter. Take the in-phase unit voltage of the fundamental wave of the AC bus fundamental wave voltage extracted in step (2.3) as the fundamental wave of the fundamental frequency modulation voltage of the commutation valve of the active filter, and multiply the amplitude by the fundamental wave to obtain the fundamental frequency modulation voltage of the active filter.

[0049] (3) Add the harmonic modulation voltage of the commutation valve obtained in step (1) and the fundamental wave modulation voltage of the commutation valve obtained in step (2) to obtain the total modulation voltage of the commutation valve in the active filter.

[0050] Embodiment III

[0051] As Figures 3 to 5 shown, the present invention also provides a design method for an active filter that compensates reactive power and controls harmonics simultaneously, including the following steps:

[0052] (1) Perform equivalent analysis on the active filter to obtain the equivalent circuit diagram of the active filter, and perform harmonic analysis and fundamental frequency analysis on the active filter based on this equivalent circuit diagram.

[0053] a. Equivalent Circuit Analysis

[0054] The connection device of the active filter designed by the present invention is composed of a connection transformer and a connected passive filter in parallel. Since the connection transformer belongs to a magnetic coupling device and the primary side and the secondary side are not directly connected by a circuit, there are problems of complex and non-intuitive analysis. Circuit equivalence of the connection transformer is helpful for the design of each device.

[0055] As Figure 3 shown, it is the equivalent circuit diagram of the active filter. Among them, the connection transformer is equivalent to a PI-type circuit. The high-voltage side of the connection transformer is equivalent to a parallel capacitor C p , the connection between the high-voltage side and the low-voltage side is equivalent to a connection inductor L s , the low-voltage side of the connection transformer is equivalent to a parallel inductor L p , and the following formula is satisfied:

[0056] C p =(k - 1) / ω 2 L (1)

[0057] L s =L / k (2)

[0058] L p =L / k(k - 1) (3)

[0059] Among them, k is the transformer turns ratio; L is the equivalent value of the transformer short-circuit inductance on the high-voltage side; ω is the frequency.

[0060] Figure 3 In it, C1 is the component capacitor of the connected passive filter, L1 is the component inductor of the connected passive filter, APF represents the commutation valve of the active filter, U s represents the equivalent voltage source of the AC system, and i s represents the harmonic current of the harmonic source.

[0061] It can be seen that the capacitance value of the parallel capacitor C p varies with the frequency, and the parallel capacitor C p can be incorporated into the system impedance; the parallel inductor L p does not affect the filtering effect, but affects the current stress flowing through the commutation valve; the connection inductor Ls and the connected passive filter need to be designed so that the capacity of the required commutation valve reaches the optimal value.

[0062] b. Harmonic Analysis

[0063] When calculating the path of the harmonic current and the harmonic capacity of the commutation valve of the active filter, the approximate equivalent circuit diagram of the active filter is as Figure 4 shown. In the figure, i s-his the reverse cancellation current of the harmonic current of the harmonic source generated by the active filter, U h is the harmonic voltage output of the converter valve of the active filter. It can be seen that U h is actually i s-h at the equivalent connection inductance L of the connecting transformer s and the opposite value of the voltage drop on the parallel structure connecting the passive filter. i T3-h is U h at the equivalent parallel inductance L of the connecting transformer p The reverse current of the current generated on it. It can be seen that the harmonic current output i of the active filter APF-h is i T3-h and i s-h The sum of. i T2-h is the current flowing through the equivalent connection inductance L of the connecting transformer s i F-h is the current flowing through the connecting filter.

[0064] c. Fundamental frequency analysis

[0065] When calculating the magnitude of the fundamental reactive power and the fundamental frequency capacity of the converter valve of the active filter, the approximate equivalent circuit diagram of the active filter is as Figure 5 shown. Since the impedance of the AC system is generally small, it can be ignored. In the figure, i Q is the total designed reactive current of the active filter, flowing into the AC system to generate the target output reactive power. It should be noted that due to the existence of the connected passive filter, the total reactive capacity generated by the active filter has a capacitive bias. From Figure 5 it can be seen that i T1-1 is the fundamental frequency current generated by the AC system voltage on the equivalent parallel capacitor C of the transformer p According to i Q and i T1-1 the fundamental frequency current i flowing into the equivalent connection inductance L of the transformer can be calculated s and the fundamental frequency current i flowing through the passive filter T2-1 and its corresponding voltage drop, and subtracting from the AC system voltage to obtain the fundamental frequency voltage output of the converter valve of the active filter. The fundamental frequency voltage output of the converter valve generates current i on the equivalent parallel inductance L of the connecting transformer F-1 i p i T3-1 i T3-1 i T2-1 and i F-1 Adding them up finally gives the fundamental current output of the active filter.

[0066] (2) Based on the equivalent circuit diagram of the active filter and the results of harmonic analysis and fundamental frequency analysis, design the equipment components of the active filter.

[0067] Specifically, it includes the following steps:

[0068] (2.1) Based on the equivalent circuit diagram of the active filter, design the tuning frequency of the connected passive filter according to the harmonic order to be filtered, and design parameters such as the turns ratio and short-circuit impedance of the connected transformer in combination with the harmonic filtering requirement and reactive power output requirement, so that the equivalent connected inductance L of the connected transformer s and the connected passive filter resonate at a frequency near the middle of the target harmonic order and the fundamental frequency.

[0069] According to Figure 3 the equivalent circuit diagram shown, the connection device of the active filter designed by the present invention can be equivalent to the parallel connection of a passive filter and an inductor. If the passive filter is designed as the simplest single-tuned filter, as Figure 3 shown, analyzing its impedance characteristics, it can be known that the connection device mainly features an inductor at low frequencies, presenting a low impedance; as the frequency increases, the parallel inductor L p and the passive filter undergo parallel resonance, and the connection device presents an extremely high impedance; as the frequency increases and approaches the resonance frequency of the passive filter, the impedance of the connection device mainly features the passive filter and finally reaches the minimum value; as the frequency continues to increase, the impedance of the connection device presents inductive and continuously increases.

[0070] It can be seen that the connection device will mainly present a low impedance at the fundamental frequency and the tuning frequency of the passive filter. Therefore, the tuning frequency of the passive filter should be designed according to the harmonic order to be filtered, and parameters such as the turns ratio and short-circuit impedance of the connected transformer should be designed in combination with the harmonic filtering requirement and reactive power output requirement.

[0071] From the harmonic analysis circuit Figure 4 it can be known that in order to filter the target harmonic current with the smallest possible capacity of the commutation valve, the passive filter should be tuned to the target harmonic order. Generally, the capacity of the passive filter is limited by the single-step switching reactive power change acceptable to the AC system, and the capacity of the passive filter should be made as close as possible to the maximum capacity acceptable to the AC system. The short-circuit impedance of the connected transformer should be as small as possible for reactive power output, but for harmonic filtering, from Figure 4 it can be known that if the short-circuit impedance of the connected transformer is too small or the turns ratio is too large, the total parallel impedance will increase near the tuning frequency of the passive filter. If the equivalent connected inductance L s and the passive filter resonate near the tuning frequency of the passive filter, it will make the filtering at the tuning frequency very difficult. In addition, if the short-circuit impedance of the connected transformer is too small or the turns ratio is too large, the current flowing through the equivalent parallel inductor L p will be too large, increasing the current output of the commutation valve of the active filter. Considering comprehensively, the equivalent connected inductance L sThe passive filter resonates at a frequency near the middle of the target harmonic order and the fundamental frequency.

[0072] (2.2) Calculate the final voltage output and current output of the active filter according to the harmonic compensation requirement and reactive power compensation requirement of the active filter design requirements, and design the capacity of the converter valve in the active filter accordingly.

[0073] Specifically, it includes the following steps:

[0074] (2.2.1) Calculate the harmonic voltage output and harmonic current output of the active filter required under this harmonic according to the maximum harmonic compensation requirement of the active filter design requirements.

[0075] From the approximate equivalent circuit of the active filter harmonic Figure 4 It can be seen that the maximum value of the harmonic source current to be filtered is i s0-h , and the active filter needs to generate a reverse current i s-h to cancel it out. The harmonic voltage output U h of the converter valve of the active filter can be calculated according to Equation (4)

[0076]

[0077] where is the impedance of the connected passive filter at the target harmonic order, is the impedance of the equivalent connection inductance L s of the connected transformer at the target harmonic order.

[0078] The harmonic current output i APF-h of the converter valve of the active filter can be calculated according to Equation (5), which is the sum of the target harmonic current i s-h and the current i T3-h generated by the converter valve output on the equivalent parallel inductance of the transformer.

[0079]

[0080] (2.2.2) Calculate the minimum fundamental voltage output and fundamental current output of the active filter converter valve according to the maximum reactive power compensation requirement of the active filter design requirements.

[0081] From the approximate equivalent circuit of the active filter fundamental frequency Figure 5 It can be seen that when the fundamental frequency voltage of the active filter converter valve is in phase with the fundamental frequency voltage of the AC bus, as the amplitude of the fundamental frequency voltage of the active filter converter valve changes linearly, the current output of the active filter converter valve and the fundamental frequency current input by the active filter to the AC system both change linearly.

[0082] From Figure 5It can be known that the current \(i\) in the equivalent parallel capacitance of the transformer T1-h is calculated as shown in Equation (6)

[0083]

[0084] The active power filter design injects reactive current \(i\) into the AC system Q , then the fundamental voltage output \(U\) of the converter valve of the active power filter APF-1 is calculated as shown in Equation (7)

[0085]

[0086] In the formula, is the fundamental impedance connecting the passive filter, is the fundamental impedance connecting the equivalent connecting inductance of the transformer.

[0087] The fundamental current output \(i\) of the converter valve of the active power filter APF-1 is calculated as shown in Equation (8):

[0088]

[0089] The fundamental frequency current on the high-voltage side of the transformer connected to the active power filter is as shown in Equation (9)

[0090]

[0091] Through analysis, it can be obtained that when the current flowing through the transformer connected to the active power filter is zero, the active power filter emits capacitive reactive power to the system, and the converter valve of the active power filter emits capacitive fundamental current; when the fundamental current output of the converter valve of the active power filter is zero, the active power filter injects capacitive reactive power into the system; when the reactive power injected by the active power filter into the system is zero, the converter valve of the active power filter emits inductive fundamental current.

[0092] In order to make full use of the capacity of the converter valve of the active power filter, generally, the point where the current output of the converter valve is zero is taken as the zero working point. At this time, the active power filter as a whole outputs capacitive reactive power. Determine the maximum inductive reactive power and maximum capacitive reactive power input by the active power filter into the AC system according to the design requirements, and calculate the fundamental voltage output and fundamental current output of the converter valve of the active power filter according to the above formulas.

[0093] (2.2.3) According to the calculated minimum fundamental voltage output and fundamental current output of the converter valve of the active power filter, as well as the calculated harmonic voltage output and harmonic current output, calculate the final voltage output and current output of the active power filter.

[0094] Among them, the final voltage output of the active filter is the sum of the harmonic voltage output and the fundamental voltage output. The final current output of the active filter is the sum of the harmonic current output and the fundamental current output.

[0095] (2.2.4) Design the converter valve capacity and the component devices of the active filter according to the final voltage output and current output of the active filter.

[0096] Embodiment 4

[0097] The following takes a specific embodiment applied to a newly built ±800 kV, 8000 MW UHVDC converter station as an example to specifically illustrate the above-mentioned design method of the active filter:

[0098] It is known that the maximum single-step switching reactive power fluctuation allowed by the AC system to which the converter station is connected is 300 MVA. The active filter is required to filter out the 11th and 13th harmonics generated by the converter station and generate 200 MVA of dynamically reactive power with smooth modulation to increase the fixed capacity of other passive reactive power compensation devices such as shunt capacitors. The maximum 11th and 13th currents generated by the converter station under full power are 328 A and 238 A respectively, and 4 active filters are used for filtering.

[0099] In this embodiment, the active filter adopts the basic topological structure as shown in Figure 1 and the basic control strategy as shown in Figure 2 . Figure 1 In, MMC Valve is the converter valve commutating arm, Uc is the voltage of the converter valve arm, i F is the current connecting the low-voltage side of the passive filter, i s is the harmonic current of the harmonic source, i T1 is the current connecting the high-voltage side of the transformer, i T2 is the target harmonic current connecting the low-voltage side of the transformer, and Zs is the impedance of the AC system. The fixed capacity of the connected passive filter is 300 MVA, and the form of the single-tuned filter is selected, and the tuning point is set at the 12th order. The specific parameters are shown in Table 1.

[0100] Table 1 Parameters of the equipment connecting the passive filter

[0101]

[0102]

[0103] The capacity of the converter station service transformer is generally 240 MVA, and the transformation ratio is 525 kV / 35 kV. The fundamental frequency impedance of the passive filter |Z C1| = 525×525 / 300 = 919Ω. Let the equivalent connection impedance of the connecting transformer and the passive filter resonate at the intermediate frequency between the 12th and fundamental frequencies, i.e., the 6th frequency of 300 Hz. Then the impedance ωL of the equivalent connection inductance of the connecting transformer s = ωL T / k should be designed to be 25.52Ω, and it is calculated that the short-circuit impedance of the connecting transformer can be taken as 33.3%.

[0104] At this time, the passive filter and the equivalent connection inductance are in parallel, equivalently increasing the impedance multiple to be

[0105]

[0106] When the current of the active filter commutation valve is zero, the total current in the equivalent connection inductance L of the transformer s and the passive filter is the same as the current in the equivalent parallel inductance L of the connecting transformer p The active filter voltage output is

[0107]

[0108] At this time, the reactive current injected into the AC system is calculated to be 0.288 kA of capacitive current.

[0109] The corresponding reactive power is -260 MVA. That is, the power zero point of the active filter generates 260 MVA of capacitive reactive power. The goal is to generate 200 MVA of dynamic reactive power, so the reactive power output range of the active filter is 160 MVA to 360 MVA of capacitive reactive power.

[0110] When generating 160 MVA of capacitive reactive power, the commutation valve output is the line voltage of 29 kV RMS and the current output is 1663 A RMS.

[0111] When generating 360 MVA, the commutation valve output is the line voltage of 39 kV RMS and the current output is 1663 A RMS.

[0112] Based on this, IGBT devices of 4500V / 2100A can be selected, the rated working voltage is selected as 2.4 kV, and the Y-connected neutral point grounding method is adopted. Each phase of the commutation valve requires 13 IGBT sub-modules.

[0113] The above embodiments are only used to illustrate the present invention. The structures, connection methods, manufacturing processes, etc. of each component can all be changed. Any equivalent transformation and improvement made on the basis of the technical solution of the present invention should not be excluded from the protection scope of the present invention.

Claims

1. A design method for an active filter that simultaneously compensates for reactive power and controls harmonics. The primary system of the active filter that simultaneously compensates for reactive power and controls harmonics includes: Turn-off device commutation valve and connection equipment; the turn-off device commutation valve is connected to the AC bus through the connection equipment, and the AC bus is respectively connected to the AC system and the harmonic source; the turn-off device commutation valve is composed of 3-phase commutation bridge arms, and each phase of the commutation bridge arm is composed of a DC support capacitor and a turn-off device. By controlling the opening and closing of the turn-off device, the output voltage of the turn-off device commutation valve is controlled; the connection equipment includes a connection transformer and a connection passive filter connected in parallel. The connection transformer is an inductive device and is used to flow through the fundamental frequency reactive current in the low frequency band; the connection passive filter is a capacitive device and is used to flow through the high frequency harmonic current in the high frequency band; it is characterized by including the following steps: (1)Perform equivalent analysis on the active filter to obtain the equivalent circuit diagram of the active filter, and perform harmonic analysis and fundamental frequency analysis on the active filter based on this equivalent circuit diagram; (2)Based on the equivalent circuit diagram of the active filter and the results of harmonic analysis and fundamental frequency analysis, design the equipment components of the active filter; In the step (1), when performing equivalent analysis on the active filter, the high-voltage side connected to the transformer is equivalently regarded as a parallel capacitor , and the low-voltage side is equivalently regarded as a parallel inductor , and the connection between the high-voltage side and the low-voltage side is equivalently regarded as a connecting inductor , and it satisfies that: Among them, is the connection transformer turns ratio; is the equivalent value of the short-circuit inductance of the connection transformer on the high-voltage side; ω is the frequency.

2. The design method for an active filter that simultaneously compensates for reactive power and controls harmonics according to claim 1, wherein In the step (2), the method of designing the equipment components of the active filter based on the equivalent circuit diagram of the active filter and the results of harmonic analysis and fundamental frequency analysis includes the following steps: (2.1)Based on the equivalent circuit diagram of the active filter, design the tuning frequency of the connection passive filter according to the harmonic order to be filtered, and design the connection equipment of the active filter in combination with the harmonic filtering requirement and the reactive power output requirement; (2.2)According to the harmonic compensation requirement and the reactive power compensation power requirement of the active filter design requirements, calculate the final voltage output and current output of the active filter, and design the capacity of the turn-off device commutation valve in the active filter accordingly.

3. The design method for an active filter that simultaneously compensates for reactive power and controls harmonics according to claim 2, wherein In the step (2.1), when designing each parameter in the connecting device, the equivalent connecting inductance of the connecting transformer and the connecting passive filter resonate at a frequency near the middle of the target harmonic order and the fundamental frequency.

4. The design method for an active filter that simultaneously compensates for reactive power and controls harmonics according to claim 2, wherein In the step (2.1), determine the specific parameters of the connection passive filter according to the system's allowable single-shot reactive power switching and the target filtering times, and determine the specific parameters of the connection transformer according to the parallel resonance times and reactive power output capacity of the connection passive filter.

5. The design method for an active filter that simultaneously compensates for reactive power and controls harmonics according to claim 2, wherein In the step (2.2), the method of designing the capacity of the turn-off device commutation valve of the active filter includes the following steps: (2.2.1)According to the maximum harmonic compensation requirement of the active filter design requirements, calculate the harmonic voltage output and harmonic current output required by the active filter under this harmonic; (2.2.2)According to the maximum reactive power compensation power requirement of the active filter design requirements, calculate the minimum fundamental voltage output and fundamental current output of the active filter commutation valve; (2.2.3)According to the calculated minimum fundamental voltage output and fundamental current output of the active filter commutation valve, and the calculated harmonic voltage output and harmonic current output, calculate the final voltage output and current output of the active filter; (2.2.4)According to the final voltage output and current output of the active filter, design the capacity and composition equipment of the turn-off device commutation valve of the active filter.

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