Ground fault identification and protection method and inverter

Abstract

The invention discloses a ground fault identification and protection method and an inverter. The inverter comprises a second controllable switch and a protection circuit. The protection circuit is configured to receive a three-phase phase current detected by the phase current sensor, a leakage current detected by the leakage current sensor and a bus current detected by the bus current sensor; based on the three-phase current, the leakage current and the bus current, a trigger signal is generated when a ground fault occurs in a branch where a cable between the negative direct current end of the inverter and the photovoltaic module and / or the energy storage battery is located, and the trigger signal is used for controlling the second controllable switch to be switched off. According to the invention, the power switch tube and the first controllable switch are protected from being damaged by the grounding fault current, the service life of the inverter is prolonged, and the maintenance cost of the inverter caused by the grounding fault is reduced.

Description

Technical Field

[0001] This application relates to the field of inverter technology, and in particular to a method for identifying and protecting against grounding faults, and an inverter. Background Technology

[0002] In recent years, new energy sources have developed rapidly, and photovoltaic (PV) and energy storage inverters, as core devices for PV-energy storage power generation, occupy an important position. Existing PV and PV-energy storage inverters, because their cables are mostly laid along the frame of the PV modules or buried underground, are easily subjected to pressure or cut by sharp objects, and the cables themselves are prone to aging and damage, leading to poor insulation to ground. When the grid is a TN power supply system and the inverter is operating, if a ground fault occurs in the PV-type cable, a common-mode current of thousands of amperes will be formed between the inverter and the ground, damaging the inverter. The fault time is approximately 20ms. Existing relay circuits typically have a breaking time of about 2ms to 10ms, which is difficult to break within a short time, and the high current breaking of the relay may cause the contacts to melt and stick together, resulting in high maintenance costs. Summary of the Invention

[0003] This application provides a method for identifying and protecting against grounding faults, and an inverter, to quickly identify faults and disconnect faulty circuits.

[0004] This application provides an inverter, wherein the DC terminal of the inverter is connected to a photovoltaic module and / or an energy storage battery via a cable, and a bus current sensor is provided on the negative DC bus of the inverter; each of the three-phase AC terminals of the inverter is connected to the power grid via a phase current sensor and a first controllable switch, and a leakage current sensor is also provided between the three-phase AC terminals of the inverter and the power grid.

[0005] The inverter also includes a second controllable switch and a protection circuit. The second controllable switch is located on the negative DC bus or between at least one of the three-phase AC terminals of the inverter and the power grid.

[0006] The protection circuit is configured to receive the three-phase phase current detected by the phase current sensor, the leakage current detected by the leakage current sensor, and the bus current detected by the bus current sensor; based on the three-phase phase current, the leakage current, and the bus current, when a ground fault occurs in the branch where the cable between the negative DC terminal of the inverter and the photovoltaic module and / or the energy storage battery is located, the trigger signal is used to control the second controllable switch to open.

[0007] This application also provides a method for identifying and protecting against grounding faults. The DC terminal of the inverter is connected to a photovoltaic module and / or a storage battery via a cable, and a bus current sensor is installed on the negative DC bus of the inverter. Each of the three AC phases of the inverter is connected to the power grid via a phase current sensor and a first controllable switch, and a leakage current sensor is also installed between the three AC phases of the inverter and the power grid. The inverter further includes a second controllable switch and a protection circuit. The second controllable switch is installed on the negative DC bus or between at least one of the three AC phases of the inverter and the power grid.

[0008] The grounding fault identification and protection methods include:

[0009] Receive the three-phase phase current detected by the phase current sensor, the leakage current detected by the leakage current sensor, and the bus current detected by the bus current sensor;

[0010] Based on the three-phase current, the leakage current, and the bus current, a trigger signal is generated when a ground fault occurs in the branch where the cable between the negative DC terminal of the inverter and the photovoltaic module and / or the energy storage battery is located, so as to control the second controllable switch to open.

[0011] The ground fault identification and protection method and inverter provided in this application can quickly identify the fault and actively control the second controllable switch to quickly disconnect the fault circuit when a ground fault occurs in the branch where the cable between the negative DC terminal of the inverter and the photovoltaic module and / or energy storage battery is located. This protects the power switch tube and the first controllable switch from damage by the ground fault current, improves the service life of the inverter, and reduces the inverter maintenance cost caused by ground faults. Attached Figure Description

[0012] Figure 1 A schematic diagram of an inverter system provided in an embodiment of this application;

[0013] Figure 2 This is a schematic diagram of the inverter connection in a TN system provided in an embodiment of this application;

[0014] Figure 3 A schematic diagram of a leakage current sensor provided in an embodiment of this application;

[0015] Figure 4 A schematic diagram of a two-stage photovoltaic inverter provided in an embodiment of this application;

[0016] Figure 5 A schematic diagram of an inverter with a protection circuit and a second controllable switch provided for an embodiment of this application;

[0017] Figure 6This is a simulation diagram of an inverter non-disconnection fault circuit provided in an embodiment of this application;

[0018] Figure 7 This is a simulation diagram of the fault current after the trigger fuse blows, provided in an embodiment of this application.

[0019] Figure 8 This is a schematic diagram of a grounding fault identification and protection method provided in an embodiment of this application.

[0020] The realization of the purpose, functional features and advantages of this application will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0021] To make the technical problems, technical solutions, and beneficial effects to be solved by this application clearer and more understandable, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of this application and are not intended to limit this application.

[0022] In the description of this application, it should be understood that the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0023] Figure 1 This is a schematic diagram of an inverter system provided in an embodiment of this application.

[0024] refer to Figure 1 To understand this, an inverter system includes an inverter (shown in the dashed box in the diagram), with the AC terminal of the inverter connected to the power grid and the DC terminal of the inverter connected to a photovoltaic array and / or energy storage battery.

[0025] An inverter includes an inverter unit (shown as DC / AC in the diagram), which is used to convert DC to AC. The inverter unit can also perform commutation with the power grid and control the power factor.

[0026] In some examples, the DC bus or DC terminal of the inverter unit can be directly connected to the photovoltaic array and / or energy storage battery. For example, both the positive and negative DC buses are directly connected to the photovoltaic array and / or energy storage battery via cables; a DC bus capacitor is connected between the positive and negative DC buses of the inverter unit.

[0027] Understandably, in this application scenario, the positive DC bus and negative DC bus of the inverter unit constitute the positive DC terminal and negative DC terminal of the inverter.

[0028] If the inverter unit is only connected to the photovoltaic array, the inverter can be called a photovoltaic inverter; if the inverter unit is only connected to the energy storage battery, the inverter can be called an energy storage inverter; if the inverter unit is connected to both the photovoltaic array and the energy storage battery, the inverter can be called a photovoltaic-energy storage inverter. In practical applications, multiple inverters can be connected to multiple photovoltaic arrays or multiple energy storage batteries through their DC terminals.

[0029] In some examples, the DC bus or DC terminal of the inverter unit can be connected to the photovoltaic array and / or energy storage battery via a DC / DC converter unit, for example, through a DC / DC converter unit and cables. It is understood that this is a two-stage inverter system, or the inverter can be called a two-stage inverter. Similarly, a DC bus capacitor is connected between the positive and negative DC buses of the inverter unit. The DC / DC converter unit is used to achieve DC-to-DC conversion, such as boost or buck conversion; the DC / DC converter unit can also achieve maximum power point tracking and bus stabilization.

[0030] Taking the diagram as an example, the DC bus or DC terminal of the inverter unit can be connected to the photovoltaic array and the energy storage battery through the first DC / DC conversion unit (shown as the first DC / DC in the diagram) and the second DC / DC conversion unit (shown as the second DC / DC in the diagram). Specifically, the input terminal of the first DC / DC unit is connected to the photovoltaic array, the input terminal of the second DC / DC unit is connected to the energy storage battery, and the output terminals of the first DC / DC unit and the second DC / DC unit are connected in parallel and then connected to the inverter unit.

[0031] Understandably, in this application scenario, the positive DC and negative DC terminals of the DC / DC conversion unit, which are close to the photovoltaic array and / or energy storage battery, constitute the positive DC and negative DC terminals of the inverter.

[0032] In some examples, the AC terminal of the inverter unit, i.e. the AC terminal of the inverter, can be connected to the power grid via a first controllable switch, such as a relay.

[0033] Relays disconnect the inverter unit from the power grid, i.e., break the power grid circuit. The core functions of a relay are "control" and "logic judgment." A relay uses a small current (such as tens of milliamps) to control the switching on and off of a larger current circuit, achieving automation or remote control. The design focus of relays is sensitive, reliable operation and long lifespan, allowing for frequent operation under normal current. Relays typically lack dedicated arc-extinguishing devices. When the relay contacts separate, excessive current can generate a high-temperature arc. If not extinguished quickly, the arc will continue to burn, causing the contacts to weld together. This means the high temperature melts the contact surface and causes them to stick together under pressure, preventing the relay from breaking and resulting in complete failure. Therefore, relays cannot break large currents and must break under small or even zero current to maximize relay protection and extend its lifespan. Secondly, the relay typically requires a certain amount of time from receiving the signal to breaking the circuit, generally between 2ms and 20ms in the industry. Therefore, if the fault lasts only a short time, the relay cannot quickly break the faulty circuit.

[0034] Figure 2 This is a schematic diagram of the inverter connection in a TN system provided in an embodiment of this application.

[0035] like Figure 2 As shown, the TN system is a low-voltage power distribution grounding system, one of the three basic systems defined in the International Electrotechnical Commission (IEC) standards (the other two being the TT and IT systems). The core characteristic of the TN system is that one point of the power system (usually the neutral point of the transformer) is directly grounded, and the exposed conductive parts of electrical equipment, such as the metal casing of an inverter, are directly connected to this grounding point via a protective earth (PE) conductor or a protective neutral (PEN) conductor. T indicates that the neutral point of the power source (transformer) is directly grounded, and N indicates that the metal casing of the electrical equipment is connected to the system's grounding point via the protective neutral conductor. The neutral point of the TN system is grounded. Based on whether the protective earth conductor and the working earth conductor are separated, the TN power supply system is divided into TN-S, TN-C, and TN-CS power supply systems. The TN-S power supply system strictly separates the protective earth conductor and the working earth conductor; the TN-C power supply system uses the working earth conductor as both the protective earth conductor and the working earth conductor; and the TN-CS power supply system is between TN-S and TN-C, with the section from the power source being a TN-C system and the section at the load end being a TN-S system.

[0036] Figure 3 A schematic diagram of a leakage current sensor provided in an embodiment of this application.

[0037] like Figure 3As shown, the structure of a leakage current sensor (RCD) is a toroidal iron core with a secondary winding (detection coil) wound around it. All the energized conductors, namely the three phase wires (L1, L2, L3 or A, B, C) and the neutral wire (N, if any), must pass through the center of the iron core together. These energized conductors or wires passing through the iron core act as the primary winding of the current transformer. The core principle of leakage current detection is based on Kirchhoff's current law, which states that in any circuit, the sum of the currents flowing into a node equals the sum of the currents flowing out of the node. Whether in a three-phase balanced load or a three-phase unbalanced load, the vector sum of the currents flowing through the RCD and all the wires passing through the current transformer is zero. This means that the magnetic fields they generate cancel each other out in the iron core, the total magnetic flux is zero, there is no induced electromotive force on the secondary winding, and the output is zero. If the cable insulation is grounded, causing a leakage current to ground in one phase (e.g., phase L1), the situation changes. This leakage current does not return through the system's neutral line, but instead flows back to the power supply neutral point through paths such as the earth (protective ground wire PE). At this point, the current I_N flowing back to the neutral line will not be equal to the sum of the three-phase currents flowing out. Therefore, the vector sum of the currents in all the conductors passing through the current transformer of the RCD will no longer be zero.

[0038] Figure 4 A schematic diagram of a two-stage photovoltaic inverter provided in an embodiment of this application.

[0039] like Figure 4 As shown, in the two-stage photovoltaic inverter, the DC / DC conversion unit is a boost converter circuit, and the inverter unit is a diode-clamped three-level inverter circuit (for simplicity, only the bridge arm of phase A is shown; the bridge arms of phases B and C are similar). The power grid is a TN system, the primary side of the transformer is Y-connected, and the neutral point is grounded. The power devices used on the boost side are MOSFETs and fast recovery diodes, and the power devices used on the inverter side are IGBTs and fast recovery diodes. In the diagram, Q1, Q2, Q3, and Q4 are IGBTs in a three-level inverter circuit with diode clamping on the inverter side; D1, D2, D3, and D4 are anti-parallel diodes or body diodes of the IGBTs; Q5 is a MOSFET in the boost circuit on the boost side; D7 is a diode in the boost circuit; L1 is a boost inductor; L2 is an inverter-side filter inductor; C1 and C2 are DC bus capacitors; C3 is an inverter-side filter capacitor; C4 is a boost input capacitor; and K1 and K2 are master-slave relays for phase A bridge arm, meaning the AC terminal of phase A bridge arm is connected to the corresponding phase A of the power grid through relays K1 and K2.

[0040] Under normal circumstances, the potential of the bus center point O (the connection point between DC bus capacitors C1 and C2) to PE should be 0V. The potential of the inverter's positive DC bus BUS+ to PE is Vbus / 2, and the potential of the cable corresponding to the inverter's negative DC bus BUS- or negative DC terminal PV- to PE is -Vbus / 2.

[0041] At the moment of fault, the potential of the bus center point O relative to PE becomes Vbus / 2, the potential of the inverter's positive DC bus BUS+ relative to PE becomes Vbus, and the potential of the cable corresponding to the inverter's negative DC bus BUS- or negative DC terminal PV- relative to PE becomes 0V because it is grounded.

[0042] Because the potential of the bus capacitor to ground increases, it can be assumed that when the bridge arm of the three-level inverter circuit is working, the current in each of the three phases of the bridge arm will generate a three-phase common-mode current to ground, i.e., leakage current. This leakage current flows from the cable breakage point, for example, the cable corresponding to the negative DC terminal PV-, to the bridge arm of the three-level inverter circuit, and then through the body diode of the lower bridge arm IGBT to the power grid. The impedance in the loop is generally below 0.1Ω, resulting in a fault current that can reach thousands of amperes. Therefore, the fault can be identified by detecting the leakage current, the sum of the three-phase currents, and the current flowing from the negative DC bus BUS- to the bridge arm. After a fault occurs, the fault current will form a circulating current through the body diode of the lower bridge arm IGBT. As long as the power grid is in the negative half-cycle and there are no disconnectable devices in the loop, this circulating current will continue. Generally, relays carrying large currents require a long time to disconnect and may even stick together and fail to disconnect. In this case, the body diode of the IGBT will overheat and fail due to the continuous high current, leading to the failure of the inverter. Therefore, it is necessary to disconnect the faulty loop as quickly as possible.

[0043] Based on this, refer to Figure 5 To understand this, each phase of the inverter's three-phase AC terminals, i.e., the output of each phase arm in the inverter unit, is connected to the power grid via a phase current sensor and a first controllable switch. For example, the output of phase A arm is connected to phase A of the power grid via phase current sensor A and master-slave relays K1 and K2; the output of phase B arm is connected to phase B of the power grid via phase current sensor B and master-slave relays K3 and K4; and the output of phase C arm is connected to phase C of the power grid via phase current sensor C and master-slave relays K5 and K6. A leakage current sensor (shown as RCD in the diagram) is also installed between the three-phase AC terminals of the inverter (i.e., the output of each phase arm in the inverter unit) and the power grid. A bus current sensor is installed on the negative DC bus of the inverter, for example, bus current sensor D is installed on the negative DC bus of the inverter unit.

[0044] The inverter also includes a second controllable switch (shown as K7 in the figure). This second controllable switch is located on the negative DC bus of the inverter and / or between at least one of the three-phase AC terminals of the inverter and the power grid. For example, the second controllable switch is located on the negative DC bus of the inverter unit. In some examples, the second controllable switch can be an electronic switch or a mechanical switch. The second controllable switch includes a fast-breaking switch with active breaking function, such as a fuse with an arc-extinguishing device. Its breaking time is less than the breaking time of the first controllable switch, and further, its breaking time is less than the single-phase sinusoidal half-wave time of the power grid. For example, a fast-breaking fuse with active breaking function, which includes an arc extinguisher, can break large currents of thousands of amperes and quickly extinguish the arc, completing the circuit breakage in less than 1 ms, much faster than the breaking speed of a relay.

[0045] The inverter also includes a protection circuit configured to receive three-phase current detected by a phase current sensor, leakage current detected by a leakage current sensor, and bus current detected by a bus current sensor; based on the three-phase current, the leakage current, and the bus current, a trigger signal is generated when a ground fault occurs in the branch where the cable between the negative DC terminal of the inverter and the photovoltaic module and / or the energy storage battery is located, and the trigger signal is used to control the second controllable switch to open.

[0046] For example, the phase current sensors detect the three-phase currents Ia, Ib, and Ic, respectively; the leakage current sensor detects the leakage current Ircd; and the bus current sensor detects the bus current Ibus. Based on the three-phase currents Ia, Ib, and Ic, the leakage current Ircd, and the bus current Ibus, a trigger signal is generated when a ground fault occurs in the branch where the cable between the negative DC terminal of the inverter and the photovoltaic module and / or energy storage battery is located. This trigger signal is used to control the second controllable switch to open. Specifically, when a ground fault occurs in the branch where the cable between the negative DC terminal of the inverter unit and the photovoltaic module and / or energy storage battery is located, a trigger signal I_ocp is generated, thereby controlling the fuse to open the fault circuit (as shown by the blue arrow in the figure).

[0047] It should be noted that, in some examples, this also applies when a ground fault occurs in the branch containing the cable between the positive DC terminal of the inverter unit and the photovoltaic module and / or energy storage battery. Alternatively, it also applies when a ground fault occurs in the photovoltaic module and / or energy storage battery itself, the DC terminal of the inverter unit, or a component connected to the cable.

[0048] In some examples, the protection circuit is configured to generate a trigger signal when the sum of the three-phase currents is greater than a first preset threshold, the leakage current is greater than a second preset threshold, and the bus current is greater than a third preset threshold.

[0049] It should be noted that in the above example, the current flowing towards the power grid is the positive direction of the current, and the first preset threshold, the second preset threshold, and the third preset threshold are positive values.

[0050] Specifically, the protection circuit includes an adder U1, a first comparator U3, a second comparator U4, a third comparator U2, and a gate circuit U5;

[0051] Adder U1 is used to add the three-phase currents Ia, Ib, and Ic to obtain the sum of the three-phase currents Ipsum;

[0052] The first comparator U3 is used to compare the sum of the three-phase currents Ipsum with a first preset threshold Ipsum_set and output a first comparison signal, such as a high level, when the sum of the three-phase currents Ipsum is greater than the first preset threshold Ipsum_set.

[0053] The second comparator U4 is used to compare the leakage current Ircd with the second preset threshold Ircd_set and output a second comparison signal, such as a high level, when the leakage current Ircd is greater than the second preset threshold Ircd_set.

[0054] The third comparator U2 is used to compare the bus current Ibus with the third preset threshold Ibus_set and output a third comparison signal, such as a high level, when the bus current Ibus is greater than the third preset threshold Ibus_set.

[0055] Gate circuit U5 is used to generate a trigger signal based on the first comparison signal, the second comparison signal, and the third comparison signal. For example, if gate circuit U5 is an AND gate, the trigger signal is generated when the first comparison signal, the second comparison signal, and the third comparison signal are all at high level, i.e., at high level.

[0056] Understandably, in practical designs, adder U1, first comparator U3, second comparator U4, and third comparator U2 can all be operational amplifiers. The three-phase currents Ia, Ib, and Ic are connected to the positive input of the operational amplifier, while the sum of the three-phase currents Ipsum, leakage current Ircd, and bus current Ibus are connected to the negative input. The first preset threshold Ipsum_set, the second preset threshold Ircd_set, and the third preset threshold Ibus_set are all connected to the positive input. The three inputs of gate circuit U5 are connected to the outputs of the first comparator U3, the second comparator U4, and the third comparator U2, respectively, and the output of gate circuit U5 outputs a trigger signal.

[0057] It is also understandable that when the sum of the three-phase currents Ipsum is less than the first preset threshold Ipsum_set, or the leakage current Ircd is less than the second preset threshold Ircd_set, or the bus current Ibus is less than the third preset threshold Ibus_set, the first comparator U3, the second comparator U4, and the third comparator U2 all output a low level. At this time, the output of the gate circuit U5 is also low. That is, when no ground fault occurs, a low level is output to ensure that the fuse is in a non-blown state.

[0058] In some examples, the inverter also includes a control unit configured to acquire a trigger signal I_ocp to block the inverter's power switches and control the first controllable switch to open.

[0059] In this process, the power switching transistors of the inverter are blocked, that is, the power switching transistors are turned off or kept off by a control signal (such as PWM).

[0060] The control unit controls the first controllable switch to open, that is, when the phase current of any phase is less than a fourth preset threshold, it controls the corresponding first controllable switch to open. It should be noted that current flowing towards the power grid is considered the positive direction of current, and the fourth preset threshold is a positive value. Specifically, it can be any of the following situations:

[0061] If the phase currents of all three phases are less than the fourth preset threshold Irly_set, then the first controllable switches corresponding to the three phases will be disconnected simultaneously.

[0062] When two phase currents in the three-phase current are less than the fourth preset threshold Irly_set, the first controllable switch corresponding to the two phases is disconnected simultaneously; then when the phase current of the other phase is less than the fourth preset threshold Irly_set, the first controllable switch corresponding to it is disconnected.

[0063] In the three-phase current, whenever the phase current of any phase is less than the fourth preset threshold Irly_set, the first controllable switch corresponding to that phase is immediately disconnected.

[0064] In some examples, the control unit is configured to report information about a ground fault occurring in the branch where the cable is located, such as reporting the ground fault to a host computer or a cloud server.

[0065] refer to Figures 6-7 To understand, such as Figure 6The diagram shows a simulation of a PV-to-ground fault (i.e., the cable between the inverter's negative DC terminal and the photovoltaic module and / or energy storage battery) in a non-disconnected fault circuit of the inverter. The simulation shows that when a PV-to-ground fault occurs, as long as the fault circuit is not disconnected, a continuous sinusoidal half-wave current of up to thousands of amperes will flow through the body diode of the lower bridge arm, causing the body diode to overheat and fail.

[0066] like Figure 7 The diagram shows a simulation of the fault current after the fuse is triggered. Before the gate circuit outputs the trigger signal I_ocp, only one phase current in the circuit is rapidly increasing. After the fuse is triggered, the current in that phase rapidly decreases. The three-phase relays are controlled to trip simultaneously when all three phase currents are less than the current threshold Irly_set. This ensures that the relays trip under low or zero current conditions, with at most two bridge arms experiencing excessive current, and the duration of this sinusoidal half-wave does not exceed 10ms. The current is within the impact current capability of the body diodes connected in parallel with the IGBTs, preventing diode damage. Ultimately, this achieves the goal of protecting power devices and extending the lifespan of the relays.

[0067] refer to Figure 8 To understand, Figure 8 This application provides a method for identifying and protecting against grounding faults, which includes the following steps:

[0068] S11. Receive the three-phase current detected by the phase current sensor, the leakage current detected by the leakage current sensor, and the bus current detected by the bus current sensor.

[0069] S12. Based on the three-phase current, the leakage current, and the bus current, a trigger signal is generated when a ground fault occurs in the branch where the cable between the negative DC terminal of the inverter and the photovoltaic module and / or the energy storage battery is located, so as to control the second controllable switch to open.

[0070] It should be noted that the foregoing description applies to the ground fault identification and protection method of this embodiment. Alternatively, any content not described in detail in the ground fault identification and protection method of this embodiment can be understood by referring to the foregoing description.

[0071] The preferred embodiments of this application have been described above with reference to the accompanying drawings, but this does not limit the scope of the claims. Any modifications, equivalent substitutions, and improvements made by those skilled in the art without departing from the scope and spirit of this application shall be within the scope of the claims.

Claims

1. An inverter, characterized in that, The DC terminal of the inverter is connected to the photovoltaic module and / or energy storage battery via a cable, and a bus current sensor is installed on the negative DC bus of the inverter; each phase AC terminal of the inverter is connected to the grid via a phase current sensor and a first controllable switch, and a leakage current sensor is also installed between the three phase AC terminals of the inverter and the grid. The inverter also includes a second controllable switch and a protection circuit. The second controllable switch is located on the negative DC bus or between at least one of the three-phase AC terminals of the inverter and the power grid. The protection circuit is configured to receive the three-phase phase current detected by the phase current sensor, the leakage current detected by the leakage current sensor, and the bus current detected by the bus current sensor; based on the three-phase phase current, the leakage current, and the bus current, when a ground fault occurs in the branch where the cable between the negative DC terminal of the inverter and the photovoltaic module and / or the energy storage battery is located, the trigger signal is used to control the second controllable switch to open.

2. The inverter according to claim 1, characterized in that, The protection circuit is configured to generate the trigger signal when the sum of the three-phase currents is greater than a first preset threshold, the leakage current is greater than a second preset threshold, and the bus current is greater than a third preset threshold.

3. The inverter according to claim 2, characterized in that, The protection circuit includes an adder, a first comparator, a second comparator, a third comparator, and a gate circuit. The adder is used to add the three-phase currents together to obtain the sum of the three-phase currents; The first comparator is used to compare the sum of the three-phase currents with the first preset threshold and output a first comparison signal when the sum of the three-phase currents is greater than the first preset threshold; The second comparator is used to compare the leakage current with the second preset threshold and output a second comparison signal when the leakage current is greater than the second preset threshold; The third comparator is used to compare the bus current with the third preset threshold and output a third comparison signal when the bus current is greater than the third preset threshold; The gate circuit is used to generate the trigger signal based on the first comparison signal, the second comparison signal, and the third comparison signal.

4. The inverter according to claim 1, characterized in that, The inverter also includes a control unit configured to acquire the trigger signal to block the power switching transistors of the inverter and control the first controllable switch to open.

5. The inverter according to claim 4, characterized in that, The control unit is configured to control the first controllable switch corresponding to any one phase to disconnect when the phase current of any one phase is less than a fourth preset threshold.

6. The inverter according to claim 4, characterized in that, The control unit is configured to report information indicating a grounding fault in the branch where the cable between the negative DC terminal of the inverter and the photovoltaic module and / or energy storage battery is located.

7. The inverter according to claim 1, characterized in that, The breaking time of the second controllable switch is shorter than the breaking time of the first controllable switch.

8. The inverter according to claim 7, characterized in that, The breaking time of the second controllable switch is less than the single-phase sinusoidal half-wave time of the power grid.

9. The inverter according to claim 1, characterized in that, The second controllable switch includes a fuse with an arc-extinguishing device; and / or, the first controllable switch includes a relay.

10. A method for identifying and protecting against ground faults, wherein the method is applied to an inverter, characterized in that, The DC terminal of the inverter is connected to the photovoltaic module and / or energy storage battery via cables, and a bus current sensor is installed on the negative DC bus of the inverter; each of the three-phase AC terminals of the inverter is connected to the grid via a phase current sensor and a first controllable switch, and a leakage current sensor is also installed between the three-phase AC terminals of the inverter and the grid; the inverter also includes a second controllable switch and a protection circuit, the second controllable switch being installed on the negative DC bus or between at least one of the three-phase AC terminals of the inverter and the grid; The grounding fault identification and protection methods include: Receive the three-phase phase current detected by the phase current sensor, the leakage current detected by the leakage current sensor, and the bus current detected by the bus current sensor; Based on the three-phase current, the leakage current, and the bus current, a trigger signal is generated when a ground fault occurs in the branch where the cable between the negative DC terminal of the inverter and the photovoltaic module and / or the energy storage battery is located, so as to control the second controllable switch to open.

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