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

Abstract

A single-phase non-leakage current non-isolated photovoltaic grid-connected system, the system comprises: a photovoltaic power module PV, a DC-DC converter, an NPC three-level inverter, an LCL filter, and an AC power grid. The photovoltaic power module PV is connected to the DC-DC converter, the DC-DC converter is connected to the NPC three-level inverter, the NPC three-level inverter is connected to the LCL filter, and the LCL filter is connected to the AC power grid. The DC-DC converter of the front stage realizes that the boost ratio can be flexibly adjusted while ensuring the automatic voltage equalization of the output capacitor, so that the input capacitor voltage of the rear stage NPC inverter and the midpoint potential between the capacitors are stable, avoiding the distortion of the inverter output AC quantity, and at the same time omitting the control of the rear stage inverter on the midpoint potential balance of the input capacitor, greatly simplifying 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.

Description

Technical Field

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

[0002] In the prior art, in order to improve the efficiency of photovoltaic grid-connected systems and reduce the cost of grid-connected systems, non-isolated grid-connected systems have been further developed. Due to the structural characteristics of the photovoltaic array itself, a large parasitic capacitance to the ground will be formed. The parasitic capacitance and the common-mode branch in the non-isolated grid-connected inverter form a loop. When there is a changing common-mode voltage in the system, the common-mode current (leakage current) generated by the loop may exceed a certain range (the current standard is that the low leakage current peak should be less than 300mA), which will cause harm to personal safety and system operation. Therefore, how to effectively suppress the common-mode current (leakage current) of the non-isolated grid-connected system is of great significance.

[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%; Figure 1 The H5 topology shown in the figure is based on the ordinary full-bridge, and an additional switch tube S5 is connected in series at the DC input end to suppress the generation of leakage current. Compared with other improved topologies, H5 only adds one switch device to achieve the purpose of suppressing leakage current, but S5 is always in a high-frequency modulation state during the entire grid cycle, and has to withstand high conduction loss and switching loss, which increases the difficulty of heat sink design; since the common-mode voltage in the half-bridge inverter system remains unchanged, the leakage current can also be suppressed. In order to further improve system efficiency and improve the quality of grid-connected power, Figure 2 A two-stage three-level half-bridge inverter system is proposed. However, the boost ratio of the front stage is limited. Although its gain is twice that of the traditional Boost converter, it is not adjustable. In some applications where the input-output transformation ratio differs by 10 times or even higher, it is obviously still difficult to meet the application requirements, and the input capacitor of the inverter needs additional voltage balancing control.

[0004] In summary, the current methods for suppressing leakage current mainly lie in improving the topology or control strategy, but this will also bring new disadvantages, such as increased losses, limited front-stage boost ratio, increased control costs, etc., and a new leakage current suppression solution needs to be proposed. Summary of the invention

[0005] In order to solve the problems of high cost and low efficiency of traditional isolated grid-connected systems, the present invention provides a single-phase non-isolated photovoltaic grid-connected system without leakage current. The front-stage DC-DC converter performs voltage boosting, and the voltage boosting ratio can be flexibly controlled while achieving automatic output voltage balancing. The back-stage adopts a three-level inverter, which reduces the distortion of the inverter output AC quantity, improves the efficiency of the system, effectively suppresses the leakage current of the traditional non-isolated grid-connected system, and improves the safety and reliability of the system operation.

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

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

[0008] Photovoltaic power module PV, DC-DC converter, NPC three-level inverter, LCL filter, AC power grid;

[0009] The photovoltaic power module PV is connected to a DC-DC converter, the DC-DC converter is connected to an NPC three-level inverter, the NPC three-level inverter is connected to an LCL filter, and the LCL filter is connected to an AC power grid.

[0010] The DC-DC converter includes inductors L1 to L5, a power switch tube S1, diodes D1 to D4, and capacitors C1 to C8;

[0011] One end of the inductor L1 is connected to the positive electrode of the photovoltaic power module PV, and the other end of the inductor L1 is respectively connected to the drain of the power switch S1 and one end of the capacitor C2. The other end of the capacitor C2 is respectively connected to the anode of the diode D1 and one end of the inductor L2. The other end of the inductor L2 is respectively connected to the cathode of the diode D3 and one end of the capacitor C5.

[0012] One end of capacitor C1 is connected to one end of capacitor C2, the other end of capacitor C1 is respectively connected to the anode of diode D3 and one end of inductor L4, the other end of inductor L4 is connected to one end of capacitor C7, the other end of capacitor C7 is respectively connected to the other end of capacitor C5, the cathode of diode D1, the source of switch tube S1, and the negative pole of photovoltaic power module PV;

[0013] One end of capacitor C3 and one end of capacitor C4 are connected to one end of capacitor C2, the other end of capacitor C3 is respectively connected to the anode of diode D4 and one end of inductor L5, the other end of inductor L5 is respectively connected to the other end of capacitor C7 and one end of capacitor C8; the other end of capacitor C4 is respectively connected to the anode of diode D2 and one end of inductor L3, the other end of inductor L3 is respectively connected to the cathode of diode D4 and one end of capacitor C6; the other end of capacitor C8 is respectively connected to the other end of capacitor C6 and the cathode of diode D2.

[0014] The DC-DC converter has the advantage of being modularly scalable to change gain.

[0015] Each expansion unit consists of 2 capacitors, 1 inductor, and 1 diode. Taking Figure 2 the red same-polarity expansion unit as an example, the inductor L pm is connected to the left end of the capacitor C pm1 and the intersection of the right end and the anode of the diode D pm The right end is connected to one end of the capacitor C pm2 The left end of the capacitor C pm1 is connected to one end of the capacitor C N11 The left end of the capacitor C pm2 is connected to the negative pole of the DC source. The blue part is the anti-polarity expansion unit.

[0016] The connection form between each same-polarity expansion unit is as follows (1 < i ≤ m - 1): The intersection of the left end of the capacitor C Pi2 and the right end of the inductor L Pi in the (i - 1)-th expansion unit is connected to the cathode of the diode D pi in the i-th expansion unit. The left end of the capacitor C Pi1 in the (i - 1)-th expansion unit is connected to the left end of the capacitor C P(i+1)1 in the i-th expansion unit, and both ends of the same-polarity expansion unit capacitor C P(i+1)2 have the same voltage magnitude as both ends of the basic Cuk converter capacitor C P12 ;

[0017] The connection form between each anti-polarity expansion unit is as follows (1 < i ≤ n): The intersection of the upper end of the capacitor C Ni2 and the right end of the inductor L Ni in the i-th expansion unit is connected to the cathode of the diode D N(i-1) in the (i - 1)-th expansion unit. The left end of the capacitor C Ni1 is connected to the left end of the capacitor C N(i-1)1 in the (i - 1)-th expansion unit. And both ends of each anti-polarity expansion unit capacitor C Ni2 have the same voltage magnitude as both ends of the basic Cuk converter capacitor C P12 ;

[0018] After expanding to m - 1 same-polarity and n anti-polarity units, the same-polarity output gain is: The anti-polarity output gain is:

[0019] The grounding point of the system power grid is connected to the negative pole of the photovoltaic power module PV through the midpoint a of the series connection of two capacitors C7 and C8.

[0020] The NPC three-level inverter includes diodes D5, D6, and switching tubes S2 to S5;

[0021] The source of the switch tube S5 is connected to one end of the capacitor C7, the drain of the switch tube S5 is connected to the source of the switch tube S4, the drain of the switch tube S4 is connected to the source of the switch tube S3, the drain of the switch tube S3 is connected to the source of the switch tube S2, and the drain of the switch tube S2 is connected to the other end of the capacitor C8;

[0022] The anode of the diode D6 is connected to the drain of the switch tube S5, the cathode of the diode D6 is connected to the anode of the diode D5, and the cathode of the diode D5 is connected to the drain of the switch tube S3; the cathode of the diode D6 is connected to the negative pole of the photovoltaic power module PV.

[0023] The LCL filter includes inductors L6 and L7 and a capacitor C9;

[0024] One end of the inductor L6 is connected to the source of the switch tube S3, and the other end of the inductor L6 is respectively connected to one end of the capacitor C9 and one end of the inductor L7. The other end of the inductor L7 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 C9 are both connected to the negative pole of the photovoltaic power module PV.

[0025] The present invention provides a single-phase non-leakage current non-isolated photovoltaic grid-connected system, and the technical effects are as follows:

[0026] 1) The system of the present invention adopts a two-stage structure. The DC-DC converter of the front stage plays a role of boosting. Its boosting ratio can be flexibly adjusted through the expansion module. At the same time, its output capacitor (intermediate capacitor) voltage u C7 、u C8 It can realize automatic balancing and ensure the input capacitor voltage u of the subsequent inverter C7 、u C8 The self-balancing makes the potential of the midpoint a of the two input capacitors of the inverter stable, avoiding the distortion of the output AC quantity caused by the imbalance of the midpoint potential of the NPC three-level inverter, and also omitting the control of the midpoint potential balance of the subsequent inverter, which greatly simplifies the control strategy of the subsequent inverter. The LCL filter is connected to the output of the inverter, and the output current and the grid voltage are kept in phase by controlling the inverter.

[0027] 2) The NPC half-bridge structure adopted in the back stage of the present invention not only ensures the effective suppression of leakage current by bypassing the common mode branch, but also greatly reduces the harmonic content of the output AC quantity and the size of the filter inductor. The back stage half-bridge structure makes the common mode branch short-circuited, effectively suppresses the leakage current of the traditional non-isolated grid-connected system, and improves the safety and reliability of system operation.

[0028] 3) The NPC three-level inverter of the present invention can realize three-level output, which greatly reduces the harmonic content of the output AC quantity and the volume of the filter.

[0029] 4) Compared with a 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.

[0030] 5) Compared with the traditional single-phase H-bridge topology, the post-stage NPC topology reduces switching losses, reduces the amount of inductance used, and has higher efficiency.

[0031] 6) Compared with the two-level inverter, the NPC three-level inverter has a smaller output waveform harmonic content and a smaller filter size and weight. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is the H5 inverter topology diagram.

[0033] Figure 2 This is the topology diagram of the existing two-stage three-level inverter.

[0034] Figure 3 This is a front-stage DC-DC circuit diagram of the present invention.

[0035] Figure 4 It is a circuit diagram of the present invention.

[0036] FIG5(1) is a waveform diagram of the intermediate total capacitor voltage of the present invention;

[0037] FIG5(2) is a voltage waveform diagram of two intermediate capacitors of the present invention.

[0038] Figure 6 It is the grid-connected voltage and current waveform diagram of the present invention.

[0039] Figure 7 1 is a common mode current waveform diagram of the present invention.

[0040] Figure 8 (1) is a conventional two-stage circuit diagram with a boost as the front stage;

[0041] FIG8(2) is a leakage current waveform diagram of a conventional front-end boost solution. DETAILED DESCRIPTION

[0042] like Figure 4 As shown, a single-phase non-leakage current non-isolated photovoltaic grid-connected system adopts a two-stage non-isolated structure. The system includes:

[0043] Photovoltaic power module PV, DC-DC converter, NPC three-level inverter, LCL filter, AC power grid;

[0044] The photovoltaic power module PV is connected to a DC-DC converter, the DC-DC converter is connected to an NPC three-level inverter, the NPC three-level inverter is connected to an LCL filter, and the LCL filter is connected to an AC power grid.

[0045] C PV C is the parasitic capacitance of the photovoltaic panel of the photovoltaic power module, and Zg is the ground impedance between the grid grounding point and the inverter housing grounding point.

[0046] The DC-DC converter includes inductors L1 to L5, power switch tube S1, diodes D1 to D4, and capacitors C1 to C8;

[0047] One end of inductor L1 is connected to the positive pole of the photovoltaic power module PV, and the other end of inductor L1 is respectively connected to the drain of power switch S1 and one end of capacitor C2. The other end of capacitor C2 is respectively connected to the anode of diode D1 and one end of inductor L2. The other end of inductor L2 is respectively connected to the cathode of diode D3 and one end of capacitor C5;

[0048] One end of capacitor C1 is connected to one end of capacitor C2, and the other end of capacitor C1 is respectively connected to the anode of diode D3 and one end of inductor L4. The other end of inductor L4 is connected to one end of capacitor C7. The other end of capacitor C7 is respectively connected to the other end of capacitor C5, the cathode of diode D1, the source of switch tube S1, and the negative pole of the photovoltaic power module PV;

[0049] One end of capacitor C3 and one end of capacitor C4 are both connected to one end of capacitor C2. The other end of capacitor C3 is respectively connected to the anode of diode D4 and one end of inductor L5. The other end of inductor L5 is respectively connected to the other end of capacitor C7 and one end of capacitor C8. The other end of capacitor C4 is respectively connected to the anode of diode D2 and one end of inductor L3. The other end of inductor L3 is respectively connected to the cathode of diode D4 and one end of capacitor C6. The other end of capacitor C8 is respectively connected to the other end of capacitor C6 and the cathode of diode D2.

[0050] As Figure 3 shown, the DC-DC converter has the advantage that the module can be extended to change the gain.

[0051] Each expansion unit consists of 2 capacitors, 1 inductor, and 1 diode. Taking the red same-polarity expansion unit in Figure 2 as an example, the left end of inductor L pm is connected to the intersection of the right end of capacitor C pm1 and the anode of diode D pm , and the right end is connected to one end of capacitor C pm2 . The left end of capacitor C pm1 is connected to one end of capacitor C N11 . Capacitor C pm2 is connected to the negative pole of the DC source. The blue part is the anti-polarity expansion unit.

[0052] The connection form between each same-polarity expansion unit is as follows (1 < i ≤ m - 1): The intersection of the left end of capacitor C Pi2 and the right end of inductor L Pi in the (i - 1)th expansion unit is connected to the diode D piis connected to the cathode, and the capacitor C in the (i - 1)-th extended unit Pi1 The left end of is connected to the capacitor C in the i-th extended unit P(i+1)1 The left end of, and the capacitors C of the same-polarity extended units P(i+1)2 Both ends are connected to the capacitor C of the basic Cuk converter P12 The voltages across both ends are of the same magnitude;

[0053] The connection forms between the anti-polarity extended units are as follows (1 < i ≤ n): The upper end of the capacitor C in the i-th extended unit Ni2 is connected to the intersection of the right end of the inductor L Ni and the cathode of the diode D in the (i - 1)-th extended unit N(i-1) is connected. The left end of the capacitor C Ni1 is connected to the left end of the capacitor C in the (i - 1)-th extended unit N(i-1)1 The left end. And the capacitors C of the anti-polarity extended units Ni2 Both ends are connected to the capacitor C of the basic Cuk converter P12 The voltages across both ends are of the same magnitude.

[0054] Furthermore, it is obtained that after expanding to m - 1 same-polarity and n anti-polarity units, the same-polarity output gain is: The anti-polarity output gain is:

[0055] The grid grounding point is connected to the negative pole of the photovoltaic power module PV through the midpoint a of the series connection of two capacitors C7 and C8.

[0056] The NPC three-level inverter includes diodes D5, D6, and switching tubes S2 to S5;

[0057] The source of the switching tube S5 is connected to one end of the capacitor C7, the drain of the switching tube S5 is connected to the source of the switching tube S4, the drain of the switching tube S4 is connected to the source of the switching tube S3, the drain of the switching tube S3 is connected to the source of the switching tube S2, and the drain of the switching tube S2 is connected to the other end of the capacitor C8;

[0058] The anode of the diode D6 is connected to the drain of the switching tube S5, the cathode of the diode D6 is connected to the anode of the diode D5, and the cathode of the diode D5 is connected to the drain of the switching tube S3; the cathode of the diode D6 is connected to the negative pole of the photovoltaic power module PV.

[0059] The LCL filter includes inductors L6, L7, and capacitor C9; one end of the inductor L6 is connected to the source of the switching tube S3, the other end of the inductor L6 is respectively connected to one end of the capacitor C9 and one end of the inductor L7, the other end of the inductor L7 is connected to one end of the AC grid, and the other end of the AC grid and the other end of the capacitor C9 are both connected to the negative pole of the photovoltaic power module PV.

[0060] The present invention proposes a single-phase non-leakage current non-isolated photovoltaic grid-connected system, which adopts a two-stage structure. The first stage is a DC-DC converter with reverse polarity output, which plays a role of boosting, and its gain can be flexibly controlled by an expansion module. When the current of the inductor L1 is continuously turned on, the DC-DC converter can be divided into two working states according to the different states of the power switch S1:

[0061] (1): The power switch S1 is turned on, and the diodes D1, D2, D3, and D4 are all turned off. At this time, the inductors L1, L2, L3, L4, and L5, and the capacitors C1, C2, C3, and C4 are charged, and the capacitors C5, C6, C7, and C8 are discharged. The voltages at the terminals of the inductors L1, L2, L3, L4, and L5 are as follows:

[0062]

[0063] Where: u in is the DC side input capacitor voltage, u C1 、u C2 、u C3 、u C4 、u C5 、u C6 、u C7 、u C8 They are the capacitor voltages on capacitors C1, C2, C3, C4, C5, C6, C7, and C8 respectively.

[0064] (2): The power switch S1 is turned off, and the diodes D1, D2, D3, and D4 are all turned on. At this time, the inductors L1, L2, L3, L4, and L5, and the capacitors C1, C2, C3, and C4 are discharged, and the capacitors C5, C6, C7, and C8 are charged. The voltages at the terminals of the inductors L1, L2, L3, L4, and L5 are as follows:

[0065]

[0066] Finally, the output capacitor (intermediate capacitor) voltage u is obtained. C7 、u C8 Both The output capacitor voltage is self-balanced, as shown in Figure 5(1), that is, the input voltage of the subsequent NPC three-level inverter is Figure 5(2) shows the magnitude of each input capacitor voltage, which is The subsequent NPC three-level inverter has three modes:

[0067] (1) “1” status:

[0068] S2 and S3 are turned on, S4 and S5 are turned off, diodes D5 and D6 are both turned off, and the inverter outputs a positive level;

[0069] (2) “0” status:

[0070] S3 and S4 are turned on, S2 and S3 are turned off, diodes D5 and D6 are both turned on, and the inverter outputs a zero level;

[0071] (3) "-1" state:

[0072] S2 and S3 are turned off, S4 and S5 are turned on, diodes D5 and D6 are both turned off, and the inverter outputs a negative level.

[0073] After passing through the LCL filter, the following can be obtained Figure 6 The grid-connected waveform diagram shown.

[0074] In Figure 4 Under the action of the red wire, the negative electrode of the photovoltaic panel is equipotential with the grid grounding point, both equal to 0V, making the common-mode branch short-circuited. As Figure 7 shown, the common-mode current i CM is 0A, thus achieving the purpose of suppressing the common-mode current.

[0075] Figure 8(1) shows a structure with a boost pre-stage and an NPC three-level inverter post-stage. It can also be seen from the leakage current waveform in Figure 8(2) that for a 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.

[0076] The DC-DC converter of the present invention has the advantage that the module can be expanded to change the gain. Each expansion unit consists of 2 capacitors, 1 inductor, and 1 diode. Taking Figure 3 the red same-polarity expansion unit in as an example, the left end of the inductor L pm is connected to the intersection of the right end of the capacitor C pm1 and the anode of the diode D pm , and the right end is connected to one end of the capacitor C pm2 . The left end of the capacitor C pm1 is connected to one end of the capacitor C N11 . The left end of the capacitor C pm2 is connected to the negative electrode of the DC source, and the blue part is the anti-polarity expansion unit.

[0077] The connection form between each same-polarity expansion unit is as follows (1 < i ≤ m - 1): The intersection of the left end of the capacitor C Pi2 and the right end of the inductor L Pi in the (i - 1)-th expansion unit is connected to the cathode of the diode D pi in the i-th expansion unit. The left end of the capacitor C Pi1 in the (i - 1)-th expansion unit is connected to the left end of the capacitor C P(i+1)1 in the i-th expansion unit, and the voltages across both ends of the same-polarity expansion unit capacitor C P(i+1)2 are the same as those across both ends of the capacitor C P12 of the basic Cuk converter;

[0078] The connection form between each anti-polarity extension unit is as follows (1 < i ≤ n): The upper end of the capacitor C in the i-th extension unit Ni2 is connected to the intersection of the right end of the inductor L Ni and the cathode of the diode D in the (i - 1)-th extension unit N(i-1) . The left end of the capacitor C Ni1 is connected to the left end of the capacitor C in the (i - 1)-th extension unit N(i-1)1 . And the voltages across the capacitors C of each anti-polarity extension unit Ni2 are the same as the voltage across the capacitor C of the basic Cuk converter P12 .

[0079] When extended to include m - 1 same-polarity extension units and n anti-polarity extension units, we can obtain: The same-polarity output gain is: The anti-polarity output gain is:

Claims

1. A single-phase non-leakage current non-isolated photovoltaic grid-connected system, characterized in that: The system includes: Photovoltaic power module PV, DC-DC converter, NPC three-level inverter, LCL filter, AC power grid; The photovoltaic power module PV is connected to the DC-DC converter, the DC-DC converter is connected to the NPC three-level inverter, the NPC three-level inverter is connected to the LCL filter, and the LCL filter is connected to the AC power grid; The DC-DC converter includes inductors L1 to L5, a power switch tube S1, diodes D1 to D4, and capacitors C1 to C8; One end of the inductor L1 is connected to the positive electrode of the photovoltaic power module PV, and the other end of the inductor L1 is respectively connected to the drain of the power switch S1 and one end of the capacitor C2. The other end of the capacitor C2 is respectively connected to the anode of the diode D1 and one end of the inductor L2. The other end of the inductor L2 is respectively connected to the cathode of the diode D3 and one end of the capacitor C5. One end of capacitor C1 is connected to one end of capacitor C2, the other end of capacitor C1 is respectively connected to the anode of diode D3 and one end of inductor L4, the other end of inductor L4 is connected to one end of capacitor C7, the other end of capacitor C7 is respectively connected to the other end of capacitor C5, the cathode of diode D1, the source of switch tube S1, and the negative pole of photovoltaic power module PV; One end of capacitor C3 and one end of capacitor C4 are connected to one end of capacitor C2, the other end of capacitor C3 is connected to the anode of diode D4 and one end of inductor L5 respectively, the other end of inductor L5 is connected to the other end of capacitor C7 and one end of capacitor C8 respectively; the other end of capacitor C4 is connected to the anode of diode D2 and one end of inductor L3 respectively, the other end of inductor L3 is connected to the cathode of diode D4 and one end of capacitor C6 respectively; the other end of capacitor C8 is connected to the other end of capacitor C6 and the cathode of diode D2 respectively; When the current of the inductor L1 is continuously conducting, the DC-DC converter can be divided into two working states according to the different states of the power switch S1: (1): The power switch S1 is turned on, and the diodes D1, D2, D3, and D4 are all turned off. At this time, the inductors L1, L2, L3, L4, and L5, and the capacitors C1, C2, C3, and C4 are charged, and the capacitors C5, C6, C7, and C8 are discharged. The voltages at the terminals of the inductors L1, L2, L3, L4, and L5 are as follows: (2): The power switch S1 is turned off, and the diodes D1, D2, D3, and D4 are all turned on. At this time, the inductors L1, L2, L3, L4, and L5, and the capacitors C1, C2, C3, and C4 are discharged, and the capacitors C5, C6, C7, and C8 are charged. The voltages at the terminals of the inductors L1, L2, L3, L4, and L5 are as follows: The output capacitor voltage u can be obtained C7 、u C8 Both The output capacitor voltage is self-balanced; The NPC three-level inverter includes diodes D5 and D6, and switch tubes S2 to S5; The source of the switch tube S5 is connected to one end of the capacitor C7, the drain of the switch tube S5 is connected to the source of the switch tube S4, the drain of the switch tube S4 is connected to the source of the switch tube S3, the drain of the switch tube S3 is connected to the source of the switch tube S2, and the drain of the switch tube S2 is connected to the other end of the capacitor C8; The anode of diode D6 is connected to the drain of switch tube S5, the cathode of diode D6 is connected to the anode of diode D5, and the cathode of diode D5 is connected to the drain of switch tube S3; the cathode of diode D6 is connected to the negative pole of photovoltaic power module PV; The NPC three-level inverter has three modes: Mode 1, "1" state: S2 and S3 are turned on, S4 and S5 are turned off, diodes D5 and D6 are both turned off, and the inverter outputs a positive level; Mode 2, "0" state: S3 and S4 are turned on, S2 and S3 are turned off, diodes D5 and D6 are both turned on, and the inverter outputs zero level; Mode 3, "-1" state: S2 and S3 are turned off, S4 and S5 are turned on, diodes D5 and D6 are both turned off, and the inverter outputs a negative level.

2. According to claim 1, a single-phase non-leakage current non-isolated photovoltaic grid-connected system is characterized by: The grid grounding point is connected to the negative pole of the photovoltaic power module PV through the midpoint a connected in series with two capacitors C7 and C8.

3. According to claim 1, a single-phase non-leakage current non-isolated photovoltaic grid-connected system is characterized by: The LCL filter includes inductors L6 and L7 and a capacitor C9; One end of the inductor L6 is connected to the source of the switch tube S3, and the other end of the inductor L6 is respectively connected to one end of the capacitor C9 and one end of the inductor L7. The other end of the inductor L7 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 C9 are both connected to the negative pole of the photovoltaic power module PV.

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