A non-isolated two-stage inverter system and a leakage current suppression method thereof

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, a half-bridge 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 half-bridge inverter, the half-bridge 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 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 half-bridge 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 single-phase non-isolated photovoltaic grid-connected system, and in particular to a non-isolated two-stage inverter system and a leakage current suppression method thereof. Background Art

[0002] Transformerless photovoltaic grid-connected inverters have been widely used in the photovoltaic market due to their high efficiency, low cost, small size, and light weight. However, since the photovoltaic panels in this system are electrically connected to the power grid, the parasitic capacitance of the photovoltaic panels to the ground, the inverter, and the power grid can form a conduction loop. Under the unipolar modulation of the traditional full-bridge inverter system, the leakage current induced by the high-frequency common-mode voltage can pass through this loop. The leakage current is undoubtedly detrimental to the system performance. It may potentially lead to a series of problems, such as harmonic currents, increased power loss, safety issues, and electromagnetic interference problems. Therefore, the leakage current must be suppressed to a certain extent to meet the standards (such as the German VDE-0126-1-1 standard stipulates that when the leakage current is higher than 300mA, the photovoltaic grid-connected system must be cut off from the power grid within 0.3 seconds) to improve the reliability of the photovoltaic system.

[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%; the H5 topology 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 achieves the purpose of suppressing leakage current by adding only one switching device, but S5 is always in a high-frequency modulation state during the entire grid cycle, and must withstand high conduction losses and switching losses, which increases the difficulty of heat sink design; since the common-mode voltage in the half-bridge inverter system remains unchanged, leakage current can also be suppressed. In order to further improve system efficiency and improve the quality of grid-connected power, Figure 1 A two-stage three-level half-bridge inverter system is proposed. However, the boost ratio of the front stage is limited and the input capacitor of the inverter needs additional voltage balancing control.

[0004] In summary, the current methods for suppressing leakage current mainly focus on improving topology or control strategy, but this will also bring new disadvantages, such as increased losses and higher control costs, and a new leakage current suppression solution needs to be proposed. Summary of the invention

[0005] The present invention provides a non-isolated two-stage inverter system and a leakage current suppression method thereof. A front-stage DC-DC converter realizes an adjustable step-up ratio and also realizes the characteristic of automatic output capacitor voltage balancing. A half-bridge inverter is adopted in the rear stage, and an LCL filter is adopted in the system, which has good high-frequency attenuation characteristics, and the volume, weight, cost and loss of the system are greatly reduced; and the generation of leakage current is greatly suppressed.

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

[0007] A non-isolated two-stage inverter system, the system comprising:

[0008] Photovoltaic power module, DC-DC converter, half-bridge inverter, LCL filter, AC power grid;

[0009] The photovoltaic power module is connected to a DC-DC converter, the DC-DC converter is connected to a half-bridge inverter, the half-bridge 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, 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 C1. The other end of the capacitor C1 is respectively connected to the anode of the diode D1, one end of the inductor L2, and one end of the capacitor C3. The other end of the inductor L2 is respectively connected to the cathode of the diode D3 and one end of the capacitor C5. The other end of the capacitor C3 is respectively connected to the anode of the diode D3 and one end of the inductor L4. The other end of the inductor L4 is connected to one end of the capacitor C7. The other end of the capacitor C7, the other end of the capacitor C5, the cathode of the diode D1, and the source of the switch tube S1 are all connected to the negative electrode of the photovoltaic power module.

[0012] One end of capacitor C2 is connected to one end of capacitor C1, and the other end of capacitor C2 is respectively connected to one end of capacitor C4, one end of inductor L3, and the anode of diode D2. The other end of capacitor C4 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 cathode of diode D4 is respectively connected to the other end of inductor L3 and one end of capacitor C6. The cathode of diode D2 is respectively connected to the other end of capacitor C6 and the other end of capacitor C8.

[0013] The half-bridge inverter includes switches S2 and S3. The source of the switch S2 is connected to one end of the capacitor C7. The drain of the switch S2 is connected to the source of the switch S3. The drain of the switch S3 is connected to the other end of the capacitor C8.

[0014] The LCL filter includes inductors L6, L7, and capacitor C9; one end of the inductor L6 is connected to the drain of the switch tube S2, 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, and 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.

[0015] The capacitor C7 is connected in series with the capacitor C8, and the midpoint a of the two capacitors C7 and C8 is connected to the negative electrode of the photovoltaic power module and the grid.

[0016] The two switch tubes S2 and S3 are connected in series to form a bridge arm on one side. The output end of the inverter is connected between the switch tubes S2 and S3, and then connected to the inductor L6 of the LCL filter. The negative pole of the photovoltaic power module and the grid grounding point are at the same potential, both of which are 0V.

[0017] The present invention provides a non-isolated two-stage inverter system and a leakage current suppression method thereof, and the technical effects are as follows:

[0018] 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 expanded. 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 circuit keeps the potential of the midpoint a of the two input capacitors of the inverter stable, avoiding the distortion of the output AC quantity of the subsequent inverter. On the other hand, the negative pole of the photovoltaic power module is connected to the grounding point of the grid, so that its potential is always kept at 0V, so that the common mode loop can be short-circuited, thereby effectively suppressing the leakage current to 0A, improving the safety and reliability of the system operation.

[0019] 2) The system adopts a two-stage non-isolated structure, which solves the problems of high cost and low efficiency of traditional isolated grid-connected systems; the DC-DC converter in the front stage realizes the boost while ensuring the automatic voltage equalization of the output capacitor, so that the input capacitor voltage of the half-bridge inverter and the midpoint potential between the capacitors are stable, avoiding the distortion of the inverter output AC quantity, and also omits the control of the input capacitor midpoint potential balance by the rear-stage inverter, which greatly simplifies the control strategy of the rear-stage inverter. The inverter output is connected to an LCL filter, and the output current and the grid voltage are kept in phase through the control of the inverter.

[0020] 3) 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. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a diagram of an existing two-stage three-level inverter system.

[0022] Figure 2 This is the front stage DC-DC circuit diagram of the present invention

[0023] Figure 3 The circuit diagram of the present invention is

[0024] FIG4(1) is a waveform diagram of the voltage of the intermediate total capacitor of the present invention;

[0025] FIG4(2) is a voltage waveform diagram of two intermediate capacitors of the present invention.

[0026] Figure 5 It is the grid-connected voltage and current waveform diagram of the present invention.

[0027] Figure 6 1 is a common mode current waveform diagram of the present invention.

[0028] Figure 7 (1) is a conventional two-stage circuit diagram with a boost as the front stage;

[0029] FIG7(2) is a leakage current waveform diagram of a conventional front-end boost solution. DETAILED DESCRIPTION

[0030] like Figure 3 As shown, a non-isolated two-stage inverter system includes a photovoltaic power module, a DC-DC converter, a half-bridge inverter, an LCL filter, and an AC power grid. The photovoltaic power module is connected to the DC-DC converter, the DC-DC converter is connected to the half-bridge inverter, the half-bridge inverter 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 power module, and Zg is the ground impedance between the grid grounding point and the inverter chassis grounding point.

[0031] The DC-DC converter includes inductors L1 to L5, a power switch tube S1, diodes D1 to D4, and capacitors C1 to C8;

[0032] One end of the inductor L1 is connected to the positive electrode of the photovoltaic power module, 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 C1. The other end of the capacitor C1 is respectively connected to the anode of the diode D1, one end of the inductor L2, and one end of the capacitor C3. The other end of the inductor L2 is respectively connected to the cathode of the diode D3 and one end of the capacitor C5. The other end of the capacitor C3 is respectively connected to the anode of the diode D3 and one end of the inductor L4. The other end of the inductor L4 is connected to one end of the capacitor C7. The other end of the capacitor C7, the other end of the capacitor C5, the cathode of the diode D1, and the source of the switch tube S1 are all connected to the negative electrode of the photovoltaic power module.

[0033] One end of capacitor C2 is connected to one end of capacitor C1, and the other end of capacitor C2 is respectively connected to one end of capacitor C4, one end of inductor L3, and the anode of diode D2. The other end of capacitor C4 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 cathode of diode D4 is respectively connected to the other end of inductor L3 and one end of capacitor C6. The cathode of diode D2 is respectively connected to the other end of capacitor C6 and the other end of capacitor C8.

[0034] As Figure 2 shown, the DC-DC converter has the advantage that the module can be expanded to change the gain.

[0035] 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 power source, and the blue part is the anti-polarity expansion unit.

[0036] Among them, the connection forms between each same-polarity expansion unit are 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 cathode of diode D pi in the i-th expansion unit. The left end of capacitor C Pi1 in the (i - 1)-th expansion unit is connected to the left end of 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 basic Cuk converter capacitor C P12 . The connection forms between each anti-polarity expansion unit are as follows (1 < i ≤ n): The intersection of the upper end of capacitor C Ni2 and the right end of inductor L Ni in the i-th expansion unit is connected to the cathode of diode D N(i-1) in the (i - 1)-th expansion unit. The left end of capacitor C Ni1 is connected to the left end of capacitor C N(i-1)1 in the (i - 1)-th expansion unit. And the voltages across both ends of each anti-polarity expansion unit capacitor C Ni2 are the same as those across both ends of the basic Cuk converter capacitor C P12 . Furthermore, it can be obtained that when expanded to include m - 1 same-polarity expansion units and n anti-polarity expansion units:

[0037] The same polarity output gain is: The reverse polarity output gain is:

[0038] The half-bridge inverter includes switches S2 and S3. The source of the switch S2 is connected to one end of the capacitor C7. The drain of the switch S2 is connected to the source of the switch S3. The drain of the switch S3 is connected to the other end of the capacitor C8.

[0039] The LCL filter includes inductors L6, L7, and capacitor C9; one end of the inductor L6 is connected to the drain of the switch tube S2, 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, and 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.

[0040] The capacitor C7 is connected in series with the capacitor C8, and the midpoint a of the two capacitors C7 and C8 is connected to the negative electrode of the photovoltaic power module and the grounding point of the power grid. The two switch tubes S2 and S3 are connected in series to form a bridge arm on one side, and the output end of the inverter is connected between the switch tubes S2 and S3, and then connected to the inductor L6 of the LCL filter. The negative electrode of the photovoltaic power module and the grounding point of the power grid are at the same potential, both of which are 0V.

[0041] The common mode current i of the system of the present invention CM is 0A.

[0042] The system of the present invention adopts a two-stage structure. The first stage is a bipolar output DC-DC converter, which plays a role of boosting. The output end is connected to two output capacitors. 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:

[0043] (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, L5, and the capacitors C7 and C8 are charged, and the capacitors C1, C2, C3, C4, C5, and C6 are discharged. The voltages at the terminals of the inductors L1, L2, L3, L4, and L5 are as follows:

[0044]

[0045] (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, L5, and the capacitors C7 and C8 are discharged, and the capacitors C1, C2, C3, C4, C5, and C6 are charged. The voltages at the terminals of the inductors L1, L2, L3, L4, and L5 are as follows:

[0046]

[0047] Finally, the output capacitor voltage u is obtainedC7 、u C8 Both As shown in Figure 4(1), the input voltage of the rear half-bridge inverter is

[0048] As shown in FIG4(2), the output capacitor of the front-stage DC-DC converter realizes automatic voltage equalization.

[0049] The half-bridge inverter uses bipolar modulation, with two modes:

[0050] (1) Status “1”:

[0051] S2 is turned on, S3 is turned off, and the inverter outputs a positive level;

[0052] (2) "-1" status:

[0053] S3 is turned on, S2 is turned off, and the inverter outputs a negative level;

[0054] After the inverter output, connect the LCL filter to filter out the harmonic components, and the grid-connected voltage and current waveforms are as follows: Figure 5 As shown, the system operates at unity power factor.

[0055] exist Figure 3 Under the action of the red wire, the potential of the negative pole of the photovoltaic panel and the grounding point of the power grid are both equal to 0V, successfully short-circuiting the green common-mode branch. Figure 6 The common mode current i CM is 0A, thus achieving the purpose of suppressing common mode current.

[0056] FIG7 (1) shows a structure in which the front stage is boost and the rear stage is a half-bridge inverter. From the leakage current waveform of FIG7 (2), it can be seen that the two-stage system based on the traditional boost scheme is not as effective in suppressing the leakage current as the scheme proposed in the present invention.

[0057] The DC-DC converter has the advantage of being modularly scalable to change gain.

[0058] Each expansion unit consists of 2 capacitors, 1 inductor, and 1 diode. Figure 2 Take the red expansion unit with the same polarity as an example, the inductor L pm The left end is connected to capacitor C pm1 The right end and diode D pm The intersection of the anode and the right end is connected to the capacitor C pm2 One end of the capacitor C pm1 The left end is connected to capacitor C N11 One end of the capacitor C pm2 Connected to the negative pole of the DC source, the blue part is the reverse polarity expansion unit.

[0059] Among them, the connection forms between each same-polarity expansion unit are as follows (1 < i ≤ m - 1): The intersection of the left end of the capacitor C in the (i - 1)-th expansion unit and the right end of the inductor L is connected to the cathode of the diode D in the i-th expansion unit. The left end of the capacitor C in the (i - 1)-th expansion unit is connected to the left end of the capacitor C in the i-th expansion unit, and the voltages across both ends of the same-polarity expansion unit capacitor C are the same as those across both ends of the basic Cuk converter capacitor C. The connection forms between each opposite-polarity expansion unit are as follows (1 < i ≤ n): The intersection of the upper end of the capacitor C in the i-th expansion unit and the right end of the inductor L is connected to the cathode of the diode D in the (i - 1)-th expansion unit. The left end of the capacitor C is connected to the left end of the capacitor C in the (i - 1)-th expansion unit. And the voltages across both ends of each opposite-polarity expansion unit capacitor C are the same as those across both ends of the basic Cuk converter capacitor C. Furthermore, it can be obtained that when expanded to include m - 1 same-polarity expansion units and n opposite-polarity expansion units: Pi2 at the left end of Pi and the right end of the inductor L is connected to the cathode of the diode D in the i-th expansion unit pi at the left end of the capacitor C in the (i - 1)-th expansion unit Pi1 is connected to the left end of the capacitor C in the i-th expansion unit P(i+1)1 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 basic Cuk converter capacitor C P12 The connection forms between each opposite-polarity expansion unit are as follows (1 < i ≤ n): The upper end of the capacitor C in the i-th expansion 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 expansion 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 expansion unit N(i-1)1 And the voltages across both ends of each opposite-polarity expansion unit capacitor C Ni2 are the same as those across both ends of the basic Cuk converter capacitor C P12 Furthermore, it can be obtained that when expanded to include m - 1 same-polarity expansion units and n opposite-polarity expansion units:

[0060] The same-polarity output gain is: The opposite-polarity output gain is:

Claims

1. A non-isolated two-stage inverter system, characterized in that: The system includes: photovoltaic power module, DC-DC converter, half-bridge inverter, LCL filter, AC power grid; The photovoltaic power module is connected to a DC-DC converter, the DC-DC converter is connected to a half-bridge inverter, the half-bridge inverter is connected to an LCL filter, and the LCL filter is connected to an 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, 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 C1. The other end of the capacitor C1 is respectively connected to the anode of the diode D1, one end of the inductor L2, and one end of the capacitor C3. The other end of the inductor L2 is respectively connected to the cathode of the diode D3 and one end of the capacitor C5. The other end of the capacitor C3 is respectively connected to the anode of the diode D3 and one end of the inductor L4. The other end of the inductor L4 is connected to one end of the capacitor C7. The other end of the capacitor C7, the other end of the capacitor C5, the cathode of the diode D1, and the source of the switch tube S1 are all connected to the negative electrode of the photovoltaic power module. One end of capacitor C2 is connected to one end of capacitor C1, the other end of capacitor C2 is respectively connected to one end of capacitor C4, one end of inductor L3, and the anode of diode D2, the other end of capacitor C4 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 cathode of diode D4 is respectively connected to the other end of inductor L3 and one end of capacitor C6, the cathode of diode D2 is respectively connected to the other end of capacitor C6 and the other end of capacitor C8; The half-bridge inverter comprises switches S2 and S3, the source of the switch S2 is connected to one end of the capacitor C7, the drain of the switch S2 is connected to the source of the switch S3, and the drain of the switch S3 is connected to the other end of the capacitor C8; The capacitor C7 is connected in series with the capacitor C8, and the midpoint a of the two capacitors C7 and C8 is connected to the negative electrode of the photovoltaic power module and the grounding point of the power grid. The two switch tubes S2 and S3 are connected in series to form a bridge arm on one side. The output end of the inverter is connected between the switch tubes S2 and S3, and then connected to the inductor L6 of the LCL filter; the negative electrode of the photovoltaic power module and the grounding point of the power grid are at the same potential, both of which are 0V; 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, L5, and the capacitors C7 and C8 are charged, and the capacitors C1, C2, C3, C4, C5, and C6 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, L5, and the capacitors C7 and C8 are discharged, and the capacitors C1, C2, C3, C4, C5, and C6 are charged. The voltages at the terminals of the inductors L1, L2, L3, L4, and L5 are as follows: The non-isolated two-stage inverter system uses bipolar modulation for half-bridge inverter, with two modes: (1) "1" status: S2 is turned on, S3 is turned off, and the inverter outputs a positive level; (2) "-1" status: S3 is turned on, S2 is turned off, and the inverter outputs a negative level.

2. A non-isolated two-stage inverter system according to claim 1, characterized in that: The LCL filter includes inductors L6, L7, and capacitor C9; one end of the inductor L6 is connected to the drain of the switch tube S2, 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, and 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.

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