An lcl filter circuit of a grid-connected inverter

Abstract

This application discloses an LCL filter circuit for a grid-connected inverter, relating to the field of power electronics technology. In this circuit, the DC input power supply provides DC energy to the grid-connected inverter; the DC bus includes several electrolytic capacitors; the grid-connected inverter consists of several three-phase bridge arms; the grid-connected inverter is connected in parallel with the DC input power supply; the LCL filter circuit includes a three-phase output inductor L... a1 Inductor L b1 and inductor L c1 and respectively with inductor L a1 Inductor L b1 and inductor L c1 Series inductor L a2 Inductor L b2 and inductor L c2 and respectively with inductor L a2 Inductor L b2 and inductor L c2 parallel capacitor C a Capacitor C b and capacitor C c Capacitor C a Capacitor C b and capacitor C c Each is connected to a damping resistor R d The inverter is connected in series; the output section consists of three-phase AC power terminals connected to the power grid. This application enables effective suppression of harmonics in the inverter output current.

Description

Technical Field

[0001] This application relates to the field of power electronics technology, and in particular to an LCL filter circuit for a grid-connected inverter. Background Technology

[0002] With the widespread application of renewable energy and the development of power electronics technology, an increasing number of grid-connected inverters are being used to convert direct current (DC) to grid-compatible alternating current (AC). However, inverters may generate harmonics during the conversion process, which can not only degrade power quality but also potentially damage the power grid and connected equipment. Therefore, designing an effective filtering circuit to reduce harmonics and electromagnetic interference is crucial for improving the performance of grid-connected inverters.

[0003] While traditional LC filters can remove some harmonics, their filtering effect is limited to a single resonant frequency and can cause resonance problems. In contrast, LCL filters are favored for their multi-resonance characteristics and wider stopband, effectively suppressing harmonics over a wider frequency range. Utility Model Content

[0004] The purpose of this application is to provide an LCL filter circuit for a grid-connected inverter. This circuit effectively suppresses harmonics in the inverter output current by optimizing the configuration of inductors and capacitors, while avoiding resonance problems and improving power quality.

[0005] To achieve the above objectives, this application provides the following solution:

[0006] In a first aspect, this application provides an LCL filter circuit for a grid-connected inverter, comprising:

[0007] DC input power supply, DC bus, grid-connected inverter, LCL filter circuit and output section;

[0008] The DC input power supply is used to provide DC energy to the grid-connected inverter;

[0009] The DC bus includes several electrolytic capacitors;

[0010] The grid-connected inverter consists of several three-phase bridge arms; the grid-connected inverter is connected in parallel with the DC input power supply;

[0011] The LCL filter circuit includes a three-phase output inductor L a1 Inductor L b1 and inductor L c1 and respectively with inductor L a1 Inductor L b1 and inductor L c1 Series inductor L a2 Inductor L b2 and inductor Lc2 and respectively with inductor L a2 Inductor L b2 and inductor L c2 parallel capacitor C a Capacitor C b and capacitor C c ;

[0012] The capacitor C a Capacitor C b and capacitor C c Each is connected to a damping resistor R d Series;

[0013] The output section is a three-phase AC power output terminal that is connected to the power grid.

[0014] Optionally, the grid-connected inverter includes three-phase bridge arm A, three-phase bridge arm B, and three-phase bridge arm C; the three-phase bridge arm A, three-phase bridge arm B, and three-phase bridge arm C are connected in parallel.

[0015] Optionally, each three-phase bridge arm consists of a number of anti-parallel diodes, a switching transistor, and a clamping diode.

[0016] Optionally, the three-phase bridge arm A is composed of a first switching transistor, a second switching transistor, a third switching transistor, a fourth switching transistor, a first clamping diode, and a second clamping diode connected in anti-parallel.

[0017] Optionally, the first and fourth switching transistors are the outer transistors of the three-phase bridge arm A, the second and third switching transistors are the inner transistors of the three-phase bridge arm A, the anode of the first clamping diode is connected to the midpoint of the first and second switching transistors, and the cathode is connected to the midpoint of the DC bus, the anode of the second clamping diode is connected to the midpoint of the DC bus, and the cathode is connected to the midpoint of the third and fourth switching transistors.

[0018] Optionally, the three-phase bridge arm B is composed of the fifth, sixth, seventh, and eighth switching transistors of the anti-parallel diodes, the third clamping diode, and the fourth clamping diode.

[0019] Optionally, the fifth and eighth switching transistors are the outer transistors of the three-phase bridge arm B, the sixth and seventh switching transistors are the inner transistors of the three-phase bridge arm B, the anode of the third clamping diode is connected to the midpoint of the fifth and sixth switching transistors, and the cathode is connected to the midpoint of the DC bus, the anode of the fourth clamping diode is connected to the midpoint of the DC bus, and the cathode is connected to the midpoint of the seventh and eighth switching transistors.

[0020] Optionally, the three-phase bridge arm C is composed of the ninth, tenth, eleventh, and twelfth switching transistors of the anti-parallel diodes, the fifth clamping diode, and the sixth clamping diode.

[0021] Optionally, the ninth and twelfth switching transistors are the outer transistors of the three-phase bridge arm C, the tenth and eleventh switching transistors are the inner transistors of the three-phase bridge arm C, the anode of the fifth clamping diode is connected to the midpoint of the ninth and tenth switching transistors, and the cathode is connected to the midpoint of the DC bus, the anode of the sixth clamping diode is connected to the midpoint of the DC bus, and the cathode is connected to the midpoint of the eleventh and twelfth switching transistors.

[0022] Optionally, the inductor L a1 Inductor L b1 and inductor L c1 One end is connected to the output terminals of three-phase bridge arms A, B, and C of the grid-connected inverter, respectively, and the other end is connected to inductor L. a2 Inductor L b2 and inductor L c2 One end of the inductor L is connected; a2 Inductor L b2 and inductor L c2 The other end is connected to the three-phase AC output terminal of the output section as the output terminal of the LCL filter circuit.

[0023] According to the specific embodiments provided in this application, the following technical effects are disclosed:

[0024] This application provides an LCL filter circuit for a grid-connected inverter. The grid-connected inverter in this application uses an LCL filter circuit, which can effectively suppress harmonics in the inverter output current. The LCL filter circuit consists of three sets of inductors L... a1 L b1 L c1 and L a2 L b2 L c2 and three sets of capacitors C a C b C c This is combined to form a third-order filter. The inductor L... a1 L b1 L c1 Primarily used for filtering high-frequency harmonics, while inductor L a2 L b2 L c2 With capacitor C a C b C c Together, they form a resonant circuit that performs deep filtering of harmonics at specific frequencies. In addition, capacitor C... a C b C cConnected in series with damping resistor Rd, the LCL filter effectively dampens the resonant peak of the LCL filter, preventing system instability. When the inverter output current contains harmonics, the inductor and capacitor in the LCL filter circuit form a high-impedance path for the harmonics, filtering out most of the harmonic current while allowing the fundamental current to pass smoothly. In this way, the LCL filter circuit can effectively reduce the harmonic content in the inverter output current, improve the waveform quality of the output current, and meet grid connection requirements. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1 This is a schematic diagram of an LCL filter circuit for a grid-connected inverter according to an embodiment of the present invention.

[0027] Figure 2 This is a comparison chart of simulated grid-connected current FFT analysis of the LCL filter circuit of a grid-connected inverter according to an embodiment of this utility model.

[0028] 1-First switching transistor, 2-Second switching transistor, 3-Third switching transistor, 4-Fourth switching transistor, 5-Fifth switching transistor, 6-Sixth switching transistor, 7-Seventh switching transistor, 8-Eighth switching transistor, 9-Ninth switching transistor, 10-Tenth switching transistor, 11-Eleventh switching transistor, 12-Twelfth switching transistor, 13-First clamping diode, 14-Second clamping diode, 15-Third clamping diode, 16-Fourth clamping diode, 17-Fifth clamping diode, 18-Sixth clamping diode. Detailed Implementation

[0029] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0030] To make the above-mentioned objectives, features and advantages of this application more apparent and understandable, the application will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0031] like Figure 1As shown, this utility model provides an LCL filter circuit for a grid-connected inverter, including: a DC input power supply, a DC bus, a grid-connected inverter, an LCL filter circuit, and an output section;

[0032] The DC input power supply is used to provide DC energy to the grid-connected inverter;

[0033] The DC bus includes several electrolytic capacitors;

[0034] The grid-connected inverter consists of several three-phase bridge arms; the grid-connected inverter is connected in parallel with the DC input power supply;

[0035] The LCL filter circuit includes a three-phase output inductor L a1 Inductor L b1 and inductor L c1 and respectively with inductor L a1 Inductor L b1 and inductor L c1 Series inductor L a2 Inductor L b2 and inductor L c2 and respectively with inductor L a2 Inductor L b2 and inductor L c2 parallel capacitor C a Capacitor C b and capacitor C c ;

[0036] The capacitor C a Capacitor C b and capacitor C c Each is connected to a damping resistor R d Series;

[0037] The output section is a three-phase AC power output terminal that is connected to the power grid.

[0038] Among them, the DC input power supply is U dc The DC bus section includes two large-capacity electrolytic capacitors, C1 and C2, which are used to smooth the DC voltage and reduce power supply ripple.

[0039] The three-phase inverter bridge section comprises 12 switching devices connected in a specific manner to form the inverter's output. Each phase consists of four switching devices, which generate three output voltage levels through different combinations. The three output phases of the three-phase inverter bridge are phase A, phase B, and phase C, each consisting of two switching transistors (upper and lower). A diode is connected in anti-parallel next to each switching transistor to provide a freewheeling path when the transistor is turned off, protecting it from damage by reverse voltage.

[0040] The clamping circuit includes six clamping diodes to limit the voltage across the switching transistor and prevent damage to the switching transistor due to excessive voltage.

[0041] Specifically, the grid-connected inverter includes three-phase bridge arms A, B, and C; these three-phase bridge arms A, B, and C are connected in parallel. Each three-phase bridge arm consists of several anti-parallel diode switching transistors and clamping diodes.

[0042] The three-phase bridge arm A is composed of a first switching transistor 1, a second switching transistor 2, a third switching transistor 3, a fourth switching transistor 4, a first clamping diode 13, and a second clamping diode 14 connected in anti-parallel.

[0043] The first switch 1 and the fourth switch 4 are the outer tubes of the three-phase bridge arm A, the second switch 2 and the third switch 3 are the inner tubes of the three-phase bridge arm A, the anode of the first clamping diode 13 is connected to the midpoint of the first switch 1 and the second switch 2, and the cathode is connected to the midpoint of the DC bus, the anode of the second clamping diode 14 is connected to the midpoint of the DC bus, and the cathode is connected to the midpoint of the third switch 3 and the fourth switch 4.

[0044] The three-phase bridge arm B is composed of anti-parallel diodes: the fifth switch 5, the sixth switch 6, the seventh switch 7, the eighth switch 8, the third clamping diode 15, and the fourth clamping diode 16. The fifth switch 5 and the eighth switch 8 are the outer diodes of the three-phase bridge arm B, and the sixth switch 6 and the seventh switch 7 are the inner diodes of the three-phase bridge arm B. The anode of the third clamping diode 15 is connected to the midpoint of the fifth switch 5 and the sixth switch 6, and the cathode is connected to the midpoint of the DC bus. The anode of the fourth clamping diode 16 is connected to the midpoint of the DC bus, and the cathode is connected to the midpoint of the seventh switch 7 and the eighth switch 8.

[0045] The three-phase bridge arm C is composed of the ninth switch (9), tenth switch (10), eleventh switch (11), twelfth switch (12), fifth clamping diode (17), and sixth clamping diode (18) connected in anti-parallel. The ninth switch (9) and twelfth switch (12) are the outer tubes of the three-phase bridge arm C, and the tenth switch (10) and eleventh switch (11) are the inner tubes of the three-phase bridge arm C. The anode of the fifth clamping diode (17) is connected to the midpoint of the ninth switch (9) and tenth switch (10), and the cathode is connected to the midpoint of the DC bus. The anode of the sixth clamping diode (18) is connected to the midpoint of the DC bus, and the cathode is connected to the midpoint of the eleventh switch (11) and twelfth switch (12).

[0046] The inductor L a1 Inductor L b1 and inductor L c1 One end is connected to the output terminals of three-phase bridge arms A, B, and C of the grid-connected inverter, respectively, and the other end is connected to inductor L. a2 Inductor L b2and inductor L c2 One end of the inductor L is connected; a2 Inductor L b2 and inductor L c2 The other end is connected to the three-phase AC output terminal of the output section as the output terminal of the LCL filter circuit.

[0047] The LCL filter circuit includes a three-phase output inductor L. a1 L b1 L c1 It is used to suppress high-frequency harmonics of current.

[0048] With inductor L a1 L b1 L c1 Series inductor L a2 L b2 L c2 This forms the second-stage inductor in the LCL filter circuit, further suppressing harmonics. A capacitor C is connected in parallel with the second-stage inductor. a C b C c The resonant capacitor, forming the LCL filter circuit, together with the inductor, determines the filter's resonant frequency, used to filter out harmonics of specific frequencies. A damping resistor R is introduced. d Connecting a damping resistor in series with the capacitor can effectively suppress resonance and improve the performance of the filter circuit.

[0049] The output section is a three-phase AC power output terminal that is connected to the power grid.

[0050] The working principle of the LCL filter circuit is as follows:

[0051] DC power is converted to three-phase AC power by a three-phase inverter bridge. An LCL filter circuit filters the AC power output from the inverter. The inductor has high impedance for high-frequency signals and low impedance for low-frequency signals. Therefore, when the input signal passes through the inductor L... a1 L b1 L c1 At this time, high-frequency harmonic components will be significantly attenuated, while fundamental frequency signals can pass through relatively easily.

[0052] Inductor L a2 L b2 L c2 and L a1 L b1 L c1 The series connection further increases the impedance to high-frequency signals, thereby improving the filtering effect. Capacitor C a C b C c With inductor L a2 Lb2 L c2 They are connected in parallel to form a resonant circuit. This resonant circuit has the lowest impedance to signals of a specific frequency, which is usually the harmonic frequency that is to be suppressed during the design. When the harmonic frequency in the input signal matches the resonant frequency, these harmonic components will be contained within the capacitor C. a C b C c A large voltage is formed across the damping resistor Rd, which is then effectively absorbed and suppressed. After processing by the LCL filter circuit, the high-frequency harmonic components of the signal are significantly suppressed, resulting in a relatively pure fundamental frequency signal. Finally, the filtered three-phase AC power is connected to the power grid through the output terminals to supply the load.

[0053] To further illustrate this utility model, this embodiment provides a detailed parameter design for the LCL filter circuit of a 500kW grid-connected inverter, and combines simulation analysis for comparison.

[0054] When designing an LCL filter, the total inductance must be carefully balanced. If the inductance value is too low, it will weaken the system's ability to suppress grid-connected current harmonics, preventing it from meeting the power quality standards for grid-connected current. Furthermore, the larger ripple current will increase inductor losses and temperature on the inverter side, thus affecting the lifespan of components. Conversely, if the inductance value is too high, it will not only increase the system's size, weight, and cost, but also lead to a larger inductor voltage drop, causing an increase in bus voltage and resulting in additional energy loss.

[0055] Inverter-side inductor L a1 The calculation formula is as follows:

[0056]

[0057] In the formula: V DC DC voltage 1100V; f sw The switching frequency is 3kHz; K is the ripple factor of 5% to 25%, taken as 20%, I MAX The peak phase current is 993A. L is calculated. a1 =0.231mH.

[0058] The formula for calculating the grid-connected side inductance La2 is as follows:

[0059] L a2 =rL a1 .

[0060] In the formula, a value of r ranging from 0.4 to 1 is more suitable. In this example, r is taken as 0.4, which yields L. a2 =0.09mH.

[0061] In grid-connected inverters, the larger the filter capacitor of an LCL filter, the more reactive power is generated, which also reduces the inverter's power conversion efficiency. Therefore, the reactive power absorbed by the filter capacitor is generally limited to no more than 5% of the system's rated power. In this calculation, 2% of the inverter's rated capacity is selected as the reactive power, so...

[0062] In the formula, P is the rated active power output of the inverter; US is the effective value of the grid phase voltage; f is the fundamental frequency of the grid; C can be calculated. a =104μF.

[0063] When the LCL filter circuit resonates, the grid-connected harmonic current will increase significantly. According to the resonance formula, the frequency at which the grid-connected current resonates is:

[0064]

[0065] To improve the stability of high-power grid-connected inverters, a passive resistor method, which has a relatively simple control approach, is used for grid connection. This involves connecting a resistor R in series with the capacitor side of the LCL filter. d This reduces the harmonic current at the resonant point. Furthermore, in the design of LCL filter parameters, the passive damping resistor Rd is generally 1 / 3 of the capacitive reactance at the resonant angular frequency of the LCL filter, i.e.:

[0066]

[0067] At this point, the power loss of the passive resistor is not significant, and R can be calculated. d =0.82Ω.

[0068] Furthermore, Total Harmonic Distortion (THD), as a key indicator of power quality, describes the amount of harmonic components in alternating current. THD is usually expressed as a percentage, and the calculation formula is as follows:

[0069]

[0070] Among them, V n Vn is the effective value (RMS) of the nth harmonic voltage, and V1 is the effective value (RMS) of the fundamental voltage. For current calculations, the formula simply requires replacing the voltage values ​​with current values.

[0071] Based on the above calculation parameters, the LCL filter circuit of a grid-connected inverter of this invention is simulated using MATLAB / SIMLINK, and the grid-connected current harmonics are analyzed using FFT (Fast Fourier Transform).

[0072] Figure 2This document presents a simulation of the LCL filter circuit of a grid-connected inverter according to an embodiment of this invention, along with a comparison chart of the grid-connected current FFT analysis. Specifically, Figure 2 (a) in the figure shows the distribution of harmonic content in the grid-connected current of a grid-connected inverter using an LC filter circuit, while Figure 2 (b) shows the distribution of grid-connected current harmonics in the grid-connected inverter using the LCL filter circuit designed with the parameters of this utility model.

[0073] observe Figure 2 As shown in (a), the total harmonic distortion (THD) of the LC filter circuit is 0.53%, indicating a relatively high harmonic content in the current waveform. However, this is still within an acceptable range for many applications. The amplitude of the fundamental frequency (50Hz) component is 216.1, representing the main frequency component and the primary component of the current. The figure shows that the LC filter has a certain suppression effect on low-order harmonics. Besides the fundamental frequency component, several other harmonic components exist, especially noticeable at lower harmonics (such as 100Hz and 150Hz).

[0074] As for Figure 2 In (b), the total harmonic distortion (THD) of the LCL filter circuit is 0.38%, indicating a lower harmonic content in the current waveform and thus better power quality. The amplitude of the fundamental frequency (50Hz) component is the same as that of the LC filter, both being 216.1, indicating that the two filters have similar effects on the fundamental frequency component. Compared with the LC filter, the LCL filter exhibits better performance in suppressing harmonics, especially in the high-order harmonic region, where harmonic components are significantly reduced. By introducing an additional series inductor and a parallel capacitor with a damping resistor, the LCL filter forms a more complex filter network, thus providing superior harmonic suppression over a wider frequency range.

[0075] In summary, this application has the following technical effects:

[0076] The LCL filter circuit for a grid-connected inverter provided in this application aims to achieve high-efficiency, low-harmonic power conversion. It is suitable for grid-connected systems of renewable energy such as solar photovoltaic power generation and wind power generation. It can significantly improve the performance and power quality of the grid-connected inverter, reduce harmonics and electromagnetic interference, improve the stability and reliability of the system, meet the requirements of power grid specifications, and is suitable for various occasions that require high power quality.

[0077] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0078] This document uses specific examples to illustrate the principles and implementation methods of this application. The descriptions of the above embodiments are only for the purpose of helping to understand the methods and core ideas of this application. Furthermore, those skilled in the art will recognize that, based on the ideas of this application, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of this application.

Claims

1. An LCL filter circuit for a grid-connected inverter, characterized in that, include: DC input power supply, DC bus, grid-connected inverter, LCL filter circuit and output section; The DC input power supply is used to provide DC energy to the grid-connected inverter; The DC bus includes several electrolytic capacitors; The grid-connected inverter consists of several three-phase bridge arms; the grid-connected inverter is connected in parallel with the DC input power supply; The LCL filter circuit includes a three-phase output inductor L a1 Inductor L b1 and inductor L c1 and respectively with inductor L a1 Inductor L b1 and inductor L c1 Series inductor L a2 Inductor L b2 and inductor L c2 and respectively with inductor L a2 Inductor L b2 and inductor L c2 parallel capacitor C a Capacitor C b and capacitor C c ; The capacitor C a Capacitor C b and capacitor C c Each is connected to a damping resistor R d Series; The output section is a three-phase AC power output terminal that is connected to the power grid.

2. The LCL filter circuit for a grid-connected inverter according to claim 1, characterized in that, The grid-connected inverter includes three-phase bridge arm A, three-phase bridge arm B, and three-phase bridge arm C; the three-phase bridge arm A, three-phase bridge arm B, and three-phase bridge arm C are connected in parallel.

3. The LCL filter circuit for a grid-connected inverter according to claim 2, characterized in that, Each three-phase bridge arm consists of a switching transistor and a clamping diode connected in anti-parallel.

4. The LCL filter circuit for a grid-connected inverter according to claim 3, characterized in that, The three-phase bridge arm A is composed of a first switching transistor, a second switching transistor, a third switching transistor, a fourth switching transistor, a first clamping diode, and a second clamping diode connected in anti-parallel.

5. The LCL filter circuit for a grid-connected inverter according to claim 4, characterized in that, The first and fourth switching transistors are the outer transistors of the three-phase bridge arm A, and the second and third switching transistors are the inner transistors of the three-phase bridge arm A. The anode of the first clamping diode is connected to the midpoint of the first and second switching transistors, and the cathode is connected to the midpoint of the DC bus. The anode of the second clamping diode is connected to the midpoint of the DC bus, and the cathode is connected to the midpoint of the third and fourth switching transistors.

6. The LCL filter circuit for a grid-connected inverter according to claim 2, characterized in that, The three-phase bridge arm B is composed of the fifth, sixth, seventh, and eighth switching transistors of the anti-parallel diodes, the third clamping diode, and the fourth clamping diode.

7. The LCL filter circuit for a grid-connected inverter according to claim 6, characterized in that, The fifth and eighth switching transistors are the outer transistors of the three-phase bridge arm B, and the sixth and seventh switching transistors are the inner transistors of the three-phase bridge arm B. The anode of the third clamping diode is connected to the midpoint of the fifth and sixth switching transistors, and the cathode is connected to the midpoint of the DC bus. The anode of the fourth clamping diode is connected to the midpoint of the DC bus, and the cathode is connected to the midpoint of the seventh and eighth switching transistors.

8. The LCL filter circuit for a grid-connected inverter according to claim 2, characterized in that, The three-phase bridge arm C is composed of the ninth, tenth, eleventh, and twelfth switching transistors of the anti-parallel diodes, the fifth clamping diode, and the sixth clamping diode.

9. The LCL filter circuit for a grid-connected inverter according to claim 8, characterized in that, The ninth and twelfth switching transistors are the outer transistors of the three-phase bridge arm C, and the tenth and eleventh switching transistors are the inner transistors of the three-phase bridge arm C. The anode of the fifth clamping diode is connected to the midpoint of the ninth and tenth switching transistors, and the cathode is connected to the midpoint of the DC bus. The anode of the sixth clamping diode is connected to the midpoint of the DC bus, and the cathode is connected to the midpoint of the eleventh and twelfth switching transistors.

10. The LCL filter circuit for a grid-connected inverter according to claim 2, characterized in that, The inductor L a1 Inductor L b1 and inductor L c1 One end is connected to the output terminals of three-phase bridge arms A, B, and C of the grid-connected inverter, respectively, and the other end is connected to inductor L. a2 Inductor L b2 and inductor L c2 One end of the inductor L is connected; a2 Inductor L b2 and inductor L c2 The other end is connected to the three-phase AC output terminal of the output section as the output terminal of the LCL filter circuit.

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