A single-phase non-isolated photovoltaic grid-connected system capable of suppressing leakage current

Abstract

A single-phase non-isolated photovoltaic grid-connected system capable of suppressing leakage current includes: a photovoltaic power module, a front-stage conversion circuit, a rear-stage inverter circuit, an LCL filter, and an AC power grid. The system adopts a two-stage non-isolated structure, which solves the problems of high cost and low efficiency of the traditional isolated grid-connected system; the DC-DC converter of the front stage realizes the flexible adjustment of the boost ratio while ensuring the automatic voltage equalization of the output capacitor, so that the input capacitor voltage of the rear-stage 3L-SNPC inverter and the midpoint potential between the capacitors are stable, avoiding the distortion of the inverter output AC quantity, and omitting the control of the rear-stage inverter on the midpoint potential balance of the input capacitor, which greatly simplifies the control strategy of the rear-stage inverter. The inverter output is connected to the LCL filter, and the output current and the grid voltage are kept in phase by controlling the inverter. On the other hand, the half-bridge structure of the rear stage makes the common-mode branch short-circuited, effectively suppressing the leakage current of the traditional non-isolated grid-connected system and improving the safety and reliability of the system operation.

Description

Technical Field

[0001] The invention relates to a single-phase non-isolated photovoltaic grid-connected system, and in particular to a single-phase non-isolated photovoltaic grid-connected system capable of suppressing leakage current. Background Art

[0002] The non-isolated photovoltaic grid-connected inverter enables an electrical connection between the photovoltaic panel and the large power grid. When the power device operates at high frequency to generate a high-frequency common-mode voltage, a high-frequency common-mode current will be formed in the loop composed of the inverter, the photovoltaic panel parasitic capacitance and the power grid. Due to the large area of ​​the photovoltaic panel, its parasitic capacitance to the ground is very large. Usually, the capacitance of the crystalline silicon cell to the ground is about 150nF / kw. In order to improve the efficiency of the inverter, its own impedance is low, and the impedance of the power grid itself is also low, which may cause the leakage current of the common-mode loop to exceed the allowable range. The existence of high-frequency leakage current will not only bring conducted interference and radiated interference, increase the harmonic content of the grid current and system losses, but also endanger the safety of related equipment and personnel. Therefore, the suppression of leakage current in non-isolated grid-connected systems has become a hot topic of research.

[0003] Research shows that in order to suppress common-mode current, it is possible to consider improving the topology of the grid-connected inverter or adopting an appropriate control strategy. For a full-bridge inverter using bipolar modulation, while suppressing leakage current, the system will generate more losses, making its highest efficiency around 95%; based on the ordinary full-bridge, a series of topologies for suppressing leakage current have been proposed, such as H5 topology, H6 topology, Heric topology, etc., but these topologies all have to bear very high conduction losses, increase the difficulty of heat sink design, and are not conducive to improving system efficiency; the half-bridge circuit naturally has the function of suppressing leakage current, but due to its low voltage utilization rate, the input voltage required on the DC side is higher, and the problem of inverter input capacitor voltage balance under two-stage inversion must also be considered; Figure 1 A non-isolated two-stage inverter system is proposed in the paper. The rear stage adopts a T-type inverter. However, the gain of the front stage DC-DC is only The gain is limited.

[0004] Therefore, the current leakage current suppression scheme for non-isolated grid-connected systems still has problems such as incomplete suppression effect, need to improve efficiency, and limited pre-stage gain. Summary of the invention

[0005] The present invention provides a single-phase non-isolated photovoltaic grid-connected system capable of suppressing leakage current. The front-stage DC-DC converter can realize flexible adjustment of the boost ratio while ensuring automatic balancing of the output capacitor voltage, thereby ensuring effective suppression of the leakage current and greatly reducing the harmonic content of the output AC quantity and the size of the filter inductor, thereby improving the system efficiency.

[0006] The technical solution adopted by the present invention is:

[0007] A single-phase non-isolated photovoltaic grid-connected system capable of suppressing leakage current, the system comprising:

[0008] Photovoltaic power module, front-stage conversion circuit, rear-stage inverter circuit, LCL filter, AC power grid;

[0009] The photovoltaic power module is connected to a front-stage conversion circuit, the front-stage conversion circuit is connected to a rear-stage inverter circuit, the rear-stage inverter circuit is connected to an LCL filter, and the LCL filter is connected to an AC power grid.

[0010] The front-stage conversion circuit includes a basic Cuk converter, a positive polarity expansion unit, and two reverse polarity expansion units;

[0011] The basic Cuk converter includes an inductor L, an P1 , capacitor C P11 , C P12 , power switch S1, diode D P1 ;

[0012] One end of the inductor L is connected to the positive electrode of the photovoltaic power module, and the other end of the inductor L is connected to the drain of the power switch S1 and the capacitor C P11 One end, capacitor C P11 The other end is connected to diode D P1 Anode, inductor L P1 One end, inductor L P1 The other end is connected to capacitor C P12 One end, capacitor C P12 The other end, diode D P1 The cathode and source of the power switch S1 are both connected to the negative pole of the photovoltaic power module;

[0013] The positive polarity expansion unit includes a capacitor C P21 , C P22 , diode D P2 , inductance L P2 ;

[0014] Capacitor C P21 One end is connected to the capacitor C in the basic Cuk converter P11 One end, capacitor C P21 The other end is connected to diode D P2 Anode, inductor L P2 One end, inductor L P2 The other end is connected to capacitor C P22 One end, capacitor C P22 The other end, diode D P2 The cathode is connected to the capacitor C in the basic Cuk converter P12 one end;

[0015] Of the two reverse polarity expansion units, the first reverse polarity expansion unit includes a capacitor C N11 , C N12 , inductance L N1 , diode D N1 ;

[0016] Capacitor C N11 One end is connected to the capacitor C in the basic Cuk converter P11 One end, capacitor C N11 The other end is connected to the inductor L N1 One end, diode D N1 Anode, inductance L N1 The other end is connected to the capacitor C in the basic Cuk converter P12 The other end, capacitor C N12 One end, capacitor C N12 The other end is connected to diode D N1 cathode;

[0017] The first reverse polarity expansion unit includes capacitor C N21 , C N22 , inductance L N2 , diode D N2 ;

[0018] Capacitor C N21 One end is connected to the capacitor C in the basic Cuk converter P11 One end, capacitor C N21 The other end is connected to the inductor L N2 One end, diode D N2 Anode, inductance L N2 The other end is connected to the capacitor C in the first reverse polarity expansion unit. N12 The other end, capacitor C N22 One end, capacitor C N22 The other end is connected to diode D N2 cathode.

[0019] The latter inverter circuit includes switch tubes S2 to S7, diodes D1 and D2; the drain of switch tube S5 is connected to capacitor C P22 At one end, the source of the switch tube S5 is connected to the drain of the switch tube S4, the source of the switch tube S4 is connected to the drain of the switch tube S3, the source of the switch tube S3 is connected to the drain of the switch tube S2, and the source of the switch tube S2 is connected to the capacitor C N22 The other end;

[0020] The source of the switch tube S5 is connected to the anode of the diode D2, the cathode of the diode D2 is connected to the anode of the diode D1, and the cathode of the diode D1 is connected to the source of the switch tube S3;

[0021] The anode of the diode D1 is connected to the source of the switch tube S6 , the drain of the switch tube S6 is connected to the drain of the switch tube S7 , and the source of the switch tube S7 is connected to the source of the switch tube S4 .

[0022] The switch tubes S2 to S5 are connected in series to form a bridge arm. The source of the switch tube S6 is connected to the midpoint a. The drain of the switch tube S6 is connected to the drain of the switch tube S7. The source of the switch tube S7 is connected to the output end of the inverter. The diodes D1 and D2 are connected in series. The anode of the diode D2 is connected to the intersection of the switch tubes S4 and S5. The cathode of the diode is connected to the intersection of the switch tubes S2 and S3. The capacitor C P12 and C P22 Can be connected in series to be equivalent to an input capacitor, capacitor C N12 and C N22 It can also be connected in series to be equivalent to an input capacitor.

[0023] Output capacitor (intermediate capacitor) voltage u CP12 、u CP22 、u CN12 、u CN22 are equal to The output capacitor voltage is self-balanced, and the two equivalent input capacitor voltages of the subsequent inverter are equal to The potential at the midpoint a of the input capacitor remains stable;

[0024] The LCL filter includes inductors L1, L2, and capacitor C1; one end of the inductor L1 is connected to the source of the switch tube S7, and the other end of the inductor L1 is respectively connected to one end of the capacitor C1 and one end of the inductor L2, and the other end of the inductor L2 is connected to one end of the AC power grid, and the other end of the AC power grid and the other end of the capacitor C1 are both connected to the negative pole of the photovoltaic power module.

[0025] The common mode current (leakage current) i of the system of the present invention CM is 0A.

[0026] The present invention provides a single-phase non-isolated photovoltaic grid-connected system capable of suppressing leakage current, and the technical effects are as follows:

[0027] 1) The back-stage inverter of the present invention adopts 3L-SNPC topology, which reduces switching loss and the amount of inductor used, and has higher efficiency compared with the traditional single-phase H-bridge topology; compared with the traditional 3L-NPC inverter, the 3L-SNPC topology has uniform loss distribution, which avoids the problem of difficulty in heat sink design caused by uneven loss distribution of the 3L-NPC inverter under high power; it can be applied to high-power occasions; compared with the traditional two-level inverter, the 3L-SNPC topology has a small harmonic content in the output waveform, and the required filter has a small volume and weight.

[0028] 2) The system of the present invention adopts a two-stage structure. The DC-DC converter of the front stage plays a role of boosting, and its boost ratio can be adjusted by the expansion unit. Its output capacitor (intermediate capacitor) voltage u CP12 、u CP22 、u CN12 、u CN22 Automatic balancing can also be achieved, ensuring the self-balancing of the equivalent input capacitor voltage of the subsequent inverter, so that the potential of the midpoint a of the two input capacitors of the inverter remains stable, avoiding the distortion of the output AC quantity due to the unbalanced midpoint potential of the 3L-SNPC three-level inverter, and also omitting the control of the midpoint potential balance of the subsequent inverter, thereby simplifying the control strategy of the subsequent inverter.

[0029] On the other hand, the 3L-SNPC half-bridge structure adopted in the later stage not only ensures the effective suppression of leakage current by short-circuiting the common-mode branch, but also greatly reduces the harmonic content of the output AC quantity and the size of the filter inductor. In addition, compared with the traditional 3L-NPC structure, the 3L-SNPC avoids the problem of uneven distribution of device losses by adopting a suitable control strategy, while also improving the switching frequency, making it more suitable for high-power occasions.

[0030] 3) 3L-SNPC three-level inverter can achieve three-level output, greatly reducing the harmonic content of the output AC and the volume of the filter. Compared with the single L filter structure, the third-order LCL filter used in the present invention has better high-frequency attenuation characteristics, and its volume, weight, cost and loss are greatly reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is the existing two-stage inverter circuit diagram

[0032] Figure 2 It is a circuit diagram of the present invention.

[0033] Figure 3 The present invention is a front-stage expandable DC-DC converter

[0034] FIG4(1) is a waveform diagram of the voltage of the intermediate total capacitor of the present invention;

[0035] FIG4(2) is a waveform diagram of the voltage of the equivalent capacitor of the back-end inverter of the present invention.

[0036] Figure 5 It is the grid-connected voltage and current waveform diagram of the present invention.

[0037] Figure 6 1 is a common mode current waveform diagram of the present invention.

[0038] Figure 7 (1) is a conventional two-stage circuit diagram with a boost as the front stage;

[0039] FIG7(2) is a leakage current waveform diagram of a conventional front-end boost solution. DETAILED DESCRIPTION

[0040] like Figure 2 As shown, a single-phase non-isolated photovoltaic grid-connected system capable of suppressing leakage current includes: a photovoltaic power module, a front-stage conversion circuit, a rear-stage inverter circuit, an LCL filter, and an AC power grid. The photovoltaic power module is connected to the front-stage conversion circuit, the front-stage conversion circuit is connected to the rear-stage inverter circuit, the rear-stage inverter circuit is connected to the LCL filter, and the LCL filter is connected to the AC power grid. PV is the parasitic capacitance of the photovoltaic panel, Z g It is the ground impedance between the grid grounding point and the inverter chassis grounding point.

[0041] The system adopts a two-stage structure. The first stage is a DC-DC converter with reverse polarity output, which plays a role of boosting. At the same time, its boost ratio can be adjusted through the expansion module, and its output voltage can also achieve self-balancing. When the current of the inductor L is continuously turned on, the duty cycle of the first-stage converter can vary between 0 and 1. According to the different states of the power switch, the circuit can be divided into two working states:

[0042] 1: Power switch S1 is turned on, diode D P1 , D P2 , D N1 , D N2 are turned off, at this time, the inductors L and L P1 , L P2 , L N1 , L N2 , capacitor C P12 , C P22 , C N12 , C N22 Charging, capacitor C P11 , C P21 , C N11 , C N21 Discharge; inductance L, L P1 , L P2 , L N1 , L N2 The terminal voltage is shown as follows:

[0043]

[0044] 2: Power switch S1 is turned off, diode D P1 , D P2 , D N1 , D N2 are both turned on, at this time, the inductors L and L P1 , L P2 , L N1 , L N2 , capacitor CP12 , C P22 , C N12 , C N22 Discharge, capacitor C P11 , C P21 , C N11 , C N21 Charge; inductors L, L P1 , L P2 , L N1 , L N2 The terminal voltage is shown as follows:

[0045]

[0046] From its working mode, it can be obtained that the DC-DC converter can achieve the voltages u CP12 , u CP22 , u CN12 , u CN22 of the four output capacitors (intermediate capacitors) are automatically balanced and all equal to As shown in Fig. 4(1) and Fig. 4(2), it can be obtained that the two equivalent input voltages of the subsequent 3L-SNPC are both equal to This makes the midpoint voltage of the input capacitor of the subsequent inverter stable, avoiding the distortion of the output AC quantity caused by the imbalance of the midpoint potential of the input capacitor of the 3L-SNPC three-level inverter. At the same time, the control of the midpoint potential balance of the subsequent inverter is also omitted, greatly simplifying the control strategy of the subsequent inverter.

[0047] In addition, as Figure 3 shown, the proposed DC-DC converter also has the advantage of flexible expansion to change the gain. Taking the positive polarity expansion unit as an example, it includes capacitors C P21 , C P22 , diode D P2 , inductor L P2 ;

[0048] The connection form between each same-polarity expansion unit is as follows:

[0049] 1 < i ≤ m - 1, the diode D pi in the (i - 1)-th same-polarity expansion unit is connected to the other end of the capacitor C P(i+1)1 in the i-th same-polarity expansion unit, and one end of the capacitor C Pi2 in the (i - 1)-th same-polarity expansion unit is connected to one end of the capacitor C P(i+1)1 in the i-th same-polarity expansion unit.

[0050] The connection form between each opposite-polarity expansion unit is as follows:

[0051] 1 < j ≤ n, the capacitor C Nj1One end of the diode D in the j-1th reverse polarity expansion unit N(j-1) The cathode and capacitor C N(j-1)1 The other end of the intersection is connected. Capacitor C N2 One end is connected to the capacitor C in the j-1th expansion unit N(j-1)2 One end is connected; then, when it is expanded to contain m-1 same-polarity expansion units and n opposite-polarity expansion units, the same-polarity output gain is: The reverse polarity output gain is: The rear inverter contains 4 capacitors C P12 , C P22 , C N12 , C N22 , 2 diodes D1 and D2, 6 switch tubes S2~S7, S2~S5 are connected in series to form a bridge arm, the source of S6 is connected to the midpoint a, the drain is connected to the drain of S7, the source of S7 is connected to the output end of the inverter, the diodes D1 and D2 are connected in series, the anode of D2 is connected to the intersection of S4 and S5, the cathode of D1 is connected to the intersection of S2 and S3, the capacitor C P12 and C P22 Can be connected in series to be equivalent to an input capacitor, capacitor C N12 and C N22 It can also be connected in series to be equivalent to an input capacitor; the inverter output is connected to the LCL filter, the inductor L2 is connected to the grid, and the other end of the grid is connected to the negative pole of the photovoltaic panel, such as Figure 5 As shown, the grid-connected current and grid voltage are in phase, and unity power factor operation can be achieved.

[0052] The 3L-SNPC three-level inverter has three modes, among which the "0" state has four implementation paths:

[0053] a) "1" state:

[0054] S2, S3, and S7 are turned on, S4, S5, and S6 are turned off, diodes D1 and D2 are both turned off, and the inverter outputs a positive level;

[0055] b) "0" state:

[0056] "01 - "path:

[0057] S4 and S7 are turned on, S2, S4, S5, and S6 are turned off, diode D2 is turned on, D1 is turned off, and the inverter outputs zero level;

[0058] "02 - "path:

[0059] S3, S5, S6 are turned on, S2, S4, S7 are turned off, diode D1 is turned on, D2 is turned off, and the inverter outputs zero level;

[0060] “03 + ” path:

[0061] S3 and S6 are turned on, S2, S4, S5, and S7 are turned off, diode D1 is turned on, D2 is turned off, and the inverter outputs zero level;

[0062] “04 + ” path:

[0063] S2, S4, and S7 are turned on, S3, S5, and S6 are turned off, diode D2 is turned on, D1 is turned off, and the inverter outputs zero level;

[0064] c) “-1” state:

[0065] S4, S5, and S6 are turned on, S2, S3, and S7 are turned off, diodes D1 and D2 are both turned off, and the inverter outputs a negative level.

[0066] Under the action of the red wire in Figure 2 the green common-mode branch is short-circuited. As shown in Figure 6 the common-mode current i CM is 0 A, thus achieving the purpose of effectively suppressing the common-mode current.

[0067] Figure 7(1) is a traditional scheme, with a boost pre-stage and a 3L-SNPC three-level inverter structure in the post-stage. From the leakage current waveform in Figure 7(2), it can be seen that for the two-stage system based on the traditional boost scheme, the effect of suppressing the leakage current is not as effective as the scheme proposed in the present invention.

[0068] As shown in Figure 3 the present invention, the pre-stage DC-DC converter can flexibly change the gain through the expansion unit. Among them, the connection forms between the same-polarity expansion units are as follows:

[0069] 1 < i ≤ m - 1. The diode D pi in the (i - 1)-th same-polarity expansion unit is connected to the other end of the capacitor C P(i+1)1 in the i-th same-polarity expansion unit. One end of the capacitor C Pi2 in the (i - 1)-th same-polarity expansion unit is connected to one end of the capacitor C P(i+1)1 in the i-th same-polarity expansion unit.

[0070] The connection forms between the opposite-polarity expansion units are as follows: 1 < j ≤ n. One end of the capacitor C Nj1 in the j-th expansion unit is connected to the intersection of the cathode of the diode D N(j-1) in the (j - 1)-th opposite-polarity expansion unit and the other end of the capacitor C N(j-1)1 . One end of the capacitor C N2 is connected to the capacitor C in the (j - 1)-th expansion unitN(j-1)2 One end is connected; then, when it is expanded to contain m-1 same-polarity expansion units and n opposite-polarity expansion units, the same-polarity output gain is: The reverse polarity output gain is:

Claims

1. A single-phase non-isolated photovoltaic grid-connected system capable of suppressing leakage current, characterized in that: The system includes a photovoltaic power module, a front-stage conversion circuit, a rear-stage inverter circuit, an LCL filter, and an AC power grid; The photovoltaic power module is connected to a front-stage conversion circuit, the front-stage conversion circuit is connected to a rear-stage inverter circuit, the rear-stage inverter circuit is connected to an LCL filter, and the LCL filter is connected to an AC power grid; The front-stage conversion circuit includes a basic Cuk converter, a positive polarity expansion unit, and two reverse polarity expansion units; The basic Cuk converter includes an inductor L, an P1 , capacitor C P11 , C P12 , power switch S1, diode D P1 ; One end of the inductor L is connected to the positive electrode of the photovoltaic power module, and the other end of the inductor L is connected to the drain of the power switch S1 and the capacitor C P11 One end, capacitor C P11 The other end is connected to diode D P1 Anode, inductor L P1 One end, inductor L P1 The other end is connected to capacitor C P12 One end, capacitor C P12 The other end, diode D P1 The cathode and source of the power switch S1 are both connected to the negative pole of the photovoltaic power module; The positive polarity expansion unit includes a capacitor C P21 , C P22、 Diode D P2， Inductance L P2 ; Capacitor C P21 One end is connected to the capacitor C in the basic Cuk converter P11 One end, capacitor C P21 The other end is connected to diode D P2 Anode, inductor L P2 One end, inductor L P2 The other end is connected to capacitor C P22 One end, capacitor C P22 The other end, diode D P2 The cathode is connected to the capacitor C in the basic Cuk converter P12 one end; In two reverse polarity expansion units, The first reverse polarity expansion unit includes capacitor C N11 , C N12 , inductance L N1 , diode D N1 ; Capacitor C N11 One end is connected to the capacitor C in the basic Cuk converter P11 One end, capacitor C N11 The other end is connected to the inductor L N1 One end, diode D N1 Anode, inductance L N1 The other end is connected to the capacitor C in the basic Cuk converter P12 The other end, capacitor C N12 One end, capacitor C N12 The other end is connected to diode D N1 cathode; The first reverse polarity expansion unit includes capacitor C N21 , C N22 , inductance L N2 , diode D N2 ; Capacitor C N21 One end is connected to the capacitor C in the basic Cuk converter P11 One end, capacitor C N21 The other end is connected to the inductor L N2 One end, diode D N2 Anode, inductance L N2 The other end is connected to the capacitor C in the first reverse polarity expansion unit. N12 The other end, capacitor C N22 One end, capacitor C N22 The other end is connected to diode D N2 cathode; When the current of the inductor L is continuously conducted, the duty cycle of the pre-stage conversion circuit varies between 0 and 1. According to the different states of the power switch S1, the circuit can be divided into two working states: 1): Power switch S1 is turned on, diode D P1 , D P2 , D N1 , D N2 are turned off, at this time, the inductors L and L P1 , L P2 , L N1 , L N2 Capacitor C P12 , C P22 , C N12 , C N22 Charging, capacitor C P11 , C P21 , C N11 , C N21 Discharge; inductance L, L P1 , L P2 , L N1 , L N2 The terminal voltage is shown as follows: 2): Power switch S1 is turned off, diode D P1 , D P2 , D N1 , D N2 are both turned on, at this time, the inductors L and L P1 , L P2 , L N1 , L N2 Capacitor C P12 , C P22 , C N12 , C N22 Discharge, capacitor C P11 , C P21 , C N11 , C N21 Charging; Inductance L, L P1 , L P2 , L N1 , L N2 The terminal voltage is shown as follows: After calculation, we can get: Automatic output voltage equalization is achieved; The latter inverter circuit includes switch tubes S2 to S7, diodes D1 and D2; The drain of switch tube S5 is connected to capacitor C P22 At one end, the source of the switch tube S5 is connected to the drain of the switch tube S4, the source of the switch tube S4 is connected to the drain of the switch tube S3, the source of the switch tube S3 is connected to the drain of the switch tube S2, and the source of the switch tube S2 is connected to the capacitor C N22 The other end; The source of the switch tube S5 is connected to the anode of the diode D2, the cathode of the diode D2 is connected to the anode of the diode D1, and the cathode of the diode D1 is connected to the source of the switch tube S3; The anode of the diode D1 is connected to the source of the switch tube S6, the drain of the switch tube S6 is connected to the drain of the switch tube S7, and the source of the switch tube S7 is connected to the source of the switch tube S4; There are three modes for three-level inverters, among which the "0" state has four implementation paths: (1) "1" status: S2, S3, S7 are turned on, S4, S5, S6 are turned off, diodes D1 and D2 are both turned off, and the inverter outputs a positive level; (2) "0" state: "01 - "path: S4 and S7 are turned on, S2, S4, S5, and S6 are turned off, diode D2 is turned on, D1 is turned off, and the inverter outputs zero level; "02 - "path: S3, S5, S6 are turned on, S2, S4, S7 are turned off, diode D1 is turned on, D2 is turned off, and the inverter outputs zero level; "03 + "path: S3 and S6 are turned on, S2, S4, S5, and S7 are turned off, diode D1 is turned on, D2 is turned off, and the inverter outputs zero level; "04 + "path: S2, S4, S7 are turned on, S3, S5, S6 are turned off, diode D2 is turned on, D1 is turned off, and the inverter outputs zero level; (3) "-1" state: S4, S5, and S6 are turned on, S2, S3, and S7 are turned off, diodes D1 and D2 are both turned off, and the inverter outputs a negative level.

2. A single-phase non-isolated photovoltaic grid-connected system capable of suppressing leakage current according to claim 1, characterized in that: The LCL filter includes inductors L1, L2, and capacitor C1; one end of the inductor L1 is connected to the source of the switch tube S7, and the other end of the inductor L1 is respectively connected to one end of the capacitor C1 and one end of the inductor L2, and the other end of the inductor L2 is connected to one end of the AC power grid, and the other end of the AC power grid and the other end of the capacitor C1 are both connected to the negative pole of the photovoltaic power module.

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