Photovoltaic inverter and its DC bus capacitor protection circuit

Abstract

The present invention provides a DC bus capacitor protection circuit in a photovoltaic inverter, wherein the DC bus capacitor protection circuit includes a positive bus capacitor overvoltage detection module, a positive bus capacitor controllable voltage balancing circuit, a negative bus capacitor overvoltage detection module, and a negative bus capacitor controllable voltage balancing circuit, wherein the input end of the positive bus capacitor overvoltage detection module is connected to the positive bus capacitor group, and the output end thereof is connected to the control end of the positive bus capacitor controllable voltage balancing circuit; the positive bus capacitor controllable voltage balancing circuit is connected between one end and the other end of the positive bus capacitor group, the input end of the negative bus capacitor overvoltage detection module is connected to the negative bus capacitor group, and the output end thereof is connected to the control end of the negative bus capacitor controllable voltage balancing circuit; the negative bus capacitor controllable voltage balancing circuit is connected between one end and the other end of the negative bus capacitor group. In this way, the DC bus capacitor can be effectively protected.

Description

Technical field

[0001] The present invention relates to the technical field of inverters, and in particular to a photovoltaic inverter and a DC bus capacitor protection circuit thereof. [Background Technology]

[0002] In a photovoltaic inverter, it is necessary to connect the inverter unit through a DC bus to achieve energy decoupling between the DC side of the photovoltaic inverter and the AC inverter side. The DC bus provides a high-amplitude pulsating current to the inverter module and generates a pulsating voltage on the bus. Electrolytic capacitors are generally used as DC bus capacitors. Due to the high operating voltage of the DC bus, it is not easy to select capacitors of this withstand voltage grade. Usually, two electrolytic capacitors with lower withstand voltage and the same withstand voltage and capacity are connected in series, and multiple groups are connected in parallel to obtain a capacitor group suitable for the bus operating voltage. Due to individual differences in electrolytic capacitors themselves and leakage current characteristics, the voltages on the two voltage-dividing capacitors will be different, one high voltage and the other low voltage. The electrolytic capacitor with a high voltage may exceed the maximum withstand voltage value of the capacitor, causing damage to the capacitor. In the prior art, it is usually adopted to directly connect resistors of the same resistance in parallel to the two series-connected capacitors to achieve the purpose of voltage balancing. However, the electrolytic capacitors used in small and medium-power photovoltaic inverters have large capacity, and a high-power resistor needs to be connected in parallel at both ends of the capacitor to achieve voltage balancing. This resistor voltage balancing discharge circuit is always working during the photovoltaic inverter's grid-connected power generation process, resulting in unnecessary energy consumption of the photovoltaic inverter itself, and the resistor heats up, causing the internal temperature of the inverter to rise.

[0003] Therefore, it is necessary to propose an improved technical solution to overcome the above problems. [Summary of the invention]

[0004] One of the objectives of the present invention is to provide a photovoltaic inverter and a DC bus capacitor protection circuit thereof, which can effectively protect the DC bus capacitor.

[0005] According to one aspect of the present invention, the present invention provides a DC bus capacitor protection circuit in a photovoltaic inverter, wherein the photovoltaic inverter further comprises a positive bus capacitor group, a negative bus capacitor group and an inverter module, wherein one end of the positive bus capacitor group is connected to the positive input terminal of the inverter module, and the other end thereof is connected to the first node, and one end of the negative bus capacitor group is connected to the first node, and the other end thereof is connected to the negative input terminal of the inverter module, characterized in that the DC bus capacitor protection circuit comprises a positive bus capacitor overvoltage detection module, a positive bus capacitor controllable voltage balancing circuit, a negative bus capacitor overvoltage detection module, a positive bus capacitor controllable voltage balancing circuit, a negative bus capacitor overvoltage detection module, a positive bus capacitor controllable voltage balancing circuit, a negative bus capacitor overvoltage detection module, a positive bus capacitor controllable voltage balancing circuit, a negative bus capacitor overvoltage detection module, a positive bus capacitor controllable voltage balancing circuit, a positive bus capacitor overvoltage detection module, a negative bus capacitor overvoltage detection module, a positive bus capacitor controllable voltage balancing circuit ... The positive bus capacitor overvoltage detection module and the negative bus capacitor controllable voltage equalizing circuit are connected. The input end of the positive bus capacitor overvoltage detection module is connected to the positive bus capacitor group, and the output end thereof is connected to the control end of the positive bus capacitor controllable voltage equalizing circuit; the positive bus capacitor controllable voltage equalizing circuit is connected between one end and the other end of the positive bus capacitor group, the input end of the negative bus capacitor overvoltage detection module is connected to the negative bus capacitor group, and the output end thereof is connected to the control end of the negative bus capacitor controllable voltage equalizing circuit; the negative bus capacitor controllable voltage equalizing circuit is connected between one end and the other end of the negative bus capacitor group.

[0006] According to another aspect of the present invention, the present invention provides a photovoltaic inverter, which includes a positive bus capacitor group, a negative bus capacitor group, an inverter module, and a DC bus capacitor protection circuit. The DC bus capacitor protection circuit includes a positive bus capacitor overvoltage detection module, a positive bus capacitor controllable voltage equalizing circuit, a negative bus capacitor overvoltage detection module, and a negative bus capacitor controllable voltage equalizing circuit, wherein the input end of the positive bus capacitor overvoltage detection module is connected to the positive bus capacitor group, and the output end thereof is connected to the control end of the positive bus capacitor controllable voltage equalizing circuit; the positive bus capacitor controllable voltage equalizing circuit is connected between one end and the other end of the positive bus capacitor group, the input end of the negative bus capacitor overvoltage detection module is connected to the negative bus capacitor group, and the output end thereof is connected to the control end of the negative bus capacitor controllable voltage equalizing circuit; the negative bus capacitor controllable voltage equalizing circuit is connected between one end and the other end of the negative bus capacitor group.

[0007] Compared with the prior art, the DC bus capacitor protection circuit in the present invention includes a bus capacitor overvoltage detection module and a voltage equalizing circuit. The bus capacitor overvoltage detection module detects the actual voltage across the positive half-cycle bus capacitor and the negative half-cycle bus capacitor respectively. When it is detected that the voltage across the bus capacitor exceeds the set threshold, the voltage equalizing circuit is controlled to discharge the bus capacitor, thereby effectively protecting the DC bus capacitor.

Brief Description of the Drawings

[0008] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. Those skilled in the art can also derive other drawings based on these drawings without inventive effort. Among them:

[0009] Figure 1 Schematic diagram of the circuit structure of a photovoltaic inverter in one embodiment of the present invention;

[0010] Figure 2 for Figure 1 The diagram shows a specific circuit diagram of a partial circuit of a photovoltaic inverter in one embodiment. [Specific implementation method]

[0011] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0012] The term "one embodiment" or "embodiment" herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in various places throughout this specification does not necessarily refer to the same embodiment, nor does it refer to separate or selective embodiments that are mutually exclusive of other embodiments. Unless otherwise specified, the terms "connected," "connected," and "connected" herein, indicating electrical connection, refer to direct or indirect electrical connection.

[0013] Please refer to Figure 1 As shown, it is a schematic diagram of the circuit structure of a photovoltaic inverter in one embodiment of the present invention. Figure 2 As shown, it is Figure 1 The diagram shows a specific circuit diagram of a partial circuit of a photovoltaic inverter in one embodiment. Figure 1 The photovoltaic inverter shown includes an MPPT (Maximum Power Point Tracking) boost module 110 , a positive bus capacitor group 120 , a negative bus capacitor group 130 , a DC bus capacitor protection circuit 140 and an inverter module 150 .

[0014] The input end of the MPPT boost module 110 is connected to the photovoltaic array 200, and the output end is connected to the input end of the inverter module 150. The MPPT boost module 110 is used to boost the DC voltage generated by the photovoltaic array (or photovoltaic power source) 200 and provide the boosted DC voltage to the inverter module 150. In other embodiments, the MPPT boost module 110 can also be replaced with other boost circuits in the prior art.

[0015] The inverter module 150 is used to convert the boosted DC voltage into an AC voltage and provide the AC voltage to the commercial power grid 300 .

[0016] One end of the positive bus capacitor bank 120 is connected to the positive input terminal BUS+ of the inverter module 150, and the other end thereof is connected to the first node BUS_N. One end of the negative bus capacitor bank 130 is connected to the first node BUS_N, and the other end thereof is connected to the negative input terminal BUS- of the inverter module 150. The positive input terminal BUS+ and the negative input terminal BUS- of the inverter module 150 are connected to the output terminal of the MPPT boost module 110.

[0017] The positive bus capacitor group 120 includes at least one first capacitor, and each first capacitor in the positive bus capacitor group 120 is connected between one end and the other end of the positive bus capacitor group 120. Figure 2 In the specific embodiment shown, the positive bus capacitor group 120 includes four first capacitors C1, C2, C3 and C4. The negative bus capacitor group 130 includes at least one second capacitor, and each second capacitor in the negative bus capacitor group 130 is connected between one end and the other end of the negative bus capacitor group 130. Figure 2 In the specific embodiment shown, the negative bus capacitor group 130 includes four second capacitors C5, C6, C7 and C8. Figure 2 In the specific embodiment shown, the first capacitors C1 , C2 , C3 , and C4 , and the second capacitors C5 , C6 , C7 , and C8 are electrolytic capacitors.

[0018] The DC bus capacitor protection circuit 140 includes a positive bus capacitor overvoltage detection module 142, a positive bus capacitor controllable voltage equalizing circuit 144, a negative bus capacitor overvoltage detection module 146, and a negative bus capacitor controllable voltage equalizing circuit 148. The input end of the positive bus capacitor overvoltage detection module 142 is connected to the positive bus capacitor bank 120, and its output end is connected to the control end of the positive bus capacitor controllable voltage equalizing circuit 144; the positive bus capacitor controllable voltage equalizing circuit 144 is connected between one end and the other end of the positive bus capacitor bank 120 (i.e., the positive bus capacitor controllable voltage equalizing circuit 144 is connected in parallel with the positive bus capacitor bank 120). The input end of the negative bus capacitor overvoltage detection module 146 is connected to the negative bus capacitor group 130, and its output end is connected to the control end of the negative bus capacitor controllable voltage equalizing circuit 148; the negative bus capacitor controllable voltage equalizing circuit 148 is connected between one end and the other end of the negative bus capacitor group 130 (that is, the negative bus capacitor controllable voltage equalizing circuit 148 is connected in parallel with the negative bus capacitor group 130).

[0019] The positive bus capacitor overvoltage detection module 142 is used to detect the voltage at both ends of the positive bus capacitor group 120 (i.e., one end and the other end of the positive bus capacitor group 120). When the voltage at both ends of the positive bus capacitor group 120 is greater than the first voltage threshold Vp, the output end of the positive bus capacitor overvoltage detection module 142 outputs a first control signal to the control end of the positive bus capacitor controllable voltage equalizing circuit 144 to control the positive bus capacitor controllable voltage equalizing circuit 144 to discharge the positive bus capacitor group 120; when the voltage at both ends of the positive bus capacitor group 120 is less than the first voltage threshold Vp, the output end of the positive bus capacitor overvoltage detection module 142 outputs a second control signal to the control end of the positive bus capacitor controllable voltage equalizing circuit 144 to control the positive bus capacitor controllable voltage equalizing circuit 144 to stop discharging the positive bus capacitor group 120.

[0020] exist Figure 2 In the illustrated embodiment, the positive bus capacitor overvoltage detection module 142 includes a differential detection circuit 1422 and a first hysteresis comparator circuit 1424. The differential detection circuit 1422 is used to detect the voltage across the positive bus capacitor group 120. The differential detection circuit 1422 includes resistors R5, R12, R13, and R14, and a first operational amplifier U2-A. The negative phase input terminal of the first operational amplifier U2-A is connected to the positive input terminal BUS+ of the inverter module 150 via resistor R13, the positive phase input terminal of the first operational amplifier U2-A is connected to the first node BUS_N via resistor R14, and the positive phase input terminal of the first operational amplifier U2-A is connected to the negative input terminal BUS- of the inverter module 150 via resistor R12. The resistor R5 is connected between the negative phase input terminal and the output terminal of the first operational amplifier U2-A. The first hysteresis comparison circuit 1424 includes resistors R4, R10, R9, R3, R11 and a first comparator U1-A, wherein the resistor R11 is connected between the output of the first operational amplifier U2-A and the negative input of the first comparator U1-A; the resistor R4 and the resistor R10 are connected in series between the first voltage source 5V and the negative input BUS- of the inverter module 150, the connection node O1 between the resistor R4 and the resistor R10 is connected to the positive input of the first comparator U1-A, the resistor R9 is connected between the positive input and the output of the first comparator U1-A, the resistor R3 is connected between the first voltage source 5V and the output of the first comparator U1-A, and the output of the first comparator U1-A is connected to the output BUS_P_OVP of the positive bus capacitor overvoltage detection module 142.

[0021] exist Figure 2In the embodiment shown, the positive bus capacitor controllable voltage balancing circuit 144 includes power discharge resistors R2, R8 and a first switch connected in series between one end and the other end of the positive bus capacitor group 120, and the control end of the first switch serves as the control end of the positive bus capacitor controllable voltage balancing circuit 144. Figure 2 In the specific embodiment shown, the first switch is a relay K1.

[0022] The negative bus capacitor overvoltage detection module 146 is used to detect the voltage across the negative bus capacitor group 130 (i.e., one end and the other end of the negative bus capacitor group 130). When the voltage across the negative bus capacitor group 130 is greater than the second voltage threshold Vn, the output end of the negative bus capacitor overvoltage detection module 146 outputs a third control signal to the control end of the negative bus capacitor controllable voltage equalizing circuit 148 to control the negative bus capacitor controllable voltage equalizing circuit 148 to discharge the negative bus capacitor group 130; when the voltage across the negative bus capacitor group 130 is less than the second voltage threshold Vn, the output end of the negative bus capacitor overvoltage detection module 146 outputs a fourth control signal to the control end of the negative bus capacitor controllable voltage equalizing circuit 148 to control the negative bus capacitor controllable voltage equalizing circuit 148 to stop discharging the negative bus capacitor group 130.

[0023] exist Figure 2 In the illustrated embodiment, the negative bus capacitor overvoltage detection module 146 includes a voltage sampling circuit 1462 and a second hysteresis comparator circuit 1464. The voltage sampling circuit 1462 is used to sample the voltage across the negative bus capacitor bank 130. The voltage sampling circuit 1462 includes resistors R23 and R30 connected in series between the first node BUS_N and the negative input terminal BUS- of the inverter module 150. The connection node O2 between the resistors R23 and R30 is the output terminal of the voltage sampling circuit 1462, and the voltage at the connection node O2 is the sampled voltage output by the voltage sampling circuit 1462. The second hysteresis comparison circuit 1464 includes resistors R29, R22, R28, R27, R21 and a second comparator U3-A. Resistors R22 and R29 are connected in series between the first voltage source 5V and the negative input terminal BUS- of the inverter module 150, respectively. Resistors R29 are connected between the output terminal of the voltage sampling circuit 1462 (i.e., the connection node O2) and the negative phase input terminal of the second comparator U3-A, resistor R27 is connected between the positive phase input terminal and the output terminal of the second comparator U3-A, resistor R21 is connected between the first voltage source 5V and the output terminal of the second comparator U3-A, and the output terminal of the second comparator U3-A is connected to the output terminal BUS_N_OVP of the negative bus capacitor overvoltage detection module 146.

[0024] exist Figure 2 In the embodiment shown, the negative bus capacitor controllable voltage balancing circuit 148 includes power discharge resistors R16 and R20 connected in series between one end and the other end of the negative bus capacitor group 130, and a second switch. The control end of the second switch serves as the control end of the negative bus capacitor controllable voltage balancing circuit 148. Figure 2 In the specific embodiment shown, the second switch is a transistor Q2.

[0025] When the output terminals PV+ and PV- of the photovoltaic array 200 have voltage, there will be voltage on the bus capacitors. The working mode of the DC bus capacitor protection circuit 140 for protecting the positive bus capacitor group 120 and the negative bus capacitor group 130 is described as follows:

[0026] For the positive bus capacitor bank 120, the differential detection circuit 1422 detects the voltage across BUS+ and BUS_N (i.e., detects the voltage across the positive bus capacitor bank 120) to output a differential detection voltage. When the voltage across the positive bus capacitor bank 120 rises during charging, the upper limit value (Vp_h) of the first threshold voltage Vp is obtained by voltage division by resistors R4 and R10 at the non-inverting input of the comparator U1-A. If the differential detection voltage output by the differential detection circuit 1422 is greater than the upper limit value (Vp_h) of the first threshold voltage Vp, the output of the comparator U1-A controls the relay K1 to close, and the power resistors R2 and R8 form a closed loop, discharging the positive bus capacitor bank 120. When the voltage across the positive bus capacitor group 120 discharges and decreases, a hysteresis resistor R9 is added to the comparator U1-A, and the first threshold voltage Vp of the comparator U1-A becomes lower, becoming the lower limit value (Vp_l) of the first threshold voltage Vp. If the differential detection voltage output by the differential detection circuit 1422 is less than the lower limit value (Vp_1) of the first threshold voltage Vp, the output of the comparator U1-A flips, the relay K1 is disconnected, and the positive bus capacitor controllable equalizing circuit 144 stops working (i.e., stops discharging the positive bus capacitor group 120).

[0027] For the negative bus capacitor bank 130, the voltage sampling circuit 1462 detects the voltage across BUS_N and BUS- (i.e., detects the voltage across the negative bus capacitor bank 130) to output a sampled voltage. When the voltage across the negative bus capacitor bank 130 increases during charging, the upper limit (Vn_h) of the second threshold voltage Vn is obtained at the non-inverting input of the comparator U3-A through voltage division by resistors R22 and R28. If the sampled voltage output by the voltage sampling circuit 1462 is greater than the upper limit (Vn_h) of the second threshold voltage Vn, the output of the comparator U3-A controls transistor Q2 to enter the saturation region, and power resistors R16 and R20 form a closed loop, discharging the negative bus capacitor bank 130. When the voltage across the negative bus capacitor group 130 discharges and decreases, a hysteresis resistor 27 is added to the comparator U3-A, and the second threshold voltage Vn of the comparator U3-A becomes lower, becoming the lower limit value (Vn_1) of the second threshold voltage Vn. If the sampling voltage output by the voltage sampling circuit 1462 is less than the lower limit value (Vn_1) of the second threshold voltage Vn, the output of the comparator U3-A flips, the transistor Q2 is cut off, and the negative bus capacitor controllable voltage equalizing circuit 148 stops working (i.e., stops discharging the negative bus capacitor group 130).

[0028] That is to say, in Figure 2In the embodiment shown, the positive bus capacitor overvoltage detection module 142 is used to detect the voltage across the positive bus capacitor group 120. When the voltage across the positive bus capacitor group 120 increases and the voltage across the positive bus capacitor group 120 is greater than the upper limit value (Vp_h) of the first threshold voltage Vp, the output end of the positive bus capacitor overvoltage detection module 142 outputs a first control signal to the control end of the positive bus capacitor controllable voltage balancing circuit 144 to control the positive bus capacitor controllable voltage balancing circuit 144 discharges the positive bus capacitor group 120; when the voltage across the positive bus capacitor group 120 decreases and the voltage across the positive bus capacitor group 120 is less than the lower limit value (Vp_l) of the first threshold voltage Vp, the output end of the positive bus capacitor overvoltage detection module 142 outputs a second control signal to the control end of the positive bus capacitor controllable voltage equalizing circuit 144 to control the positive bus capacitor controllable voltage equalizing circuit 144 to stop discharging the positive bus capacitor group 120. The negative bus capacitor overvoltage detection module 146 is used to detect the voltage across the negative bus capacitor group 130. When the voltage across the negative bus capacitor group 130 increases and the voltage across the negative bus capacitor group 130 is greater than the upper limit value (Vn_h) of the second threshold voltage Vn, the output end of the negative bus capacitor overvoltage detection module 146 outputs a third control signal to the control end of the negative bus capacitor controllable voltage equalizing circuit 148 to control the negative bus capacitor controllable voltage equalizing circuit 148 to discharge the negative bus capacitor group 130; when the voltage across the negative bus capacitor group 130 decreases and the voltage across the negative bus capacitor group 130 is less than the lower limit value (Vn_1) of the second threshold voltage Vn, the output end of the negative bus capacitor overvoltage detection module 146 outputs a fourth control signal to the control end of the negative bus capacitor controllable voltage equalizing circuit 148 to control the negative bus capacitor controllable voltage equalizing circuit 148 to stop discharging the negative bus capacitor group 130.

[0029] In summary, the present invention provides a photovoltaic inverter and a DC bus capacitor protection circuit thereof. The DC bus capacitor protection circuit 140 includes a positive bus capacitor overvoltage detection module 142, a positive bus capacitor controllable voltage balancing circuit 144, a negative bus capacitor overvoltage detection module 146, and a negative bus capacitor controllable voltage balancing circuit 148. The positive bus capacitor overvoltage detection module 142 and the negative bus capacitor overvoltage detection module 146 respectively detect the actual voltage across the positive bus capacitor group 120 and the negative bus capacitor group 130. When it is detected that the voltage across the bus capacitor groups 120 and 130 exceeds a set threshold voltage upper limit, the corresponding voltage balancing circuits 144 and 148 are controlled to discharge the bus capacitor groups 120 and 130. A certain hysteresis loop is set. When it is detected that the voltage across the discharged bus capacitor groups 120 and 130 drops to a set threshold voltage lower limit, the corresponding voltage balancing circuits 144 and 148 are controlled to stop working, thereby effectively protecting the electrolytic capacitors. Because the bus voltage of a photovoltaic inverter decreases after it is actually connected to the grid, this portion of the voltage-sharing circuit is inoperative during grid-connected operation, eliminating energy consumption and improving the inverter's power generation efficiency without causing internal heating. The DC bus capacitor protection circuit in this invention is a pure hardware circuit, devoid of complex control circuitry. Even in the event of a fault in the inverter's internal control circuitry, it can still protect the bus electrolytic capacitors.

[0030] In the present invention, words such as “connect,” “connected,” “connect,” and “connected” that represent electrical connection, unless otherwise specified, represent direct or indirect electrical connection.

[0031] It should be noted that any changes made by those skilled in the art to the specific embodiments of the present invention do not depart from the scope of the claims of the present invention. Accordingly, the scope of the claims of the present invention is not limited to the aforementioned specific embodiments.

Claims

1. A DC bus capacitor protection circuit in a photovoltaic inverter, the photovoltaic inverter further comprising a positive bus capacitor group, a negative bus capacitor group, and an inverter module, wherein one end of the positive bus capacitor group is connected to the positive input terminal of the inverter module and the other end thereof is connected to a first node, and one end of the negative bus capacitor group is connected to the first node and the other end thereof is connected to the negative input terminal of the inverter module, characterized in that: The DC bus capacitor protection circuit includes a positive bus capacitor overvoltage detection module, a positive bus capacitor controllable voltage equalizing circuit, a negative bus capacitor overvoltage detection module, and a negative bus capacitor controllable voltage equalizing circuit. The input end of the positive bus capacitor overvoltage detection module is connected to the positive bus capacitor group, and the output end is connected to the control end of the positive bus capacitor controllable voltage balancing circuit; the positive bus capacitor controllable voltage balancing circuit is connected between one end and the other end of the positive bus capacitor group. The input end of the negative bus capacitor overvoltage detection module is connected to the negative bus capacitor group, and the output end is connected to the control end of the negative bus capacitor controllable voltage balancing circuit; the negative bus capacitor controllable voltage balancing circuit is connected between one end and the other end of the negative bus capacitor group. The positive bus capacitor overvoltage detection module is used to detect the voltage across the positive bus capacitor group. When the voltage across the positive bus capacitor group is greater than a first threshold voltage, the output end of the positive bus capacitor overvoltage detection module outputs a first control signal to the control end of the positive bus capacitor controllable voltage balancing circuit to control the positive bus capacitor controllable voltage balancing circuit to discharge the positive bus capacitor group; when the voltage across the positive bus capacitor group is less than the first threshold voltage, the output end of the positive bus capacitor overvoltage detection module outputs a second control signal to the control end of the positive bus capacitor controllable voltage balancing circuit to control the positive bus capacitor controllable voltage balancing circuit to stop discharging the positive bus capacitor group. The negative bus capacitor overvoltage detection module is used to detect the voltage across the negative bus capacitor group. When the voltage across the negative bus capacitor group is greater than the second threshold voltage, the output end of the negative bus capacitor overvoltage detection module outputs a third control signal to the control end of the negative bus capacitor controllable voltage balancing circuit to control the negative bus capacitor controllable voltage balancing circuit to discharge the negative bus capacitor group; when the voltage across the negative bus capacitor group is less than the second threshold voltage, the output end of the negative bus capacitor overvoltage detection module outputs a fourth control signal to the control end of the negative bus capacitor controllable voltage balancing circuit to control the negative bus capacitor controllable voltage balancing circuit to stop discharging the negative bus capacitor group.

2. The DC bus capacitor protection circuit in the photovoltaic inverter according to claim 1, characterized in that: The positive bus capacitor overvoltage detection module is used to detect the voltage across the positive bus capacitor group. When the voltage across the positive bus capacitor group increases and the voltage across the positive bus capacitor group is greater than the upper limit of the first threshold voltage, the output end of the positive bus capacitor overvoltage detection module outputs a first control signal to the control end of the positive bus capacitor controllable voltage balancing circuit to control the positive bus capacitor controllable voltage balancing circuit to discharge the positive bus capacitor group. When the voltage across the positive bus capacitor group decreases and the voltage across the positive bus capacitor group is less than the lower limit of the first threshold voltage, the output end of the positive bus capacitor overvoltage detection module outputs a second control signal to the control end of the positive bus capacitor controllable voltage balancing circuit to control the positive bus capacitor controllable voltage balancing circuit to stop discharging the positive bus capacitor group. The negative bus capacitor overvoltage detection module is used to detect the voltage across the negative bus capacitor group. When the voltage across the negative bus capacitor group increases and the voltage across the negative bus capacitor group is greater than the upper limit value of the second threshold voltage, the output end of the negative bus capacitor overvoltage detection module outputs a third control signal to the control end of the negative bus capacitor controllable voltage balancing circuit to control the negative bus capacitor controllable voltage balancing circuit to discharge the negative bus capacitor group; when the voltage across the negative bus capacitor group decreases and the voltage across the negative bus capacitor group is less than the lower limit value of the second threshold voltage, the output end of the negative bus capacitor overvoltage detection module outputs a fourth control signal to the control end of the negative bus capacitor controllable voltage balancing circuit to control the negative bus capacitor controllable voltage balancing circuit to stop discharging the negative bus capacitor group.

3. The DC bus capacitor protection circuit in a photovoltaic inverter according to claim 1 or 2, characterized in that: The positive bus capacitor group includes at least one first capacitor, and each first capacitor in the positive bus capacitor group is connected between one end and the other end of the positive bus capacitor group; The negative bus capacitor group includes at least one second capacitor, and each second capacitor in the negative bus capacitor group is connected between one end and the other end of the negative bus capacitor group.

4. The DC bus capacitor protection circuit in the photovoltaic inverter according to claim 3, characterized in that: The positive bus capacitor controllable voltage balancing circuit includes a first power discharge resistor and a first switch connected in series between one end and the other end of the positive bus capacitor group, and the control end of the first switch serves as the control end of the positive bus capacitor controllable voltage balancing circuit; The negative bus capacitor controllable voltage balancing circuit includes a second power discharge resistor and a second switch connected in series between one end and the other end of the negative bus capacitor group, and the control end of the second switch serves as the control end of the negative bus capacitor controllable voltage balancing circuit.

5. The DC bus capacitor protection circuit in the photovoltaic inverter according to claim 4, characterized in that: The first capacitor and the second capacitor are electrolytic capacitors; The first switch is a relay; The second switch is a transistor.

6. The DC bus capacitor protection circuit in the photovoltaic inverter according to claim 2, characterized in that: The positive bus capacitor overvoltage detection module includes a differential detection circuit and a first hysteresis comparison circuit. The differential detection circuit includes resistors R5, R12, R13, R14 and a first operational amplifier. The negative phase input terminal of the first operational amplifier is connected to the positive input terminal of the inverter module via the resistor R13, the positive phase input terminal thereof is connected to the first node via the resistor R14, and the positive phase input terminal thereof is connected to the negative input terminal of the inverter module via the resistor R12; the resistor R5 is connected between the negative phase input terminal and the output terminal of the first operational amplifier. The first hysteresis comparison circuit includes resistors R4, R10, R9, R3, R11 and a first comparator, wherein the resistor R11 is connected between the output of the first operational amplifier and the negative input of the first comparator; the resistor R4 and the resistor R10 are sequentially connected in series between the first voltage source and the negative input of the inverter module, the connection node between the resistor R4 and the resistor R10 is connected to the positive input of the first comparator, the resistor R9 is connected between the positive input and the output of the first comparator, the resistor R3 is connected between the first voltage source and the output of the first comparator, and the output of the first comparator is connected to the output of the positive bus capacitor overvoltage detection module.

7. The DC bus capacitor protection circuit in a photovoltaic inverter according to claim 2, characterized in that: The negative bus capacitor overvoltage detection module includes a voltage sampling circuit and a second hysteresis comparison circuit. The voltage sampling circuit includes resistors R23 and R30 connected in series between the first node and the negative input terminal of the inverter module, and the connection node between the resistors R23 and R30 is the output terminal of the voltage sampling circuit. The second hysteresis comparison circuit includes resistors R29, R22, R28, R27, R21 and a second comparator. Resistors R22 and R29 are connected in series between the first voltage source and the negative input terminal of the inverter module, resistor R29 is connected between the output terminal of the voltage sampling circuit and the negative phase input terminal of the second comparator, resistor R27 is connected between the positive phase input terminal and the output terminal of the second comparator, resistor R21 is connected between the first voltage source and the output terminal of the second comparator, and the output terminal of the second comparator is connected to the output terminal of the negative bus capacitor overvoltage detection module.

8. A photovoltaic inverter, characterized in that: It includes a positive bus capacitor group, a negative bus capacitor group, an inverter module, and a DC bus capacitor protection circuit as described in any one of claims 1 to 7.

9. The photovoltaic inverter according to claim 8, characterized in that: It also includes a boost module, The input end of the boost module is connected to the photovoltaic array, and the output end thereof is connected to the input end of the inverter module. The boost module is used to boost the DC voltage generated by the photovoltaic array and provide the boosted DC voltage to the inverter module; The inverter module is used to convert the boosted DC voltage into an AC voltage.

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