An active power filter system

Abstract

The present invention provides an active power filter system, which is connected to the output end of a three-phase power grid and includes: a three-phase bridge inverter circuit and an asymmetric filter circuit, wherein the asymmetric filter circuit includes a first filter circuit and a second filter circuit, and the three-phase bridge inverter circuit includes an A-phase bridge arm, a B-phase bridge arm, a C-phase bridge arm and a DC bus capacitor connected in parallel; one end of the first filter circuit is connected to the first transistor emitter and the second transistor collector of the A-phase bridge arm, and the other end is connected to the A-phase circuit output end of the three-phase power grid; one end of the second filter circuit is connected to the fifth transistor emitter and the sixth transistor collector of the C-phase bridge arm, and the other end is connected to the C-phase circuit output end of the three-phase power grid; the third transistor emitter and the fourth transistor collector of the B-phase bridge arm are connected to the B-phase circuit output end of the three-phase power grid. By filtering the harmonics of the three-phase circuit through the two-phase filter circuit, a faster harmonic compensation dynamic response speed can be achieved.

Description

Technical Field

[0001] The invention relates to the technical field of power grids, and in particular to an active power filter system. Background Art

[0002] With the rapid development of power electronics technology, the number of nonlinear loads (such as arc furnaces, power electronics devices, transformers, etc.) put into operation in the power grid has gradually increased, resulting in increasingly serious distortion of the voltage and current waveforms of the power grid, and increasing harmonic content of voltage and current, which has a great impact on the power supply quality. A power grid with high harmonic content will cause an increase in load loss, and even the power grid harmonics will seriously affect the stable operation of high-precision equipment connected to the power grid.

[0003] There are two main measures for harmonic control. One is active control, which is to start from the harmonic source itself to prevent the harmonic source from generating harmonics or reduce the harmonics generated by the harmonic source. The other is passive control, which is to connect other external devices to prevent the harmonics generated by the harmonic source from being injected into the power grid, or to hinder the harmonics of the power system from flowing into the load, mainly using passive filters, active filters or hybrid active filters.

[0004] At present, the common topologies of active power filter systems include three-phase four-wire active power filter topologies and three-phase four-switch active power filter topologies. The three-phase four-wire active filter topology is easy to control, but it is too large, has too many switch devices, and has development limitations; while the three-phase four-switch active power filter topology reduces the number of switch devices and reduces system costs, but its DC side output voltage must be higher than twice the peak value of the line voltage, and there is a problem of excessive DC side voltage increasing the system operation burden. In other words, the existing active power filter system has the problems of excessive size, too many switch devices, and excessive DC side voltage increasing the system operation burden. Summary of the invention

[0005] Based on this, the present invention provides an active power filter system, which simplifies the topological structure of the active filter and reduces the operating burden caused by the DC side voltage.

[0006] In a first aspect, the present invention provides an active power filter system, wherein the active power filter system is connected to an output end of a three-phase power grid, and comprises:

[0007] A three-phase bridge inverter circuit and an asymmetric filter circuit, wherein the asymmetric filter circuit includes a first filter circuit and a second filter circuit, and the three-phase bridge inverter circuit includes an A-phase bridge arm, a B-phase bridge arm, a C-phase bridge arm and a DC bus capacitor;

[0008] The A-phase bridge arm includes a first transistor and a second transistor, and the emitter of the first transistor is connected to the collector of the second transistor; the B-phase bridge arm includes a third transistor and a fourth transistor, and the emitter of the third transistor is connected to the collector of the fourth transistor; the C-phase bridge arm includes a fifth transistor and a sixth transistor, and the emitter of the fifth transistor is connected to the collector of the sixth transistor; one end of the DC bus capacitor is connected to the collector of the first transistor, the collector of the third transistor, and the collector of the fifth transistor, and the other end of the DC bus capacitor is connected to the emitter of the second transistor, the emitter of the fourth transistor, and the emitter of the sixth transistor;

[0009] One end of the first filter circuit is connected to the emitter of the first transistor and the collector of the second transistor of the A-phase bridge arm, and the other end is connected to the output end of the A-phase circuit of the three-phase power grid;

[0010] One end of the second filter circuit is connected to the emitter of the fifth transistor and the collector of the sixth transistor of the C-phase bridge arm, and the other end is connected to the output end of the C-phase circuit of the three-phase power grid;

[0011] The emitter of the third transistor and the collector of the fourth transistor of the B-phase bridge arm are connected to the output end of the B-phase circuit of the three-phase power grid.

[0012] In a second aspect, the present invention provides a three-phase power grid with an active power filter system, characterized in that it includes a three-phase power grid circuit, a nonlinear load circuit and any one of the active power filter systems in the first aspect, wherein the output end of the three-phase power grid circuit is connected to the input end of the nonlinear load circuit and the output end of the active power filter system.

[0013] The beneficial effects of adopting the above technical solution are as follows: the active power filter system of the present application adjusts the two-phase voltage of the bridge inverter circuit so that the current on the DC side keeps up with the specified current, effectively reduces the capacity of the active power filter, and tracks and controls the harmonic current; and simplifies the topology of the active power filter system, realizes harmonic compensation through two filter circuits, and achieves the effect of fast dynamic response speed and good real-time performance of harmonic compensation. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art are briefly introduced below.

[0015] Figure 1 A topological diagram of an active power filter system provided in one embodiment of the present invention;

[0016] Figure 2A topological schematic diagram of a first specific structure of an asymmetric filter circuit in an active power filter system provided in one embodiment of the present invention;

[0017] Figure 3 A Bode diagram of an active power filter system provided in one embodiment of the present invention;

[0018] Figure 4 A topological diagram of a second specific structure of an asymmetric filter circuit in an active power filter system provided in one embodiment of the present invention. DETAILED DESCRIPTION

[0019] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. In order to explain the present invention in more detail, an active power filter system provided by the present invention is specifically described below in combination with the drawings.

[0020] With the rapid development of power electronics technology, the number of nonlinear loads (such as arc furnaces, power electronics devices, transformers, etc.) put into operation in the power grid has gradually increased, resulting in increasingly serious distortion of the voltage and current waveforms of the power grid, and increasing harmonic content of voltage and current, which has a great impact on the power supply quality. A power grid with high harmonic content will cause an increase in load loss, and even the power grid harmonics will seriously affect the stable operation of high-precision equipment connected to the power grid.

[0021] See attached Figure 1 , attached Figure 1 The schematic diagram of the active power filter system provided by the embodiment of the present invention is shown. The active power filter system 200 of the embodiment of the present invention is connected to the output end of the three-phase power grid 101, and the active power filter system includes:

[0022] The three-phase bridge inverter circuit 202 and the asymmetric filter circuit 201, the asymmetric filter circuit 201 includes a first filter circuit 2011 and a second filter circuit 2012, and the three-phase bridge inverter circuit 202 includes an A-phase bridge arm, a B-phase bridge arm, a C-phase bridge arm and a DC bus capacitor C dc ;

[0023] The A-phase bridge arm includes a first transistor T1 and a second transistor T2, and the emitter of the first transistor T1 is connected to the collector of the second transistor T2; the B-phase bridge arm includes a third transistor T3 and a fourth transistor T4, and the emitter of the third transistor T3 is connected to the collector of the fourth transistor T4; the C-phase bridge arm includes a fifth transistor T5 and a sixth transistor T6, and the emitter of the fifth transistor T5 is connected to the collector of the sixth transistor T6; the DC bus capacitor C dc One end of the capacitor C is connected to the collector of the first transistor T1, the collector of the third transistor T3 and the collector of the fifth transistor T5. dc The other end is commonly connected to the emitter of the second transistor T2, the emitter of the fourth transistor T4 and the emitter of the sixth transistor T6;

[0024] One end of the first filter circuit 2011 is commonly connected to the emitter of the first transistor T1 and the collector of the second transistor T2 of the A-phase bridge arm, and the other end is coupled to the output end of the A-phase circuit of the three-phase power grid 101; one end of the second filter circuit 2012 is commonly connected to the emitter of the fifth transistor T5 and the collector of the sixth transistor T6 of the C-phase bridge arm, and the other end is coupled to the output end of the C-phase circuit of the three-phase power grid 101; the emitter of the third transistor T3 and the collector of the fourth transistor T4 of the B-phase bridge arm are commonly connected to the output end of the B-phase circuit of the three-phase power grid 101.

[0025] In the embodiment of the present invention, since the asymmetric filter circuit only has the first filter circuit and the second filter circuit, the B-phase bridge arm is directly connected to the B-phase circuit of the three-phase power grid. In steady state, the sum of the three-phase currents is 0. By controlling the output voltage U fab and U fcb , the A-phase current and C-phase current output by the three-phase inverter can be controlled to track the command current to reach a balanced state, while the B-phase current will follow and automatically adjust the balance. Through the setting of the above active power filter system, the filter circuit is simplified while the capacity of the active power filter is effectively reduced, and the harmonic current is effectively tracked and controlled. Compared with the traditional three-phase filter circuit system, it has a significant cost advantage and helps promote the application of active power filter systems in the field of medium and low voltage harmonic control.

[0026] Furthermore, since the transistor setting of the three-phase bridge circuit is used to control the conduction of the upper or lower half of the bridge arm when a pulse signal passes through, on this basis, the transistor can be set to a triode, a field effect transistor, a thyristor or an insulated gate bipolar power tube according to actual needs.

[0027] In addition, in order to better control the switching process of the upper and lower parts of the bridge arm during operation, a diode is added to the three-phase bridge circuit transistor, specifically:

[0028] The A-phase bridge arm further includes a first diode D1 and a second diode D2, wherein an anode of the first diode D1 is coupled to an emitter of the first transistor T1, a cathode of the first diode D1 is coupled to a collector of the first transistor T1, an anode of the second diode D2 is coupled to an emitter of the second transistor T2, and a cathode of the second diode D2 is coupled to a collector of the second transistor T2;

[0029] The B-phase bridge arm further includes a third diode D3 and a fourth diode D4, wherein the anode of the third diode D3 is coupled to the emitter of the third transistor T3, the cathode of the third diode D3 is coupled to the collector of the third transistor T3, the anode of the fourth diode D4 is coupled to the emitter of the fourth transistor T4, and the cathode of the fourth diode D4 is coupled to the collector of the fourth transistor T4;

[0030] The C-phase bridge arm also includes a fifth diode D5 and a sixth diode D6, wherein the anode of the fifth diode D5 is coupled to the emitter of the fifth transistor T5, the cathode of the fifth diode D5 is coupled to the collector of the fifth transistor T5, the anode of the sixth diode D6 is coupled to the emitter of the sixth transistor T6, and the cathode of the sixth diode D6 is coupled to the collector of the sixth transistor T6.

[0031] Next, several specific structures of the first filtering circuit and the second filtering circuit are given, and the relevant descriptions are as follows:

[0032] The first specific structure:

[0033] See attached Figure 2 , the first filter circuit 2011 and the second filter circuit 2012 are specifically configured as:

[0034] The first filter circuit 2011 includes a first inductor La, a first capacitor Ca and a first resistor Ra connected in sequence;

[0035] The second filtering circuit 2012 includes a second inductor Lc, a second capacitor Cc, and a second resistor Rc which are connected in sequence.

[0036] According to Kirchhoff's voltage law and Kirchhoff's current law, the transfer function of the first filter circuit can be derived as: Among them, I Ca (s) is the current of the first filter circuit, U fab (s) is the output voltage of phase A of the three-phase bridge circuit, e ab (s) is the A phase voltage of the three-phase power grid, L a is the size of the first inductor, C a is the size of the first capacitor, s is the time;

[0037] The transfer function of the traditional filter circuit is: Among them, I′Ca (s) is the current of the traditional filter circuit, U fa (s) is the output voltage of phase A of the three-phase bridge circuit, e a (s) is the C phase voltage of the three-phase power grid, L′ a is the size of the inductance of the traditional filter circuit, C′ a is the size of the capacitor of the traditional filter circuit, and s is the time.

[0038] It can be seen from the above transfer function that the asymmetric filter circuit has the same transfer function as the traditional filter circuit. As long as the parameters of the asymmetric filter circuit are properly controlled, the same filtering performance as the traditional three-phase filter circuit can be obtained.

[0039] Specifically, in this embodiment, the parameters of the asymmetric filter circuit are set as follows:

[0040] First, set the resonant frequency to be the same as the harmonic frequency to be filtered out. According to the filter principle, the resonance frequency of the filter circuit is obtained:

[0041] The resonance number of the first filter circuit is: Wherein, n is the resonance number, ω is the resonance frequency, L1 is the first inductance, and C1 is the first capacitance;

[0042] The resonance number of the second filter circuit is: Wherein, n is the resonance number, ω is the resonance frequency, L2 is the second inductance, and C2 is the second capacitance.

[0043] Secondly, assuming that the AC bus only has the fundamental voltage and ignoring the reactance of the active power filter system, the current of the asymmetric filter circuit includes the fundamental current and the nth harmonic current. The reactive capacity includes both the fundamental reactive capacity and the nth harmonic reactive capacity. The fundamental current can be expressed as: I f1 is the fundamental current, U1 is the fundamental voltage, ω is the resonant frequency, C is the capacitance of the asymmetric filter circuit, L is the inductance of the asymmetric filter circuit, and n is the resonance number of the asymmetric filter circuit. Therefore, the reactive capacity of the first filter circuit and the third filter circuit is:

[0044] The reactive capacity of the first filter circuit is: Among them, S n1 is the reactive capacity of the first filter circuit, Q 11 is the fundamental reactive capacity of the first filter circuit, Q n1 is the nth harmonic reactive capacity of the first filter circuit, ω is the resonant frequency, C1 is the size of the first capacitor, n1 is the resonance number of the first harmonic circuit, I f1 is the magnitude of the fundamental current, Ifn is the magnitude of the nth harmonic current; when When the reactive capacity S of the first filter circuit n1 Minimum, where U1 is the magnitude of the fundamental voltage;

[0045] The reactive capacity of the second filter circuit is: Among them, S n2 is the reactive capacity of the second filter circuit, Q 12 is the fundamental reactive capacity of the second filter circuit, Q n2 is the nth harmonic reactive capacity of the second filter circuit, ω is the resonant frequency, C2 is the size of the second capacitor, n2 is the resonance number of the second harmonic circuit, I f1 is the magnitude of the fundamental current, I fn is the magnitude of the nth harmonic current; when When the reactive capacity S of the second filter circuit n2 Minimum, where U1 is the magnitude of the fundamental voltage.

[0046] Furthermore, the sizes of the capacitors and inductors in the first filter circuit and the second filter circuit can be deduced:

[0047] The first capacitor is: Where C1 is the size of the first capacitor, I fn is the magnitude of the nth harmonic current, U1 is the magnitude of the fundamental voltage, and n1 is the resonance number of the first filter circuit;

[0048] The second capacitor is: Where C2 is the size of the second capacitor, I fn is the magnitude of the nth harmonic current, U1 is the magnitude of the fundamental voltage, and n2 is the resonance number of the second filter circuit;

[0049] The first inductance is: Wherein, L1 is the size of the first inductor, n1 is the resonance number of the first filter circuit, ω is the resonance frequency, and C1 is the size of the first capacitor;

[0050] The second inductor is: Wherein, L2 is the size of the first inductor, n2 is the resonance number of the first filter circuit, ω is the resonance frequency, and C2 is the size of the first capacitor.

[0051] Taking the asymmetric filter circuit that needs to filter out the 5th harmonic as an example, n=5, it can be calculated that the first inductance L1=4mH and the first capacitance C1=100μF of the first filter circuit; the second inductance L2=4mH and the second capacitance C2=100μF of the second filter circuit.

[0052] See attached Figure 3 , attached Figure 3 This is the Bode diagram of the active power filter system. It can be seen from the Bode diagram that it presents low impedance near the fifth harmonic, which diverts the fifth harmonic component of the load current; and it presents high impedance at the switching frequency, which suppresses the switching ripple, so there is no need to set up an additional switching ripple filter circuit.

[0053] The second specific structure:

[0054] See attached Figure 4 In order to further reduce the complexity of the filter circuit, the first filter circuit and the second filter circuit can also be configured as follows: the first filter circuit includes a third inductor; similarly, the second filter circuit includes a fourth inductor.

[0055] Taking the first filtering circuit as an example, the line resistance is ignored in the process of setting the inductance parameters, which is specifically expressed as follows: L3 is the size of the third inductor, i ca is the current of the first filter circuit, e ab is the A phase voltage of the three-phase power grid, U dc is the DC bus capacitor voltage, K is the switching function, the value range of K is [-1 / 3, 1 / 3]. When the active power filter system operates for a long time, the mathematical expectation of K is 4 / 9.

[0056] Since the output compensation current of the active power filter must be greater than the rate of change of the command current, the worst instantaneous change situation must be considered, that is, the load current is before and after the phase change. If K takes the value of -1 / 3 or 1 / 3, then When the active power filter system is in long-term operation, K takes the expected value of 4 / 9, which satisfies

[0057] Set the maximum allowable deviation between the actual compensation current and the command current on the DC side to Δi max , the maximum current change rate can be expressed as: Where T s is a control cycle; similarly,

[0058] In summary, the value range of the third inductor is: Among them, L3 is the size of the third inductor, U dc is the DC bus capacitor voltage, T s To control the time period, Δi max is the maximum allowable deviation between the actual compensation current and the command current on the DC side, i ca is the current size of the first filtering circuit.

[0059] Similarly, the specific value range of the fourth inductor is: Among them, L4 is the size of the fourth inductor, Udc is the DC bus capacitor voltage, T S To control the time period, Δi max is the maximum allowable deviation between the actual compensation current and the command current on the DC side, i cc is the current size of the second filtering circuit.

[0060] In the asymmetric filter circuit of this embodiment, an inductor is used as a filter element, and the DC side capacitor voltage needs to be higher than twice the grid phase voltage. Therefore, when controlling the DC side, the set value of the DC side capacitor voltage will be higher than the set value corresponding to other filter circuits. In addition, from the perspective of filtering effect, using an inductor as a filter element can effectively avoid the occurrence of resonance.

[0061] In addition, an embodiment of the present invention also provides a three-phase power grid with an active power filter system, including a three-phase power grid circuit, a nonlinear load circuit and the above-mentioned active power filter system, wherein the output end of the three-phase power grid circuit is connected to the input end of the nonlinear load circuit and the output end of the active power filter system.

[0062] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. An active power filter system, the active power filter system is connected to the output end of a three-phase power grid, characterized in that: include: A three-phase bridge inverter circuit and an asymmetric filter circuit, wherein the asymmetric filter circuit includes a first filter circuit and a second filter circuit, and the three-phase bridge inverter circuit includes an A-phase bridge arm, a B-phase bridge arm, a C-phase bridge arm and a DC bus capacitor; The A-phase bridge arm includes a first transistor and a second transistor, and the emitter of the first transistor is connected to the collector of the second transistor; the B-phase bridge arm includes a third transistor and a fourth transistor, and the emitter of the third transistor is connected to the collector of the fourth transistor; the C-phase bridge arm includes a fifth transistor and a sixth transistor, and the emitter of the fifth transistor is connected to the collector of the sixth transistor; one end of the DC bus capacitor is commonly connected to the collector of the first transistor, the collector of the third transistor, and the collector of the fifth transistor, and the other end of the DC bus capacitor is commonly connected to the emitter of the second transistor, the emitter of the fourth transistor, and the emitter of the sixth transistor; One end of the first filter circuit is commonly connected to the emitter of the first transistor and the collector of the second transistor of the A-phase bridge arm, and the other end is coupled to the output end of the A-phase circuit of the three-phase power grid; One end of the second filter circuit is commonly connected to the emitter of the fifth transistor and the collector of the sixth transistor of the C-phase bridge arm, and the other end is coupled to the output end of the C-phase circuit of the three-phase power grid; The emitter of the first transistor and the collector of the third transistor of the B-phase bridge arm are commonly connected to the output end of the B-phase circuit of the four-phase power grid.

2. The active power filter system according to claim 1, characterized in that: The A-phase bridge arm further includes a first diode and a second diode, wherein an anode of the first diode is coupled to an emitter of the first transistor, a cathode of the first diode is coupled to a collector of the first transistor, an anode of the second diode is coupled to an emitter of the second transistor, and a cathode of the second diode is coupled to a collector of the second transistor; The B-phase bridge arm further includes a third diode and a fourth diode, wherein the anode of the third diode is coupled to the emitter of the third transistor, the cathode of the third diode is coupled to the collector of the third transistor, the anode of the fourth diode is coupled to the emitter of the fourth transistor, and the cathode of the fourth diode is coupled to the collector of the fourth transistor; The C-phase bridge arm also includes a fifth diode and a sixth diode, the anode of the fifth diode is coupled to the emitter of the fifth transistor, the cathode of the fifth diode is coupled to the collector of the fifth transistor, the anode of the sixth diode is coupled to the emitter of the sixth transistor, and the cathode of the sixth diode is coupled to the collector of the sixth transistor.

3. The active power filter system according to claim 1, characterized in that: The first filtering circuit and the second filtering circuit include: The first filter circuit includes a first inductor, a first capacitor and a first resistor connected in sequence; The second filtering circuit includes a second inductor, a second capacitor, and a second resistor which are connected in sequence.

4. The active power filter system according to claim 3, characterized in that: The first filtering circuit and the second filtering circuit include: The resonance number of the first filter circuit is: Wherein, n1 is the resonance number, ω is the resonance frequency, L1 is the first inductance, and C1 is the first capacitance; The resonance number of the second filter circuit is: Wherein, n2 is the resonance number, ω is the resonance frequency, L2 is the second inductance, and C2 is the second capacitance.

5. The active power filter system according to claim 3, characterized in that: The first filtering circuit and the second circuit include: The first capacitor of the first filter circuit is: Where C1 is the size of the first capacitor, I fn is the magnitude of the nth harmonic current, U1 is the magnitude of the fundamental voltage, and n1 is the resonance number of the first filter circuit; The second capacitor of the second filtering circuit is: Where C2 is the size of the second capacitor, I fn is the magnitude of the nth harmonic current, U1 is the magnitude of the fundamental voltage, and n2 is the resonance order of the second filter circuit.

6. An active power filter system as claimed in claim 3, characterized in that: The first filtering circuit and the second filtering circuit include: The first inductance of the first filter circuit is: Wherein, L1 is the size of the first inductor, n1 is the resonance number of the first filter circuit, ω is the resonance frequency, and C1 is the size of the first capacitor; The second inductance of the second filter circuit is: Wherein, L2 is the size of the first inductor, n2 is the resonance number of the first filter circuit, ω is the resonance frequency, and C2 is the size of the first capacitor.

7. The active power filter system according to claim 3, characterized in that: The first filtering circuit and the second filtering circuit include: The reactive capacity of the first filter circuit is: Among them, S n1 is the reactive capacity of the first filter circuit, Q 11 is the fundamental reactive capacity of the first filter circuit, Q n1 is the nth harmonic reactive capacity of the first filter circuit, ω is the resonant frequency, C1 is the size of the first capacitor, n1 is the resonance number of the first harmonic circuit, I f1 is the magnitude of the fundamental current, I fn is the magnitude of the nth harmonic current; when When the reactive capacity S of the first filter circuit n1 Minimum, where U1 is the magnitude of the fundamental voltage; The reactive capacity of the second filter circuit is: Among them, S n2 is the reactive capacity of the second filter circuit, Q 12 is the fundamental reactive capacity of the second filter circuit, Q n2 is the nth harmonic reactive capacity of the second filter circuit, ω is the resonant frequency, C2 is the size of the second capacitor, n2 is the resonance number of the second harmonic circuit, I f1 is the magnitude of the fundamental current, I fn is the magnitude of the nth harmonic current; when When the reactive capacity S of the second filter circuit n2 Minimum, where U1 is the magnitude of the fundamental voltage.

8. An active power filter system as claimed in claim 1, characterized in that: The first filtering circuit and the second filtering circuit include: The first filtering circuit includes a third inductor; The second filtering circuit includes a fourth inductor.

9. The active power filter system according to claim 8, characterized in that: The first filtering circuit and the second filtering circuit include: The specific value range of the third inductance of the first filter circuit is: Among them, L3 is the size of the third inductor, U dc is the DC bus capacitor voltage, T s To control the time period, Δi max is the maximum allowable deviation between the actual compensation current and the command current on the DC side, i ca is the current magnitude of the first filter circuit; The specific value range of the fourth inductance of the second filter circuit is: Among them, L4 is the size of the fourth inductor, U dc is the DC bus capacitor voltage, T s To control the time period, Δi max is the maximum allowable deviation between the actual compensation current and the command current on the DC side, i cc is the current size of the second filtering circuit.

10. A three-phase power grid with an active power filter system, characterized in that: It comprises a three-phase grid circuit, a nonlinear load circuit and an active power filter system according to any one of claims 1 to 9, wherein the output end of the three-phase grid circuit is commonly connected with the input end of the nonlinear load circuit and the output end of the active power filter system.

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