Inverter system and ground short circuit protection method thereof

Abstract

The invention discloses an inverter system and a ground short circuit protection method thereof, and the inverter system comprises an inverter circuit which comprises a DC conversion module and an inversion module, the input end of the DC conversion module is connected with a photovoltaic string, and the output end of the DC conversion module is connected with the input end of the inversion module; the grid-connected switch comprises a semiconductor switch, and the semiconductor switch is connected between the output end of the inversion module and a power grid; and the control circuit is connected with the inverter circuit and the grid-connected switch, and is used for controlling the inverter circuit to seal waves and / or controlling the grid-connected switch to be disconnected when the photovoltaic string is short-circuited to the ground. According to the inverter system, the semiconductor switches are adopted for grid-connected connection, rapid breaking can be carried out to protect the system safety when short circuit to ground occurs in the photovoltaic string, and the system safety is higher.

Description

Technical Field

[0001] The present invention relates to the technical field of photovoltaic inverters, and in particular to an inverter system and a short-circuit protection method thereof. Background Art

[0002] With the advancement of photovoltaic power generation technology, PV string inverters are increasingly used in TN (Television-based) power grid systems. As the power output of inverters increases, the number of PV strings connected to the input is also increasing. When improper on-site implementation leads to friction between the PV input cables and the mounting bracket, this can easily cause the input cables to break. This can cause a PV+ / PV- short circuit to ground on the PV input side. Once the inverter is connected to the TN grid, a high current loop is generated within the inverter system, causing damage to the equipment and potentially irreversible consequences. While detecting which PV string is shorted to ground is simple, effectively protecting the inverter system is a challenge. Existing protection methods rely on disconnecting a grid-connected relay between the inverter output and the grid. However, when a PV string is shorted to ground, the grid-connected relay is damaged by the high current flowing through the loop, making it ineffective in protecting the inverter system. Summary of the Invention

[0003] The embodiments of the present invention provide an inverter system and a ground short-circuit protection method thereof, which solve the problem that traditional protection methods are difficult to effectively protect the inverter system when a photovoltaic string is short-circuited to the ground.

[0004] In a first aspect, an embodiment of the present invention provides an inverter system, comprising:

[0005] The inverter circuit includes a DC conversion module and an inverter module, wherein the input end of the DC conversion module is connected to the photovoltaic string, and the output end is connected to the input end of the inverter module;

[0006] A grid-connected switch, comprising a semiconductor switch connected between the output terminal of the inverter module and the grid;

[0007] A control circuit is connected to the inverter circuit and the grid-connected switch, and is used to control the inverter circuit to block the wave and / or control the grid-connected switch to disconnect when the photovoltaic string is short-circuited to the ground.

[0008] In the inverter system provided in an embodiment of the present invention, the inverter module includes a power topology circuit, a bus capacitor and an inverter inductor. The input end of the power topology circuit is connected to the output end of the DC conversion module, and the output end is connected to the semiconductor switch through the inverter inductor. The bus capacitor is connected to the input end of the power topology circuit, and the inverter circuit is connected to the power topology circuit.

[0009] In the inverter system provided in an embodiment of the present invention, the power topology circuit is a three-level I-type topology circuit, which includes a first switching tube, a second switching tube, a third switching tube, and a fourth switching tube connected in series, and a first diode and a second diode connected in series. The control circuit is connected to the gates of the first switching tube, the second switching tube, the third switching tube, and the fourth switching tube. The positive output terminal of the DC conversion module is connected to the drain of the first switching tube, the negative output terminal of the DC conversion module is connected to the source of the fourth switching tube, the anode of the first diode is connected to the drain of the fourth switching tube, the cathode of the second diode is connected to the source of the first switching tube, and the midpoint of the bus capacitor is connected between the first diode and the second diode.

[0010] In the inverter system provided in an embodiment of the present invention, the power topology circuit is a three-level T-type topology circuit, which includes a first switching tube, a second switching tube, and a third switching tube and a fourth switching tube connected in series in sequence, the drain of the third switching tube is connected to the drain of the fourth switching tube, the control circuit is connected to the gates of the first switching tube, the second switching tube, the third switching tube, and the fourth switching tube, the positive output terminal of the DC conversion module is connected to the drain of the first switching tube, the negative output terminal of the DC conversion module is connected to the source of the second switching tube, the source of the third switching tube is connected between the first switching tube and the second switching tube, and the source of the fourth switching tube is connected to the midpoint of the bus capacitor.

[0011] In the inverter system provided in an embodiment of the present invention, the DC conversion module includes a power switch tube, a freewheeling diode, a boost inductor and a chopper capacitor; wherein, one end of the boost inductor is connected to the positive electrode of the photovoltaic string, and the other end is connected to the anode of the freewheeling diode, the cathode of the freewheeling diode is connected to the positive input terminal of the inverter module, the gate of the power switch tube is connected to the control circuit, the drain is connected to the anode of the freewheeling diode, and the source is connected to the negative electrode of the photovoltaic string, and one end of the chopper capacitor is connected to the cathode of the freewheeling diode, and the other end is connected to the negative input terminal of the inverter module.

[0012] In the inverter system provided in an embodiment of the present invention, the grid-connected switch further includes a grid-connected relay, which is connected to the semiconductor switch. One of the semiconductor switch and the grid-connected relay is connected to the output end of the inverter module, and the other is connected to the power grid.

[0013] In the inverter system provided in an embodiment of the present invention, the semiconductor switch includes a plurality of switching tube devices, and the plurality of switching tube devices are connected in series in sequence. The output end of the inverter module is connected to the first switching tube device, the grid-connected relay is connected to the last switching tube device, and the control circuit is connected to all the switching tube devices.

[0014] In the inverter system provided in an embodiment of the present invention, the semiconductor switch includes a plurality of switching tube devices, the two ends of the plurality of switching tube devices are connected in parallel, the output end of the inverter module is connected to one end of all the switching tube devices connected in parallel, the grid-connected relay is connected to the other end of all the switching tube devices connected in parallel, and the control circuit is connected to all the switching tube devices.

[0015] In a second aspect, an embodiment of the present invention provides a method for protecting an inverter system from short circuit to ground, which is applied to the inverter system described in the first aspect above, and the method includes: if a photovoltaic string is detected to be short circuited to ground, obtaining the current value of the output end of the inverter module; if the current value of the output end of the inverter module is greater than a preset protection threshold, obtaining the type of short circuit to ground of the photovoltaic string and the voltage waveform of the power grid; selecting an inverter circuit sealing instruction and / or a grid-connected switch disconnecting instruction as a target control instruction according to the type of short circuit to ground of the photovoltaic string and the voltage waveform of the power grid; and controlling the inverter circuit and the grid-connected switch with the target control instruction.

[0016] An embodiment of the present invention provides an inverter system and a method for protecting the photovoltaic string from short circuit to ground. The inverter system includes: an inverter circuit, including a DC conversion module and an inverter module, wherein the input end of the DC conversion module is connected to a photovoltaic string, and the output end is connected to the input end of the inverter module; a grid-connected switch, including a semiconductor switch, wherein the semiconductor switch is connected between the output end of the inverter module and the power grid; a control circuit, connecting the inverter circuit and the grid-connected switch, and used to control the inverter circuit to block the wave and / or control the grid-connected switch to disconnect when the photovoltaic string is short-circuited to ground. The inverter system of the present application uses a semiconductor switch for grid connection, and can quickly disconnect to protect the system safety when a photovoltaic string is short-circuited to ground, thereby improving the system safety. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0018] Figure 1 A schematic structural diagram of an inverter system provided in an embodiment of the present invention;

[0019] Figure 2 A circuit diagram of an inverter system provided by an embodiment of the present invention;

[0020] Figure 3 Another circuit diagram of an inverter system provided by an embodiment of the present invention;

[0021] Figure 4 This is a fault circuit diagram of a PV string with a positive pole short-circuited to ground and the grid voltage in the positive half cycle;

[0022] Figure 5 This is a fault circuit diagram where the positive pole of a PV string is short-circuited to ground and the grid voltage is in the negative half cycle;

[0023] Figure 6 This is a fault circuit diagram when the negative pole of the PV string is short-circuited to ground and the grid voltage is in the positive half cycle;

[0024] Figure 7 This is a schematic diagram of the fault circuit when the negative pole of the PV string is short-circuited to ground and the grid voltage is in the negative half cycle;

[0025] Figure 8 This is a schematic diagram of the fault circuit when the negative pole of the PV string is short-circuited to ground and the grid voltage is in the negative half cycle;

[0026] Figure 9 A schematic flow chart of the steps of the method provided in an embodiment of the present invention;

[0027] Figure 10 A schematic flow chart of the sub-steps of the method provided in an embodiment of the present invention.

[0028] The reference numerals in the figures are:

[0029] 1. Inverter circuit; 11. DC conversion module; 12. Inverter module; 121. Power topology circuit; 122. Bus capacitor; 2. Grid-connected switch; 21. Semiconductor switch; 22. Grid-connected relay; 3. Control circuit; 4. Power grid; 5. PV string. DETAILED DESCRIPTION

[0030] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0031] Directional terms used herein, such as "upper," "lower," "front," "back," "left," "right," "inner," "outer," and "side," refer only to directions in the accompanying drawings. Therefore, these directional terms are intended to illustrate and facilitate understanding of the present invention and are not intended to limit the present invention. Furthermore, in the accompanying drawings, similar or identical structures are denoted by the same reference numerals.

[0032] In order to facilitate understanding of the present invention, the inverter system provided by the embodiment of the present invention is first described. Figures 1 to 8 , please refer to Figure 1 An embodiment of the present invention provides an inverter system, which includes: an inverter circuit 1, including a DC conversion module 11 and an inverter module 12, wherein the input end of the DC conversion module 11 is connected to the photovoltaic string 5, and the output end is connected to the input end of the inverter module 12; a grid-connected switch 2, including a semiconductor switch 21 and a grid-connected relay 22, wherein the semiconductor switch 21 is connected to the output end of the inverter module 12, and the grid-connected relay 22 is connected after the semiconductor switch 21 and connected to the power grid 4; a control circuit 3, connected to the inverter circuit 1 and the grid-connected switch 2, and used for controlling the inverter circuit 1 to block the wave and / or controlling the grid-connected switch 2 to disconnect when the photovoltaic string 5 is short-circuited to the ground.

[0033] In traditional inverter systems, the input side of the inverter is typically connected to multiple PV strings, while the output side of the inverter is connected to the grid via a three-phase grid-connected relay. Each phase of the output is connected to the grid via one or more grid-connected relays. Grid systems can generally be divided into three categories: TN, TT, and IT. In practical applications, PV string inverters are often used in TN systems. After the inverter is connected to the TN system grid, when a short circuit occurs to the ground on the positive side (PV+) or the negative side (PV-) of the PV string, multiple different current fault loops will form within the system. The high current in the loop can easily damage the machine. The current protection method is to detect the short circuit and then control the grid-connected relay between the inverter output and the grid to disconnect for protection. However, when the grid-connected relay is disconnected, it is easily damaged by the impact of the high current in the loop, making it difficult to effectively protect the system.

[0034] To solve the above problems, this embodiment provides an inverter system, which includes: an inverter circuit 1, including a DC conversion module 11 and an inverter module 12, wherein the input end of the DC conversion module 11 is connected to the photovoltaic string 5, and the output end is connected to the input end of the inverter module 12; a grid-connected switch 2, including a semiconductor switch 21 and a grid-connected relay 22, wherein the semiconductor switch 21 is connected to the output end of the inverter module 12, and the grid-connected relay 22 is connected after the semiconductor switch 21 and connected to the power grid 4; a control circuit 3, connected to the inverter circuit 1 and the grid-connected switch 2, and used to control the inverter circuit 1 to block the wave and / or control the grid-connected switch 2 to disconnect when the photovoltaic string 5 is short-circuited to ground.

[0035] In specific implementation, there is no restriction on the type of power grid system to which the inverter system can be applied. The inverter system can be applied to any of the TN power grid system, TT power grid system, and IT power grid system. This embodiment mainly takes the application of the TN power grid system as an example. Figure 1As shown, the inverter system of this embodiment includes an inverter circuit 1, a grid-connected switch 2, and a control circuit 3. The inverter circuit 1 is the circuit inside the inverter of the photovoltaic string 5. The inverter circuit 1 is mainly composed of a DC conversion module 11 and an inverter module 12. The input end of the DC conversion module 11 is connected to the photovoltaic string 5, and the output end of the DC conversion module 11 is connected to the input end of the inverter module 12. Specifically, the DC conversion module 11 and the inverter module 12 are connected by a DC bus. The DC conversion module 11 is mainly used for high-low voltage conversion of DC power. The DC conversion module 11 is specifically a circuit that can achieve step-up and step-down functions, such as a boost circuit, a buck circuit, or other circuit that can achieve step-up and step-down functions. The main function of the DC conversion module 11 is to step up and step-down the DC voltage output by the photovoltaic string 5 and output the processed DC voltage to the inverter module 12. The inverter module 12 is mainly used to convert the DC power output by the DC conversion module 11 into AC power and output it to the grid 4. The inverter module 12 is mainly a circuit module composed of power switches (such as IGBTs), capacitors, diodes, and other components. The output of the inverter module 12 is connected to the power grid 4 via the grid-connected switch 2. Specifically, the grid-connected switch 2 primarily includes a semiconductor switch 21, but may also include other types of switches, such as relays and AC contactors. The semiconductor switch 21 may be a MOS transistor, an IGBT, or other switching device. The semiconductor switch 21 is connected to the output of the inverter module 12, and the electrical energy output by the inverter module 12 must pass through the semiconductor switch 21 before entering the power grid 4. The grid-connected switches 2 are typically provided in multiple groups, one for each of the three phases at the output of the inverter module 12. There is no limit on the number of semiconductor switches 21 in the grid-connected switch 2, meaning that the grid-connected switch 2 must contain at least one semiconductor switch 21. The grid-connected switch 2 is controlled by a control circuit 3. The control circuit 3 connects the inverter circuit 1 and the grid-connected switch 2. The control circuit 3 is the core control system. It controls whether the inverter circuit 1 needs to be blocked, and controls the on / off state of the grid-connected switch 2, based on the grounding conditions of the photovoltaic strings 5. In actual applications, the determination of the short circuit of the photovoltaic string 5 to the ground is obtained by a special detection circuit. For example, by connecting a resistor between the output of the photovoltaic string 5 and the ground, the voltage to the ground can be detected to determine the short circuit of the photovoltaic string 5 to the ground. When the control circuit 3 determines that the photovoltaic string 5 is short-circuited to the ground, it controls the inverter circuit 1 to block the wave and / or controls the grid-connected switch 2 to disconnect according to the control strategy set by the system. Specifically, the control circuit 3 can control the blocking of the inverter circuit 1 and the disconnection of the grid-connected switch 2 at the same time, or only control the blocking of the inverter circuit 1 or only control the disconnection of the grid-connected switch 2. The specific control method is determined by the control strategy set by the system. The control circuit 3 controls the disconnection of the grid-connected switch 2 specifically by controlling the disconnection of the semiconductor switch 21, and the control of the blocking of the inverter circuit 1 includes controlling the blocking of the DC conversion module 11 and controlling the blocking of the inverter module 12.

[0036] In actual applications, when the photovoltaic string 5 is short-circuited to the ground, multiple different fault loops will be formed for the TN power grid 4 system. The control circuit 3 can block part of the fault loop by controlling the inverter circuit 1 to block the wave to achieve protection. For the other part of the fault loop that cannot be blocked by the inverter circuit 1 blocking the wave, the control circuit 3 can directly block the current fault loop by controlling the disconnection of the grid-connected switch 2. The disconnection time of the semiconductor switch 21 is usually in microseconds. When a system fault occurs, the control circuit 3 sends a disconnection signal to the grid-connected switch 2, so that the semiconductor switch 21 is quickly disconnected, thereby cutting off the connection between the inverter module 12 and the power grid 4 at the fastest speed, achieving rapid blocking of the system current fault loop and protecting system safety.

[0037] In one embodiment, referring to Figure 2 and Figure 3 The inverter module 12 includes a power topology circuit 121, a bus capacitor 122, and an inverter inductor L2. The input of the power topology circuit 121 is connected to the output of the DC conversion module 11, and the output is connected to the semiconductor switch 21 via the inverter inductor L2. The bus capacitor 122 is connected to the input of the power topology circuit 121, and the inverter circuit is connected to the power topology circuit 121. In a specific implementation, the inverter module 12 mainly consists of the power topology circuit 121, the bus capacitor 122, and the inverter inductor L2. The power topology circuit 121 is the core for achieving AC / DC power conversion. The power topology circuit 121 can be any of the following circuits: a two-level topology circuit, a three-level NPC topology circuit, a three-level ANPC topology circuit, a three-level I-type topology circuit, or a three-level T-type topology circuit, without limitation. The input of the power topology circuit 121 is connected to the output of the DC conversion module 11, and the output is connected to the semiconductor switch 21 via the inverter inductor L2. The control circuit 3 is connected to the power topology circuit 121. The inverter inductor L2 can suppress the high frequency noise at the output of the power topology circuit 121, improve the current output quality, and can also be used for system current detection. The bus capacitor 122 is connected to the input of the power topology circuit 121. The bus capacitor 122 is a capacitor connected in series in pairs, such as Figure 2 and Figure 3 As shown, bus capacitor 122 includes capacitors C1 and C2. Bus capacitor 122 has the functions of absorbing inrush current and smoothing current waveforms. In actual applications, control circuit 3 controls the operation of power topology circuit 121 by outputting PWM pulses, and cooperates with bus capacitor 122 to achieve DC to AC conversion.

[0038] Further, refer to Figure 2The power topology circuit 121 is a three-level I-type topology circuit, comprising a first switching transistor Q1, a second switching transistor Q2, a third switching transistor Q3, and a fourth switching transistor Q4, and a first diode D6 and a second diode D5 connected in series. The control circuit 3 is connected to the gates of the first switching transistor Q1, the second switching transistor Q2, the third switching transistor Q3, and the fourth switching transistor Q4. The positive output terminal of the DC conversion module 11 is connected to the drain of the first switching transistor Q1, the negative output terminal of the DC conversion module 11 is connected to the source of the fourth switching transistor Q4, the anode of the first diode D6 is connected to the drain of the fourth switching transistor Q4, the cathode of the second diode D5 is connected to the source of the first switching transistor Q1, and the midpoint of the bus capacitor 122 is connected between the first diode D6 and the second diode D5. In a specific implementation, the power topology circuit 121 of the inverter module 12 is designed to adopt a three-level I-type topology circuit. Taking one phase of the three-level I-type topology circuit as an example, a first switching transistor Q1, a second switching transistor Q2, a third switching transistor Q3, and a fourth switching transistor Q4 connected in series, and a first diode D6 and a second diode D5 connected in series, constitute one phase of the three-level I-type topology circuit. The control circuit 3 is connected to the gates of the first switching transistor Q1, the second switching transistor Q2, the third switching transistor Q3, and the fourth switching transistor Q4. The positive output terminal of the DC conversion module 11 is connected to the drain of the first switching transistor Q1, the negative output terminal of the DC conversion module 11 is connected to the source of the fourth switching transistor Q4, the anode of the first diode D6 is connected to the drain of the fourth switching transistor Q4, the cathode of the second diode D5 is connected to the source of the first switching transistor Q1, and the midpoint of the bus capacitor 122 is connected between the first diode D6 and the second diode D5.

[0039] In one embodiment, referring to Figure 3The power topology circuit 121 is a three-level T-type topology circuit, which includes a first switch tube Q1, a second switch tube Q2, and a third switch tube Q3 and a fourth switch tube Q4 connected in series. The drain of the third switch tube Q3 is connected to the drain of the fourth switch tube Q4. The control circuit 3 is connected to the gates of the first switch tube Q1, the second switch tube Q2, the third switch tube Q3, and the fourth switch tube Q4. The positive output terminal of the DC conversion module 11 is connected to the drain of the first switch tube Q1, the negative output terminal of the DC conversion module 11 is connected to the source of the second switch tube Q2, the source of the third switch tube Q3 is connected between the first switch tube Q1 and the second switch tube Q2, and the source of the fourth switch tube Q4 is connected to the midpoint of the bus capacitor 122. In a specific implementation, the power topology circuit 121 of the inverter module 12 is designed to adopt a three-level T-type topology circuit. Taking one phase of the three-level T-type topology circuit as an example, a first switching transistor Q1, a second switching transistor Q2, and a third switching transistor Q3 and a fourth switching transistor Q4 connected in series in sequence constitute one phase of the three-level T-type topology circuit. The drain of the third switching transistor Q3 is connected to the drain of the fourth switching transistor Q4, forming an anti-series connection. The control circuit 3 is connected to the gates of the first switching transistor Q1, the second switching transistor Q2, the third switching transistor Q3, and the fourth switching transistor Q4. The positive output terminal of the DC conversion module 11 is connected to the drain of the first switching transistor Q1, the negative output terminal of the DC conversion module 11 is connected to the source of the second switching transistor Q2, and the source of the third switching transistor Q3 is connected between the first switching transistor Q1 and the second switching transistor Q2, specifically to the connection point of the first switching transistor Q1 and the second switching transistor Q2 in series. The source of the fourth switching transistor Q4 is connected to the midpoint of the bus capacitor 122.

[0040] In one embodiment, referring to Figure 2 and Figure 3The DC conversion module 11 includes a power switch tube Q5, a freewheeling diode D7, a boost inductor L1 and a chopping capacitor C4; wherein, one end of the boost inductor L1 is connected to the positive electrode of the photovoltaic string 5, and the other end is connected to the anode of the freewheeling diode D7, and the cathode of the freewheeling diode D7 is connected to the positive input terminal of the inverter module 12; the gate of the power switch tube Q5 is connected to the control circuit 3, the drain is connected to the anode of the freewheeling diode D7, and the source is connected to the negative electrode of the photovoltaic string 5; one end of the chopping capacitor C4 is connected to the cathode of the freewheeling diode D7, and the other end is connected to the negative input terminal of the inverter module 12. In a specific implementation, the DC conversion module 11 is mainly composed of a power switch tube Q5, a freewheeling diode D7, a boost inductor L1 and a chopping capacitor C4. One end of the boost inductor L1 is connected to the positive electrode of the photovoltaic string 5, and the other end is connected to the anode of the freewheeling diode D7. The cathode of the freewheeling diode D7 is connected to the positive input terminal of the inverter module 12. The gate of the power switch tube Q5 is connected to the control circuit 3, the drain of the power switch tube Q5 is connected to the anode of the freewheeling diode D7, and the source of the power switch tube Q5 is connected to the negative electrode of the photovoltaic string 5. One end of the chopping capacitor C4 is connected to the cathode of the freewheeling diode D7, and the other end is connected to the negative input terminal of the inverter module 12. Through the above connections, a Boost boost circuit is formed, which can stably boost the output voltage of the photovoltaic string 5 and output it to the inverter module 12.

[0041] In one embodiment, referring to Figure 1 and Figure 2The grid-connected switch also includes a grid-connected relay 22, which is connected to the semiconductor switch 21. One of the semiconductor switch 21 and the grid-connected relay 22 is connected to the output of the inverter module 12, while the other is connected to the grid 4. In a specific embodiment, the grid-connected switch 2 also includes the grid-connected relay 22, that is, the grid-connected switch 2 is composed of at least two different types of switches, the semiconductor switch 21 and the grid-connected relay 22. The grid-connected relay 22 is connected to the semiconductor switch 21, and one of the semiconductor switch 21 and the grid-connected relay 22 is connected to the output of the inverter module 12, while the other is connected to the grid 4. Specifically, the semiconductor switch 21 can be connected to the output of the inverter module 12, and the grid-connected relay 22 can be connected after the semiconductor switch 21 to connect to the grid 4. Alternatively, the semiconductor switch 21 can be connected to the output of the inverter module 12, and the grid-connected relay 22 can be connected after the semiconductor switch 21 to connect to the grid 4. In other words, the order in which the semiconductor switch 21 and the grid-connected relay 22 are connected between the inverter module 12 and the grid 4 is not limited. The number of grid-connected relays 22 is not limited; if multiple sets of grid-connected relays 22 are provided, they can be connected in series. Overall, the semiconductor switch 21 and the grid-connected relay 22, two different types of switches, are connected in series between the output of the inverter module 12 and the power grid 4, forming a multi-switch system. The AC power output by the inverter module 12 must pass through the semiconductor switch 21 and the grid-connected relay 22 before entering the power grid 4. The control circuit 3 is connected to the grid-connected relay 22 and controls the on / off state of the grid-connected relay 22. In actual applications, when a short circuit to ground occurs in the system, the control circuit 3 controls the grid-connected switch 2 to disconnect by sending a disconnect signal to the grid-connected switch 2. Controlling the disconnection of the grid-connected switch 2 may include controlling the disconnection of the semiconductor switch 21 and the grid-connected relay 22 at the same time, or controlling the disconnection of the semiconductor switch 21 or the grid-connected relay 22 alone. By controlling the disconnection of the semiconductor switch 21 and the grid-connected relay 22, the connection between the inverter circuit 12 and the power grid 4 can be disconnected, and the shutdown speed of the grid-connected relay 22 is much lower than the shutdown speed of the semiconductor switch 21. Even if the control circuit 3 sends a disconnection control instruction at the same time, the grid-connected relay 22 can be protected from being damaged by a large current impact, and the system current fault loop can be quickly blocked to protect the system safety.

[0042] In one embodiment, referring to Figure 1 and Figure 2, the semiconductor switch 21 includes a plurality of switch tube devices, and the plurality of switch tube devices are connected in series in sequence. The output end of the inverter module 12 is connected to the first switch tube device, the grid-connected relay 22 is connected to the last switch tube device, and the control circuit 3 is connected to all the switch tube devices. In a specific implementation, the semiconductor switch 21 includes a plurality of switch tube devices, the number of which is at least two, and all the switch tube devices are connected in series. For example, the semiconductor switch 21 is connected in front and the grid-connected relay 22 is connected in the back between the inverter module 12 and the power grid 4. The output end of the inverter module 12 is connected to the first switch tube device, and the grid-connected relay 22 is connected to the last switch tube device. The control circuit 3 is connected to all the switch tube devices, which can control the on and off of all the switch tube devices. In actual applications, by connecting a plurality of switch tube devices in series, the control circuit 3 controls the synchronous on and off of the plurality of switch tube devices to realize the switching function, thereby improving the reliability of the disconnection protection.

[0043] In one embodiment, the semiconductor switch 21 includes multiple switching devices, with their ends connected in parallel. The output of the inverter module 12 is connected to one end of all the switching devices connected in parallel, the grid-connected relay 22 is connected to the other end of all the switching devices connected in parallel, and the control circuit 3 is connected to all the switching devices. In a specific implementation, the semiconductor switch 21 includes multiple switching devices, with at least two switching devices, with their ends connected in parallel. For example, with the semiconductor switch 21 connected in front and the grid-connected relay 22 connected in the back between the inverter module 12 and the grid 4, the output of the inverter module 12 is connected to one end of all the switching devices connected in parallel, the grid-connected relay 22 is connected to the other end of all the switching devices connected in parallel, and the control circuit 3 is connected to all the switching devices to control their on and off. In practical applications, by connecting multiple switching devices in parallel, the control circuit 3 controls the synchronous on and off of the multiple switching devices to achieve switching functionality. Even if a switching device is damaged, the other switching devices can still maintain normal disconnection function, thereby improving the reliability of disconnection protection.

[0044] In one embodiment, the semiconductor switch 21 includes multiple switching devices, some of which are connected in series, while others are connected in parallel at both ends. The series switching devices are connected to the parallel switching devices. The output end of the inverter module 12 is connected to one of the series switching devices and the parallel switching devices. The grid-connected relay 22 is connected to the other of the series switching devices and the parallel switching devices. The control circuit 3 is connected to all of the switching devices. In a specific implementation, the semiconductor switch 21 includes multiple switching devices, the number of which is not limited. Some of the switching devices are connected in series, while others are connected in parallel. The two ends are connected in parallel, and the series switching devices are connected to the parallel switching devices. Taking the example of a semiconductor switch 21 connected in front and a grid-connected relay 22 connected in back between the inverter module 12 and the grid 4, the output of the inverter module 12 is connected to one of the series switching devices or the parallel switching devices, and the grid-connected relay 22 is connected to the other of the series switching devices or the parallel switching devices. Specifically, when the output of the inverter module 12 is connected to the series switching devices, the grid-connected relay 22 is connected to the parallel switching devices. When the output of the inverter module 12 is connected to the parallel switching devices, the grid-connected relay 22 is connected to the series switching devices. The control circuit 3 is connected to all the switching devices and can control the on and off of all the switching devices. In practical applications, by connecting multiple switching devices in parallel, the control circuit 3 controls the synchronous on and off of the multiple switching devices to achieve the switching function, which can improve the reliability of the disconnection protection.

[0045] The inverter system provided in the embodiment of the present application adopts semiconductor switches for grid connection, which can quickly disconnect the photovoltaic string when a short circuit to the ground occurs to protect the system safety, thereby improving the system safety.

[0046] like Figure 9 As shown, the present application provides a method for protecting an inverter system from short circuit to ground. The method is applied to the inverter system described in the above embodiment to protect the system from short circuit to ground faults. The method includes steps: S110-S140.

[0047] S110: If a short circuit to ground of the photovoltaic string is detected, a current value at an output end of the inverter module is obtained.

[0048] In a specific implementation, the system can detect whether a photovoltaic string is short-circuited to the ground through a detection circuit. When a photovoltaic string is short-circuited to the ground is detected, the current value at the output end of the inverter module is obtained, specifically the current value on the inverter inductor is obtained. The current value at the output end of the inverter module can be obtained through a current sensor.

[0049] S120: If the current value at the output end of the inverter module is greater than a preset protection threshold, obtaining the ground short-circuit type of the photovoltaic string and the voltage waveform of the grid.

[0050] In a specific implementation, after the system obtains the current value at the inverter module output, it compares the current value at the inverter module output with a preset protection threshold. The preset protection threshold can be set according to the actual system protection requirements, and the preset protection threshold is used as a condition for determining whether protective measures need to be implemented. If the current value at the inverter module output is greater than the preset protection threshold, it indicates that there is a large inrush current in the fault circuit, which will damage the system and require protection measures. The system then obtains the type of ground short circuit of the photovoltaic string and the voltage waveform of the power grid. Specifically, the type of ground short circuit of the photovoltaic string can be divided into positive pole ground short circuit and negative pole ground short circuit. Different ground short circuit types have different system current fault circuits. The system obtains the voltage waveform of the power grid mainly to determine whether the power grid voltage waveform is in the positive half-cycle or the negative half-cycle. The system current fault circuits corresponding to the positive and negative half-cycles of the power grid voltage waveform are also different. Therefore, the system needs to implement different protection strategies based on the type of ground short circuit and the voltage waveform of the power grid.

[0051] S130 , selecting an inverter circuit blocking instruction and / or a grid-connected switch disconnection instruction as a target control instruction according to the ground short-circuit type of the photovoltaic string and the voltage waveform of the power grid.

[0052] In a specific implementation, after the system obtains the ground short-circuit type of the photovoltaic string and the voltage waveform of the grid, it selects the inverter circuit sealing instruction and / or the grid-connected switch disconnection instruction as the target control instruction according to the ground short-circuit type of the photovoltaic string and the voltage waveform of the grid. Specifically, the system can select only the inverter circuit sealing instruction as the target control instruction, or only the grid-connected switch disconnection instruction as the target control instruction, or simultaneously select the inverter circuit sealing instruction and the grid-connected switch disconnection instruction as the target control instruction. There can be a total of three different target control instructions, corresponding to different protection strategies. The target control instruction is an instruction used to control the inverter circuit and the grid-connected switch. The target control instruction can be used to control the inverter circuit sealing and the grid-connected switch disconnection. The system selects different target control instructions to block different current fault circuits, thereby achieving the purpose of system protection.

[0053] Furthermore, if Figure 10 As shown, the step S130 includes steps: S131-S133.

[0054] S131. If the short-circuit type of the photovoltaic string to ground is a positive pole short-circuit to ground and the voltage waveform of the grid is in a positive half cycle or a negative half cycle, selecting an inverter circuit wave blocking instruction as the target control instruction;

[0055] In specific implementation, Figure 4 and Figure 5 As shown, the inverter module adopts a three-level I-type topology as an example, and the system is connected to the TN grid. Figure 4 As shown in the figure, when the PV+ positive terminal of the PV string is short-circuited to ground and the grid voltage is in the positive half-cycle, a current fault loop exists in the system. This current fault loop is: PV+ positive terminal of the PV string - negative terminal of the grid - positive terminal of the grid - grid-connected switches K2 and K1 - inductor L2 - switch Q3 - diode D6 - capacitor C2 - PV- negative terminal of the PV string. The current in this loop passes through the switch Q3 itself, so the blocking function of Q3 blocks the current fault loop, achieving protection.

[0056] like Figure 5 As shown in the figure, when the PV+ positive terminal of the PV string is short-circuited to ground and the grid voltage is in the negative half-cycle, the system has two current fault loops. One fault loop is: grid positive terminal - PV+ positive terminal of the PV string - inductor L1 - switch Q5 - diode D4 - diode D3 - inverter inductor L2 - grid-connected switches K1 and K2 - grid negative terminal. The current in this loop passes through the body of switch Q5 and the diodes D3 and D4 inside switches Q3 and Q4. The blocking of switches Q3 and Q4 cannot block the loop, but the blocking of switch Q5 can block the loop, achieving protection. The other current fault loop is: grid positive terminal - PV+ positive terminal of the PV string - inductor L1 - switch Q5 - capacitor C2 - diode D5 - switch Q2 - inductor L2 - grid-connected switches K1 and K2 - grid negative terminal. The current in this loop passes through the bodies of switches Q5 and Q2. The blocking of switches Q5 or Q2 can block the current fault loop, achieving protection.

[0057] In actual applications, when the positive pole of the photovoltaic string is short-circuited to the ground and the grid voltage is in the positive half-cycle or the negative half-cycle, the system selects the inverter circuit blocking instruction as the target control instruction. The target control instruction can control the inverter circuit blocking. The blocking may include blocking the switching tubes Q5, Q2, and Q3, which can block the above-mentioned current fault circuit and achieve protection.

[0058] S132: If the short-circuit type of the photovoltaic string to ground is a negative pole short-circuit to ground and the voltage waveform of the grid is in the positive half cycle, select an inverter circuit wave-sealing instruction as the target control instruction.

[0059] In specific implementation, Figure 6 As shown, the inverter module adopts a three-level I-type topology as an example, and the system is connected to the TN grid. Figure 6As shown in the figure, when the negative pole PV- of the PV string is short-circuited to ground and the grid voltage is in the positive half-cycle, a current fault loop exists in the system. This current fault loop is: grid positive pole - grid-connected switches K2 and K1 - inductor L2 - switch tube Q3 - diode D6 - capacitor C2 - PV string negative pole PV- - grid negative pole. The current in this loop passes through the switch tube Q3. Therefore, the current fault loop can be blocked by the blocking of Q3 to achieve protection.

[0060] In actual applications, when the negative pole of the photovoltaic string is short-circuited to the ground and the grid voltage is in the positive half cycle, the system selects the inverter circuit blocking instruction as the target control instruction. The target control instruction can control the inverter circuit blocking. The inverter circuit blocking may include blocking the switch tube Q3, which can block the above-mentioned system current fault circuit to achieve protection.

[0061] S133: If the short-circuit type of the photovoltaic string to ground is a negative pole short-circuit and the voltage waveform of the grid is in the negative half cycle, select an inverter circuit blocking instruction and a grid-connected switch disconnecting instruction as the target control instruction.

[0062] In specific implementation, refer to Figure 7 and Figure 8 , the inverter module takes the three-level I-type topology as an example, Figure 7 and Figure 8 As shown, the system is connected to a TN grid. When the negative pole of the PV string PV- is short-circuited to ground and the grid voltage is in the negative half-cycle, two current fault loops exist in the system. One fault loop runs from the positive grid pole to the negative pole of the PV string PV-—diode D4—diode D3—inductor L2—grid switches K1 and K2—to the negative grid pole. This current loop does not pass through the main body of the switching transistors in the inverter circuit, and the blocking of the switching transistors cannot interrupt the current loop. Therefore, the only way to interrupt the loop and achieve protection is to open the grid switches K1 and K2. The other fault loop runs from the positive grid pole to the negative pole of the PV string PV-—capacitor C2—diode D5—switch Q2—inductor L2—grid switches K1 and K2—to the negative grid pole. The current in this loop passes through the main body of switch Q2, and the blocking of Q2 can interrupt the current fault loop and achieve protection.

[0063] In actual applications, when the negative pole of the photovoltaic string is short-circuited to the ground and the grid voltage is in the negative half-cycle, the system selects the inverter circuit blocking instruction and the grid-connected switch disconnection instruction as the target control instruction at the same time. The target control instruction can simultaneously control the inverter circuit blocking and the grid-connected switch disconnection. The inverter circuit blocking may include the blocking of the switch tube Q2, and the grid-connected switch disconnection may include the disconnection of the semiconductor switch K1 and the disconnection of the grid-connected relay K2, which can block the above-mentioned system current fault circuit and achieve protection.

[0064] S140: Control the inverter circuit and the grid-connected switch using the target control instruction.

[0065] In a specific implementation, the system selects the corresponding target control instruction based on the ground short-circuit type of the photovoltaic string and the voltage waveform of the power grid, and then controls the inverter circuit and the grid-connected switch with the target control instruction. Specifically, when the target control instruction is the inverter circuit blocking instruction, the system controls the inverter circuit blocking with the target control instruction. The inverter circuit blocking may include the blocking of the switching tube devices in the DC conversion module and the blocking of the switching tube devices in the inverter module. After the inverter circuit is blocked, the system current fault circuit can be accurately blocked. When the target control instruction is the grid-connected switch disconnection instruction, the system controls the grid-connected switch disconnection with the target control instruction. The grid-connected switch disconnection may include the disconnection of the semiconductor switch and the disconnection of the grid-connected relay. After the grid-connected switch disconnection, the system current fault circuit can be accurately disconnected. By controlling the inverter circuit and the grid-connected switch with different target control instructions, the system implements the protection strategy according to the fault type, can accurately block the system current fault circuit, effectively realize system protection, and improve the safety and reliability of the system.

[0066] The method of the present application detects a short circuit to ground of a photovoltaic string and obtains the current value at the output end of the inverter module. When the current value at the output end of the inverter module is greater than a preset protection threshold, the ground short circuit type of the photovoltaic string and the voltage waveform of the power grid are obtained. Then, a target control instruction is selected based on the ground short circuit type of the photovoltaic string and the voltage waveform of the power grid. The target control instruction is used to control the inverter circuit and the grid-connected switch. When the inverter system is applied to a TN power grid system, even if a ground short circuit fault occurs in the system, the system current fault loop can be accurately disconnected, thereby achieving precise protection of the system and improving the safety and reliability of the system.

[0067] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and such modifications or substitutions are intended to be within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be subject to the scope of protection of the claims.

Claims

1. An inverter system, characterized in that: include: The inverter circuit includes a DC conversion module and an inverter module, wherein the input end of the DC conversion module is connected to the photovoltaic string, and the output end is connected to the input end of the inverter module; A grid-connected switch, comprising a semiconductor switch connected between the output terminal of the inverter module and the grid; A control circuit is connected to the inverter circuit and the grid-connected switch, and is used to control the inverter circuit to block the wave and / or control the grid-connected switch to disconnect when the photovoltaic string is short-circuited to the ground.

2. The inverter system according to claim 1, characterized in that: The inverter module includes a power topology circuit, a bus capacitor and an inverter inductor. The input end of the power topology circuit is connected to the output end of the DC conversion module, and the output end is connected to the semiconductor switch through the inverter inductor. The bus capacitor is connected to the input end of the power topology circuit, and the inverter circuit is connected to the power topology circuit.

3. The inverter system according to claim 2, characterized in that: The power topology circuit is a three-level I-type topology circuit, which includes a first switching tube, a second switching tube, a third switching tube, and a fourth switching tube connected in series, and a first diode and a second diode connected in series. The control circuit is connected to the gates of the first switching tube, the second switching tube, the third switching tube, and the fourth switching tube. The positive output terminal of the DC conversion module is connected to the drain of the first switching tube, the negative output terminal of the DC conversion module is connected to the source of the fourth switching tube, the anode of the first diode is connected to the drain of the fourth switching tube, the cathode of the second diode is connected to the source of the first switching tube, and the midpoint of the bus capacitor is connected between the first diode and the second diode.

4. The inverter system according to claim 2, characterized in that: The power topology circuit is a three-level T-type topology circuit, which includes a first switching tube, a second switching tube, and a third switching tube and a fourth switching tube connected in series in sequence. The drain of the third switching tube is connected to the drain of the fourth switching tube. The control circuit is connected to the gates of the first switching tube, the second switching tube, the third switching tube, and the fourth switching tube. The positive output terminal of the DC conversion module is connected to the drain of the first switching tube, the negative output terminal of the DC conversion module is connected to the source of the second switching tube, the source of the third switching tube is connected between the first switching tube and the second switching tube, and the source of the fourth switching tube is connected to the midpoint of the bus capacitor.

5. The inverter system according to claim 1, characterized in that: The DC conversion module includes a power switch tube, a freewheeling diode, a boost inductor and a chopper capacitor; wherein, one end of the boost inductor is connected to the positive electrode of the photovoltaic string, and the other end is connected to the anode of the freewheeling diode, the cathode of the freewheeling diode is connected to the positive input terminal of the inverter module, the gate of the power switch tube is connected to the control circuit, the drain is connected to the anode of the freewheeling diode, and the source is connected to the negative electrode of the photovoltaic string, and one end of the chopper capacitor is connected to the cathode of the freewheeling diode, and the other end is connected to the negative input terminal of the inverter module.

6. The inverter system according to any one of claims 1 to 5, characterized in that: The grid-connected switch further includes a grid-connected relay, which is connected to the semiconductor switch. One of the semiconductor switch and the grid-connected relay is connected to the output end of the inverter module, and the other is connected to the power grid.

7. The inverter system according to claim 6, characterized in that: The semiconductor switch includes multiple switching tube devices, which are connected in series in sequence. The output end of the inverter module is connected to the first switching tube device, the grid-connected relay is connected to the last switching tube device, and the control circuit is connected to all the switching tube devices.

8. The inverter system according to claim 6, characterized in that: The semiconductor switch includes multiple switching tube devices, both ends of the multiple switching tube devices are connected in parallel, the output end of the inverter module is connected to one end of all the switching tube devices connected in parallel, the grid-connected relay is connected to the other end of all the switching tube devices connected in parallel, and the control circuit is connected to all the switching tube devices.

9. A method for protecting an inverter system from short circuit to ground, characterized in that: Applied to the inverter system according to any one of claims 1 to 8, the method comprises: If a short circuit to ground is detected in the photovoltaic string, the current value at the output end of the inverter module is obtained; If the current value at the output end of the inverter module is greater than a preset protection threshold, the ground short circuit type of the photovoltaic string and the voltage waveform of the grid are obtained; selecting an inverter circuit blocking instruction and / or a grid-connected switch disconnecting instruction as a target control instruction according to the type of ground short circuit of the photovoltaic string and the voltage waveform of the grid; The inverter circuit and the grid-connected switch are controlled using the target control instruction.

10. The method according to claim 9, characterized in that The step of selecting an inverter circuit blocking instruction and / or a grid-connected switch disconnection instruction as a target control instruction according to the ground short-circuit type of the photovoltaic string and the voltage waveform of the grid includes: If the short-circuit type of the photovoltaic string to ground is a positive pole short-circuit to ground and the voltage waveform of the grid is in a positive half cycle or a negative half cycle, selecting an inverter circuit wave sealing instruction as the target control instruction; If the short-circuit type of the photovoltaic string to ground is a negative pole short-circuit to ground and the voltage waveform of the grid is in a positive half cycle, selecting an inverter circuit wave-sealing instruction as the target control instruction; If the short-circuit type of the photovoltaic string is a negative-pole short-circuit and the voltage waveform of the grid is in a negative half cycle, an inverter circuit wave blocking instruction and a grid-connected switch disconnection instruction are selected as the target control instruction.

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