Inverter system and grid-connected relay fault detection method thereof

Abstract

The invention discloses an inverter system and a grid-connected relay fault detection method thereof, and the method comprises the steps: controlling one of a first three-phase grid-connected relay and a second three-phase grid-connected relay to be switched on, and controlling the other three-phase grid-connected relay to be switched off; acquiring voltage to ground of a power grid and leakage current flowing through the three-phase grid-connected relay; and obtaining a fault detection result according to the voltage to ground of the power grid and the leakage current flowing through the three-phase grid-connected relay. The method is applied to the inverter system, the short-circuit fault of the grid-connected relay in the inverter system can be accurately and effectively identified without depending on the type of a power grid, and the reliability of the system is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of photovoltaic power supply, and in particular to an inverter system and a grid-connected relay fault detection method thereof. Background Art

[0002] The three-phase grid-connected inverter is an important part of the photovoltaic power supply system. Based on safety considerations, the three-phase grid-connected inverter needs to be able to identify the short circuit of the grid-connected relay and make the system report the corresponding fault, and does not allow grid-connected operation. At present, the identification scheme for the short circuit of the grid-connected relay in the industry is mainly to collect the RCD leakage current in the three-phase grid-connected inverter or collect the AC component of the DC bus to determine the relay state, but this scheme can only effectively identify the power grid system as a TT or TN system. When the power grid system is an IT system, even if a single-phase relay short circuit occurs, because the impedance of the grid-connected end to the ground is large, the AC component content of the DC bus of the three-phase grid-connected inverter to the ground will be very small. It is difficult to extract the AC component and it is easy to falsely report the fault. Therefore, it is difficult to effectively identify the short circuit fault of the grid-connected relay in the photovoltaic power supply system with the traditional method. Summary of the invention

[0003] The embodiment of the present invention provides an inverter system and a grid-connected relay fault detection method thereof, aiming to solve the problem that it is difficult to effectively identify a short-circuit fault of a grid-connected relay in an inverter system in a traditional manner.

[0004] In a first aspect, an embodiment of the present invention provides a grid-connected relay fault detection method for an inverter system, wherein the inverter system includes a three-phase grid-connected inverter, a positive DC bus, a negative DC bus, and a bus midpoint arranged on the DC side of the three-phase grid-connected inverter, a positive bus capacitor connected between the positive DC bus and the bus midpoint, a negative bus capacitor connected between the negative DC bus and the bus midpoint, an AC filter capacitor and a three-phase grid-connected relay arranged on the AC side of the three-phase grid-connected inverter, the AC filter capacitor is star-connected and the center point of the star connection is connected to the bus midpoint, and the three-phase grid-connected relay includes a first three-phase grid-connected relay and a second three-phase grid-connected relay connected in series; the grid-connected relay fault detection method includes:

[0005] Controlling one of the first three-phase grid-connected relay and the second three-phase grid-connected relay to be closed and the other three-phase grid-connected relay to be opened;

[0006] Obtaining the voltage of the power grid to ground and the leakage current flowing through the three-phase grid-connected relay;

[0007] A fault detection result is obtained according to the voltage of the power grid to ground and the leakage current flowing through the three-phase grid-connected relay.

[0008] In a second aspect, an embodiment of the present invention provides an inverter system, the system comprising: a three-phase grid-connected inverter, a positive DC bus, a negative DC bus and a bus midpoint arranged on the DC side of the three-phase grid-connected inverter, a positive bus capacitor connected between the positive DC bus and the bus midpoint, a negative bus capacitor connected between the negative DC bus and the bus midpoint, an AC filter capacitor and a three-phase grid-connected relay arranged on the AC side of the three-phase grid-connected inverter, the AC filter capacitor being star-connected and the center point of the star connection being connected to the bus midpoint, the three-phase grid-connected relay comprising a first three-phase grid-connected relay and a second three-phase grid-connected relay connected in series; the system also comprises a sampling control unit connected to the three-phase grid-connected inverter for controlling the three-phase grid-connected inverter to generate grid-connected power.

[0009] In the system provided in an embodiment of the present invention, the three-phase grid-connected inverter also includes a current detection unit, which is connected to the leakage current branch flowing through the three-phase grid-connected relay to collect the leakage current flowing through the three-phase grid-connected relay, and the sampling control unit is connected to the current detection unit to obtain the leakage current.

[0010] In the system provided by the embodiment of the present invention, the current detection unit includes a leakage current Hall, the leakage current Hall is arranged on the leakage current branch flowing through the three-phase grid-connected relay, and the sampling control unit is connected to the leakage current Hall.

[0011] In the system provided in an embodiment of the present invention, the three-phase grid-connected relay includes a first three-phase grid-connected relay and a second three-phase grid-connected relay, the first three-phase grid-connected relay is connected to the AC filter capacitor, the second three-phase grid-connected relay is connected to the first three-phase grid-connected relay, and the sampling control unit is connected to the first three-phase grid-connected relay and the second three-phase grid-connected relay, wherein the second three-phase grid-connected relay is used to access the power grid.

[0012] In the system provided by the embodiment of the present invention, the system further comprises a safety capacitor, one end of which is connected between the three-phase grid-connected relay and the power grid, and the other end of which is grounded.

[0013] In the system provided by the embodiment of the present invention, the safety capacitor is a Y capacitor.

[0014] An embodiment of the present invention provides an inverter system and a grid-connected relay fault detection method thereof, the method comprising: controlling one of the first three-phase grid-connected relay and the second three-phase grid-connected relay to be closed and the other three-phase grid-connected relay to be disconnected; obtaining the voltage to ground of the power grid and the leakage current flowing through the three-phase grid-connected relay; and obtaining a fault detection result according to the voltage to ground of the power grid and the leakage current flowing through the three-phase grid-connected relay. The method of the present application controls the three-phase grid-connected relay to obtain the voltage to ground of the power grid and the leakage current flowing through the three-phase grid-connected relay, and obtains the fault detection result according to the voltage to ground of the power grid and the leakage current flowing through the three-phase grid-connected relay. It can accurately and effectively identify the short-circuit fault of the grid-connected relay in the inverter system without relying on the type of the power grid, thereby improving the reliability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying any creative work.

[0016] Figure 1 A schematic diagram of the structure of an inverter system provided by an embodiment of the present invention;

[0017] Figure 2 A schematic diagram of fault identification of an inverter system provided by an embodiment of the present invention;

[0018] Figure 3 A fault identification equivalent circuit diagram of an inverter system provided by an embodiment of the present invention;

[0019] Figure 4 Another fault identification schematic diagram of the inverter system provided by an embodiment of the present invention;

[0020] Figure 5 Another fault identification equivalent circuit diagram of the inverter system provided by an embodiment of the present invention;

[0021] Figure 6 A flowchart of the steps of a grid-connected relay fault detection method provided by an embodiment of the present invention;

[0022] Figure 7 A flow chart of sub-steps of a grid-connected relay fault detection method provided by an embodiment of the present invention;

[0023] Figure 8 A flow chart of sub-steps of a grid-connected relay fault detection method provided by an embodiment of the present invention;

[0024] The reference numerals in the figures are:

[0025] 100. Inverter system; 10. Three-phase grid-connected inverter; 11. DC bus capacitor; 12. AC filter capacitor; 13. DC / DC unit; 14. DC / AC unit; 20. Grid-connected relay; 21. First three-phase grid-connected relay; 22. Second three-phase grid-connected relay; 30. Sampling control unit; 40. Current detection unit; 50. Safety capacitor; 110. Photovoltaic power generation unit; 120. Power grid. DETAILED DESCRIPTION

[0026] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0027] The directional terms mentioned in the present invention, such as "upper", "lower", "front", "back", "left", "right", "inner", "outer", "side", etc., are only referenced to the directions of the attached drawings. Therefore, the directional terms used are used to illustrate and understand the present invention, but are not used to limit the present invention. In addition, in the drawings, structures with similar or identical structures are represented by the same reference numerals.

[0028] The embodiment of the present invention proposes an inverter system and a grid-connected relay fault detection method thereof, which solves the problem that it is difficult to effectively identify the short-circuit fault of the grid-connected relay in the inverter system in the traditional way. The specific ideas are as follows:

[0029] See also Figures 1 to 5 The embodiment of the present invention shows an inverter system 100, which includes: a power grid 120; a photovoltaic power generation unit 110, used to generate direct current power; a three-phase grid-connected inverter 10, connected between the photovoltaic power generation unit 110 and the power grid 120 to convert the direct current power into alternating current power and output it to the power grid 120, wherein the three-phase grid-connected inverter 10 includes a direct current bus capacitor 11, an alternating current filter capacitor 12 and a three-phase grid-connected relay 20, the midpoint of the direct current bus capacitor 11 is connected to the midpoint of the alternating current filter capacitor 12, and the three-phase grid-connected relay 20 is connected to the alternating current filter capacitor 12 and connected to the power grid 120; a sampling control unit 30, connected to the three-phase grid-connected inverter 10, used to control the three-phase grid-connected inverter 10 to generate power in the grid.

[0030] In the specific implementation, the inverter system 100 is mainly composed of a power grid 120, a photovoltaic power generation unit 201, a three-phase grid-connected inverter 10 and a sampling control unit 30. At present, the system of the power grid 120 can be divided into three categories: TN system, TT system, and IT system. Among them, the TT system refers to a protection system that directly grounds the metal shell of the electrical equipment, also known as a protective grounding system; the TN system is a protection system that connects the metal shell of the electrical equipment to the working neutral line, called a zero protection system; the IT system refers to a system in which the neutral point of the power supply is not grounded or grounded through a high impedance, and the metal shell of the electrical equipment is directly grounded, also known as an ungrounded power supply system. The power grid 120 in this embodiment can be any one of the IT, TT, and TN systems. The photovoltaic power generation unit 110 is mainly composed of one or more photovoltaic components that can realize photovoltaic power generation, such as a solar photovoltaic power generation unit composed of multiple solar panels. The three-phase grid-connected inverter 10 is connected between the power grid 120 and the photovoltaic power generation unit 110, and is used to convert the DC power generated by the photovoltaic power generation unit 110 into AC power and output it to the power grid 120. Specifically, the input end of the three-phase grid-connected inverter 10 receives the DC power generated by the photovoltaic power generation unit 110, and the output end of the three-phase grid-connected inverter 10 is connected to the input end of the power grid 120 to output the AC power to the power grid 120. The three-phase grid-connected inverter 10 includes a DC bus capacitor 11, an AC filter capacitor 12, and a three-phase grid-connected relay 20. In addition, the three-phase grid-connected inverter 10 can also include a DC / DC unit 13 for DC-DC conversion and a DC / AC unit 14 for DC-AC conversion. The DC / DC unit 13 can realize the voltage and current adjustment of DC power; the DC / AC unit 14 is the main part for realizing the DC to AC inversion, which is mainly composed of IGBT devices and other power devices. The DC bus capacitor 11 is a capacitor device connected in parallel between the output side of the DC / DC unit 13 and the input side of the DC / AC unit 14. Specifically, a pair of DC bus capacitors 11 are connected in parallel between the output side of the DC / DC unit 13 and the input side of the DC / AC unit 14. The DC bus capacitor 11 can smooth the bus voltage, reduce voltage noise, etc. The DC / AC unit 14 is usually a three-phase output, and the AC filter capacitor 12 is connected to the output side of the DC / AC unit 14. Specifically, a corresponding number of AC filter capacitors 12 have one end connected to each phase of the output side of the DC / AC unit 14, and the other end is connected together as the midpoint of the AC filter capacitor 12. The AC filter capacitor 12 can reduce the harmonics of the output voltage, balance the output voltage, etc. The midpoint of the DC bus capacitor 11 is connected together with the midpoint of the AC filter capacitor 12, such as Figure 1 and Figure 2As shown, O1 represents the midpoint of the DC bus capacitor 11 inside the three-phase grid-connected inverter 10, and O2 represents the midpoint of the AC filter capacitor 12 inside the three-phase grid-connected inverter 10. O1 and O2 are connected together to reduce the fluctuation of the output voltage. The three-phase grid-connected relay 20 is connected to the output side of the AC filter capacitor 12 and the DC / AC unit 14, and is connected to the input side of the power grid 120. The three-phase grid-connected relay 20 can isolate the three-phase grid-connected inverter 10 from the power grid 120 to provide protection for the power grid 120. Only when the three-phase grid-connected relay 20 is closed and turned on, the AC power output from the output side of the DC / AC unit 14 can enter the power grid 120. The three-phase grid-connected relay 20 is integrally provided with a first three-phase grid-connected relay and a second three-phase grid-connected relay, and the first three-phase grid-connected relay and the second three-phase grid-connected relay are connected in series. The first three-phase grid-connected relay includes three grid-connected relays, and the second three-phase grid-connected relay includes three grid-connected relays. The three grid-connected relays of the second three-phase grid-connected relay are correspondingly connected to the corresponding phases of the input end of the power grid 120. The sampling control unit 30 is the control core of the inverter system 100, which is specifically an IC module or a computer with logic operation and control functions. The sampling control unit 30 is connected to the three-phase grid-connected inverter 10. The sampling control unit 30 can be integrated in the three-phase grid-connected inverter 10, or can be independently set outside the three-phase grid-connected inverter 10, which is not limited here. The sampling control unit 30 is mainly used to control the three-phase grid-connected inverter 10 to connect to the grid for power generation and maintain the normal operation of the inverter system 100. Specifically, the sampling control unit 30 can control the on-off of the three-phase grid-connected relay 20 in the three-phase grid-connected inverter 10. In actual applications, the sampling control unit 30 controls the on-off of the three-phase grid-connected relay 20 by detecting faults and reports the results.

[0031] For the inverter system 100 of the power grid 120 using the IT system, Figure 1 As shown, PV represents a photovoltaic power generation unit 110, which can be a group of photovoltaic panels connected in series to form a string, RCD represents a leakage current Hall, which is located between the AC filter capacitor 12 and the three-phase grid-connected relay 20, and the three-phase grid-connected inverter 10 is connected to the A, B, and C phases of the IT system power grid 120 through the three-phase grid-connected relay 20. Figure 2 As shown in the figure, the solar panel in the system has parasitic capacitance to the ground, which is represented by capacitor C P , capacitor C N It indicates that the grid 120 also has parasitic capacitance to the ground, and the output of the three-phase grid-connected inverter 10 generally has Y capacitance to the ground. Figure 1 and Figure 2 The equivalent capacitor C YA , capacitor C YB , capacitor C YC The AC filter capacitor 12 includes a capacitor C with a "Y" connection.LA , capacitor C LB , capacitor C LC , the DC bus capacitor 11 includes the capacitor C LU and capacitor C LD , DC bus capacitor 11 (capacitor C CU With capacitor C CD ) is connected to the midpoint O1 of the AC filter capacitor 12 (capacitor C LA , capacitor C LB , C LC ) is the midpoint O2. The three-phase grid-connected relay 20 includes a relay K A1 、Relay K B1 、Relay K C1 、Relay K A2 、Relay K B2 、Relay K C2 When one of the three-phase grid-connected relays 20 is closed and another of the three-phase grid-connected relays 20 is disconnected, assuming that the C-phase grid-connected relay (including the relay K C1 、Relay K C2 ) of one of the relays K C1 , disconnect the other relay K in the C phase grid-connected relay C2 If another relay K connected to the C-phase grid-connected relay of the grid 120 C2 When a short circuit occurs, there will be a current loop of "grid 120 to ground PE - ground PE to PV - DC bus capacitor 11 - AC filter capacitor 12 - leakage current Hall RCD - C phase grid-connected relay - grid 120". The equivalent circuit diagram is as follows: Figure 3 The parasitic capacitance of the solar panel to the ground is generally between 50nF and 150nF / kW, the parasitic capacitance of the PV to the ground can reach 500nF to 1500nF, the DC bus capacitor 11 and the AC filter capacitor 12 have a minimum capacity of tens to hundreds of uF, and the parasitic capacitance of the power grid 120 to the ground is relatively small. Even if the inverter is considered for EMC, the output end adds a Y capacitor to the ground, that is, Figure 3 The equivalent capacitance C at the grid end is shown YA , capacitor C YB and capacitor C YC Due to the limitation of system leakage current, the applied AC to ground Y capacitor is generally within a few nF, which is much smaller than the parasitic capacitance of PV to ground PE, DC bus capacitor 11 and AC filter capacitor 12 in the above current loop. Therefore, in the other relay K of the C-phase grid-connected relay connected to the grid 120, C2After a short circuit occurs, the impedance of the C phase of the power grid 120 to the ground will change significantly, thereby causing a significant change in the voltage of the C phase of the power grid 120 to the ground. If the impedance of the C phase of the power grid 120 to the ground is low, even if the C phase grid-connected relay is short-circuited, the voltage of the C phase of the power grid 120 to the ground will not change significantly, but at this time, the leakage current flowing through the C phase grid-connected relay will change significantly. At this time, the fault detection result can be obtained by detecting the leakage current flowing through the C phase grid-connected relay. Therefore, it can be effectively identified whether the three-phase grid-connected relay 20 in the inverter system 100 has a short circuit fault when the power grid 120 system is an IT system.

[0032] For the inverter system 100 using the TN or TT system power grid 120, the N line in the TN system and TT system power grid is directly grounded, such as Figure 4 As shown, here, taking the TN system as an example, when one of the three-phase grid-connected relays 20 is closed and turned on, another three-phase grid-connected relay in the three-phase grid-connected relay 20 is disconnected, assuming that the C-phase grid-connected relay (including relay K C1 、Relay K C2 ) of one of the relays K C1 , disconnect the other relay K in the C phase grid-connected relay C2 If another relay K connected to the C-phase grid-connected relay of the grid 120 C2 When a short circuit occurs, there will be a current loop of "grid 120 to ground PE - ground PE to PV - DC bus capacitor 11 - AC filter capacitor 12 - leakage current Hall RCD - C phase grid-connected relay - grid 120". The equivalent circuit diagram is as follows: Figure 5 The parasitic capacitance of the solar panel to the ground is generally between 50nF and 150nF / kW, the parasitic capacitance of the PV to the ground can reach 500nF to 1500nF, the DC bus capacitor 11 and the AC filter capacitor 12 have a minimum capacity of tens to hundreds of uF, and the parasitic capacitance of the power grid 120 to the ground is relatively small. Even if the inverter is considered for EMC, the output end adds a Y capacitor to the ground, that is, Figure 4 The equivalent capacitance C at the grid end is shown YA , capacitor C YB and capacitor C YC Due to the limitation of system leakage current, the applied AC to ground Y capacitor is generally within a few nF, and its capacity is much smaller than the parasitic capacitance of PV to ground PE, DC bus capacitor 11 and AC filter capacitor 12 in the above current loop. When the three-phase grid-connected relay 20 is short-circuited, such as Figure 5As shown, the voltage to ground of each phase of the power grid 120 will not change, but the leakage current flowing through the three-phase grid-connected relay 20 will change significantly. The leakage current flowing between the grid-connected relay 20 and the AC filter capacitor 12 mainly depends on the equivalent parasitic capacitance of the DC bus to the ground. Generally, the capacitance is more than several hundred nF. When a frequency of 50 Hz is applied to the capacitive reactance, the equivalent leakage current can reach several tens of mA. Therefore, the sampling control unit 30 collects the leakage current flowing between the three-phase grid-connected relay 20 and the AC filter capacitor 12 by collecting current. According to the change of the leakage current between the grid-connected relay 20 and the filter capacitor, it can be judged whether the grid-connected relay 20 in the inverter system 100 of the power grid 200 using the TN and TT systems has a short circuit fault.

[0033] The inverter system of this embodiment can control the on and off of the grid-connected relay through the sampling control unit, and then identify the fault of the grid-connected relay according to the ground voltage of the power grid and the leakage current flowing through the grid-connected relay when the grid-connected relay is on and off, so that the reliability of the system is higher.

[0034] In one embodiment, referring to Figure 1 and Figure 2 , the three-phase grid-connected inverter 10 also includes a current detection unit 40, the current detection unit 40 is connected between the grid-connected relay 20 and the AC filter capacitor 12 to collect the leakage current between the two, and the sampling control unit 30 is connected to the current detection unit 40 to obtain the leakage current. In a specific implementation, the three-phase grid-connected inverter 10 also includes a current detection unit 40, the current detection unit 40 is connected between the grid-connected relay 20 and the AC filter, and the current detection unit 40 can be specifically designed with a Hall element. The current detection unit 40 can collect the leakage current between the grid-connected relay 20 and the AC filter capacitor 12, and after the collection, it is input to the sampling control unit 30 in the form of an output electrical signal, so that the sampling control unit 30 can obtain the leakage current between the grid-connected relay 20 and the AC filter capacitor 12.

[0035] Further, refer to Figure 1 , Figure 2 , Figure 4, the current detection unit 40 includes a leakage current Hall RCD, the leakage current Hall RCD is arranged in the loop between the grid-connected relay 20 and the AC filter capacitor 12, and the sampling control unit 30 is connected to the leakage current Hall RCD. In the specific implementation, the current detection unit 40 is designed with a leakage current Hall RCD, and the leakage current Hall RCD is arranged in the loop between the grid-connected relay 20 and the AC filter capacitor 12. The leakage current Hall RCD is a sensor based on the Hall effect. In practical applications, when leakage current is generated in the loop between the grid-connected relay 20 and the AC filter capacitor 12, a magnetic field change will be generated around the detection element inside the leakage current Hall RCD. The Hall element inside the leakage current Hall RCD senses the magnetic field change and converts the magnetic field change into a corresponding electrical signal and outputs it to the sampling control unit 30. The sampling control unit 30 can accurately obtain the leakage current between the grid-connected relay 20 and the AC filter capacitor 12.

[0036] In one embodiment, referring to Figure 1, the grid-connected relay 20 includes a first three-phase grid-connected relay 21 and a second three-phase grid-connected relay 22, the first three-phase grid-connected relay 21 is connected to the AC filter capacitor 12, the second three-phase grid-connected relay 22 is connected to the first three-phase grid-connected relay 21, the sampling control unit 30 is connected to the first three-phase grid-connected relay 21 and the second three-phase grid-connected relay 22, wherein the second three-phase grid-connected relay 22 is used to access the power grid 120. In a specific implementation, a plurality of groups of three-phase grid-connected relays 20 are provided, each group of grid-connected relays 20 is connected to one of the phases of the power grid 120, each group of grid-connected relays 20 includes a first three-phase grid-connected relay 21 and a second three-phase grid-connected relay 22, the first three-phase grid-connected relay 21 is connected to the AC filter capacitor 12, the second three-phase grid-connected relay 22 is connected to the first three-phase grid-connected relay 21 to form a series connection therewith, and the second three-phase grid-connected relay 22 is used to access the phase input end of the power grid 120. The sampling control unit 30 is connected to the first three-phase grid-connected relay 21 and the second three-phase grid-connected relay 22 to control the on-off of the two. As long as one of the first three-phase grid-connected relay 21 and the second three-phase grid-connected relay 22 is in the disconnected state, there is no power input to the power grid 120. The power grid 120 can only have power input when the first three-phase grid-connected relay 21 and the second three-phase grid-connected relay 22 are both turned on at the same time. The sampling control unit 30 can control the first three-phase grid-connected relay 21 and the second three-phase grid-connected relay 22 separately. The sampling control unit 30 can detect whether a short circuit fault occurs in the first three-phase grid-connected relay 21 and the second three-phase grid-connected relay 22 by controlling the first three-phase grid-connected relay 21 and the second three-phase grid-connected relay 22 to be alternately on and off. When detecting a short-circuit fault of the first three-phase grid-connected relay 21, the sampling control unit 30 controls the switching of the on-off state of the first three-phase grid-connected relay 21 on the premise of controlling the second three-phase grid-connected relay 22 to be turned on; and when detecting a short-circuit fault of the second three-phase grid-connected relay 22, the sampling control unit 30 controls the switching of the on-off state of the second three-phase grid-connected relay 22 on the premise of controlling the first three-phase grid-connected relay 21 to be turned on.

[0037] In one embodiment, referring to Figure 2 and Figure 4 , the three-phase grid-connected inverter 10 also includes a safety capacitor 50, one end of which is connected between the grid-connected relay 20 and the grid 120, and the other end is grounded. In a specific implementation, the three-phase grid-connected inverter 10 also includes a safety capacitor 50, and the number of safety capacitors 50 is multiple. Each phase of the grid 120 is connected to at least one safety capacitor 50, one end of the safety capacitor 50 is connected between the grid-connected relay 20 and the grid 120, and the other end of the safety capacitor 50 is grounded to form a ground connection. Specifically, the safety capacitor 50 is a Y capacitor, such as Figure 3 and Figure 5 As shown, the Y capacitor C YA 、Y capacitor C YB 、Y capacitor C YC By using a Y capacitor as the safety capacitor 50, common mode interference can be effectively suppressed, and the three-phase grid-connected inverter 10 can be improved in terms of its ability to resist grid 120 voltage shock and EMC interference resistance.

[0038] like Figure 6 As shown, the present invention provides a grid-connected relay fault detection method, which is applied to the inverter system described in the above embodiment, and is mainly used to detect a grid-connected relay short-circuit fault in the inverter system. The method includes steps: S110-S140.

[0039] S110 , controlling one of the first three-phase grid-connected relay and the second three-phase grid-connected relay to be closed, and the other three-phase grid-connected relay to be opened.

[0040] In a specific implementation, the fault detection of the inverter system is mainly performed in the stage when the system performs self-inspection on the grid-connected relay. The sampling control unit will control one of the first three-phase grid-connected relay and the second three-phase grid-connected relay to be closed, and the other three-phase grid-connected relay to be disconnected.

[0041] S120, obtaining the voltage of the power grid to ground and the leakage current flowing through the three-phase grid-connected relay.

[0042] In a specific implementation, the sampling control unit respectively obtains the voltage to ground of the power grid before one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed and the other three-phase grid-connected relay is disconnected, and after one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed and the other three-phase grid-connected relay is disconnected, that is, the sampling control unit collects the voltage to ground of the power grid before one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed and the other three-phase grid-connected relay is disconnected, and also collects the voltage to ground of the power grid after one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed.

[0043] The state before one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed and the other three-phase grid-connected relay is disconnected refers to the state where both the first three-phase grid-connected relay and the second three-phase grid-connected relay are disconnected.

[0044] In a specific implementation, a leakage current Hall is set between the AC filter capacitor of the photovoltaic relay and the grid-connected relay to collect the leakage current between the grid-connected relay and the AC filter capacitor, and the leakage current Hall is connected to the sampling control unit. The sampling control unit obtains the leakage current between the grid-connected relay and the AC filter capacitor through the leakage current Hall before one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed and the other three-phase grid-connected relay is disconnected, and after one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed and the other three-phase grid-connected relay is disconnected. Specifically, the sampling control unit obtains the leakage current between the grid-connected relay and the AC filter capacitor through the leakage current Hall before one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed and the other three-phase grid-connected relay is disconnected. After one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed and the other three-phase grid-connected relay is disconnected, the leakage current between the grid-connected relay and the AC filter capacitor is also obtained through the leakage current Hall.

[0045] S130. Obtain a fault detection result according to the voltage of the power grid to ground and the leakage current flowing through the three-phase grid-connected relay.

[0046] In a specific implementation, for an inverter system of a power grid using an IT system, if the ground impedance of the power grid is small, even if the grid-connected relay is short-circuited, the ground voltage of the power grid 120 will not change significantly, but the leakage current flowing through the grid-connected relay will change significantly at this time, and the fault detection result can be obtained by detecting the leakage current flowing through the grid-connected relay. For an inverter system of a power grid using a TN system or a TT system, since the N line of the power grid of the TN system and the TT system is grounded, when the grid-connected relay is short-circuited, the ground voltage of each phase of the power grid will not change, but the leakage current flowing through the leakage current hall will change significantly. Therefore, for the inverter system of the power grid using any of the three systems of TN, TT, and IT, after obtaining the power grid's ground voltage and the leakage current flowing through the three-phase grid-connected relay before one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed and the other three-phase grid-connected relay is disconnected, and after one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed and the other three-phase grid-connected relay is disconnected, the sampling control unit can determine whether the grid-connected relay is short-circuited through the changes in the ground voltage of the power grid before and after and the changes in the leakage current flowing through the three-phase grid-connected relay, thereby obtaining the fault detection result.

[0047] In one embodiment, if Figure 7As shown, the step S130 includes steps: S131-S133.

[0048] S131. Compare the difference between the voltage of the power grid to the ground when one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed and before the other three-phase grid-connected relay is disconnected and when one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed and after the other three-phase grid-connected relay is disconnected with a preset threshold voltage value.

[0049] In a specific implementation, after the sampling control unit obtains the voltage to ground of the power grid when one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed and the other three-phase grid-connected relay is disconnected, and after one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed and the other three-phase grid-connected relay is disconnected, the voltage to ground of the power grid before and after is subtracted to calculate the difference in the voltage to ground of the power grid, the difference in the voltage to ground of the power grid represents the amount of change in the voltage to ground of the power grid, and the difference in the voltage to ground of the power grid is compared with a preset threshold voltage value, and the preset threshold voltage value is used as a comparison standard, so as to determine whether the voltage to ground of the power grid changes significantly between the two states when one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed and the other three-phase grid-connected relay is disconnected, and after one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed and the other three-phase grid-connected relay is disconnected, so as to determine whether the grid-connected relay in the disconnected state is in the short-circuit state.

[0050] S132: Determine whether the difference between the voltages of the power grid to the ground is greater than or equal to the preset threshold voltage value.

[0051] In a specific implementation, the difference in the voltage of the power grid to the ground is compared with a preset threshold voltage value to determine whether the difference in the voltage of the power grid to the ground is greater than or equal to the preset threshold voltage value. It can be obtained whether the change in the voltage of the power grid to the ground is obvious compared with the preset threshold voltage value in the two states: before one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed and the other three-phase grid-connected relay is disconnected, and after one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed and the other three-phase grid-connected relay is disconnected, thereby determining whether a short circuit fault occurs in the grid-connected relay.

[0052] S133: If the difference between the voltages of the power grid to the ground is greater than or equal to a preset threshold voltage value, it is determined that another three-phase grid-connected relay in the disconnected state has a short-circuit fault.

[0053] In a specific implementation, if the impedance of the power grid to the ground is small, one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed, and the other three-phase grid-connected relay is disconnected before the other three-phase grid-connected relay is disconnected, and one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed, and the other three-phase grid-connected relay is disconnected. In the case of a short circuit in the grid-connected relay, if the impedance of the power grid to the ground is large, one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed, and the other three-phase grid-connected relay is disconnected before the other three-phase grid-connected relay is disconnected, and one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed, and the other three-phase grid-connected relay is disconnected. The voltage to the ground of the power grid will change significantly in two states. The preset threshold voltage value is designed according to the actual detection requirements. If the difference in the voltage to the ground of the power grid is greater than or equal to the preset threshold voltage value, it can be determined that the grid-connected relay has a short circuit fault. If the difference in the voltage to the ground of the power grid is less than the preset threshold voltage value, it can be determined that the grid-connected relay is normal and no short circuit fault has occurred. Specifically, taking the detection of the grid-connected relay connected to the C phase of the grid as an example, when the grid-connected relay is disconnected, the sampling control unit collects the voltage of the C phase grid to the ground as U C1 , and then close one of the relays K of the C-phase grid-connected relay connected to the grid 120 C1 , disconnect the other relay K in the C phase grid-connected relay C2 The sampling control unit collects the voltage of the C phase grid to ground as U C2 , using the absolute value of the two voltage differences |U C1 -U C2 | is compared with the preset threshold current value Uth. C1 -U C2 |≥Uth, it can be determined that the grid-connected relay K connected to the grid phase C C2 Is there a short circuit fault? C1 -U C2 |<Uth, it can be determined that the grid-connected relay K connected to the grid phase C C2 Normal. Among them, when the value of the preset threshold voltage value Uth is not considered, its minimum value can be 0. When the actual voltage sampling accuracy is taken into account, its minimum value can be the minimum identification voltage of the sampling accuracy. When the preset threshold voltage value is larger, in the face of |U C1 -U C2 When the change is not obvious, there may be a possibility of missed recognition, so the optimal value of the preset threshold voltage value is the minimum recognition voltage value of the actual sampling circuit accuracy.

[0054] In one embodiment, if Figure 8As shown, the step S130 also includes steps: S134-S136.

[0055] S134. Compare the difference between the leakage current flowing through the three-phase grid-connected relay when one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed and before the other three-phase grid-connected relay is disconnected and when one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed and after the other three-phase grid-connected relay is disconnected with a preset threshold current value.

[0056] In a specific implementation, the sampling control unit obtains the leakage current flowing through the grid-connected relay through the leakage current Hall when one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed and the other three-phase grid-connected relay is disconnected, and when one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed and the other three-phase grid-connected relay is disconnected, and then the leakage current flowing through the three-phase grid-connected relay before and after is subtracted to calculate the difference, which represents the change amount of the leakage current flowing through the grid-connected relay, and the difference of the leakage current flowing through the grid-connected relay is compared with the preset threshold current value, and the preset threshold current value is used as the comparison standard, so as to judge whether the leakage current flowing through the grid-connected relay changes significantly between the two states when one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed and the other three-phase grid-connected relay is disconnected, and when one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed and the other three-phase grid-connected relay is disconnected, so as to judge whether the grid-connected relay is in a short-circuit state.

[0057] S135. Determine whether the difference between the leakage currents is greater than or equal to the preset threshold current value.

[0058] In a specific implementation, the difference of the leakage current flowing through the grid-connected relay is compared with a preset threshold current value to determine whether the difference of the leakage current flowing through the grid-connected relay is greater than or equal to the preset threshold current value. It can be obtained whether the change of the leakage current flowing through the grid-connected relay is obvious compared with the preset threshold current value in two states: before one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed and the other three-phase grid-connected relay is disconnected, and after one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed and the other three-phase grid-connected relay is disconnected, thereby determining whether a short-circuit fault occurs in the grid-connected relay.

[0059] S136: If the difference between the leakage currents is greater than or equal to the preset threshold current value, it is determined that another three-phase grid-connected relay in the disconnected state has a short circuit fault.

[0060] In a specific implementation, a preset threshold current value is designed according to actual detection requirements. If one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed and the other three-phase grid-connected relay is disconnected before the other three-phase grid-connected relay is disconnected, and if one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed and the other three-phase grid-connected relay is disconnected, the difference in leakage current flowing through the grid-connected relay and the AC filter capacitor is greater than or equal to the preset threshold current value, it can be determined that the grid-connected relay in the disconnected state has a short-circuit fault. If the difference in leakage current flowing through the grid-connected relay before and after is less than the preset threshold current value, it can be determined that the grid-connected relay is normal and no short-circuit fault has occurred. Specifically, taking the detection of the grid-connected relay connected to the C phase of the power grid as an example, when the grid-connected relay is disconnected, the sampling control unit collects the leakage current between the grid-connected relay and the AC filter capacitor as I C1 , and then close one of the relays K of the C-phase grid-connected relay connected to the grid 120 C1 , disconnect the other relay K in the C phase grid-connected relay C2 The sampling control unit collects the leakage current flowing through the grid-connected relay as I C2 , using the absolute value of the difference between the two currents |I C1 -I C2 | is compared with the preset threshold current value Ith. When |I C1 -I C2 |≥Ith, it can be determined that the grid-connected relay K connected to the grid phase C C2 Is there a short circuit fault? C1 -I C2 |<Ith, the grid-connected relay K can be determined C2 Normal. Among them, the preset threshold current value comparison threshold difference minimum value can be 0, and when the actual sampling accuracy is considered, the minimum value of the preset threshold current value can be the lowest lower limit value that can be recognized by the actual leakage current Hall sampling accuracy.

[0061] The grid-connected relay fault detection method of the present application obtains the ground voltage of the power grid and the leakage current flowing through the three-phase grid-connected relay by controlling the three-phase grid-connected relay, and obtains the fault detection result according to the ground voltage of the power grid and the leakage current flowing through the three-phase grid-connected relay. It does not rely on the type of power grid and can accurately and effectively identify the short-circuit fault of the grid-connected relay in the inverter system, thereby improving the reliability of the system.

[0062] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can easily think of various equivalent modifications or replacements within the technical scope disclosed by the present invention, and these modifications or replacements should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention shall be based on the protection scope of the claims.

Claims

1. A grid-connected relay fault detection method for an inverter system, characterized in that: The inverter system includes a three-phase grid-connected inverter, a positive DC bus, a negative DC bus and a bus midpoint arranged on the DC side of the three-phase grid-connected inverter, a positive bus capacitor connected between the positive DC bus and the bus midpoint, a negative bus capacitor connected between the negative DC bus and the bus midpoint, an AC filter capacitor and a three-phase grid-connected relay arranged on the AC side of the three-phase grid-connected inverter, the AC filter capacitor is star-connected and the center point of the star connection is connected to the bus midpoint, and the three-phase grid-connected relay includes a first three-phase grid-connected relay and a second three-phase grid-connected relay connected in series; the grid-connected relay fault detection method includes: Controlling one of the first three-phase grid-connected relay and the second three-phase grid-connected relay to be closed and the other three-phase grid-connected relay to be opened; Obtaining the voltage of the power grid to ground and the leakage current flowing through the three-phase grid-connected relay; A fault detection result is obtained according to the voltage of the power grid to ground and the leakage current flowing through the three-phase grid-connected relay.

2. The grid-connected relay fault detection method according to claim 1, characterized in that: The step of controlling one of the first three-phase grid-connected relay and the second three-phase grid-connected relay to be closed and the other three-phase grid-connected relay to be opened comprises: Control at least one grid-connected relay of one of the first three-phase grid-connected relay and the second three-phase grid-connected relay to be closed, and at least another grid-connected relay of the other three-phase grid-connected relay to be opened, and the at least one grid-connected relay and the at least another grid-connected relay are connected in series accordingly.

3. The grid-connected relay fault detection method according to claim 1, characterized in that: The step of obtaining the fault detection result according to the ground voltage of the power grid and the leakage current flowing through the three-phase grid-connected relay comprises: According to the situation that one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed and the other three-phase grid-connected relay is disconnected, and the situation that one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed and the other three-phase grid-connected relay is disconnected, the difference between the voltage to ground of the power grid is compared with a preset threshold voltage value; Determining whether the difference between the voltages of the power grid to the ground is greater than or equal to the preset threshold voltage value; If the difference between the voltages of the power grid to the ground is greater than or equal to a preset threshold voltage value, it is determined that another three-phase grid-connected relay in the disconnected state has a short-circuit fault.

4. The grid-connected relay fault detection method according to claim 1, characterized in that: The step of obtaining the fault detection result according to the ground voltage of the power grid and the leakage current flowing through the three-phase grid-connected relay comprises: According to the difference between the leakage current flowing through the three-phase grid-connected relay when one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed and before the other three-phase grid-connected relay is disconnected and after the one of the first three-phase grid-connected relay and the second three-phase grid-connected relay is closed and the other three-phase grid-connected relay is disconnected, the difference is compared with a preset threshold current value; Determining whether the difference between the leakage currents is greater than or equal to the preset threshold current value; If the difference between the leakage currents is greater than or equal to the preset threshold current value, it is determined that another three-phase grid-connected relay in the disconnected state has a short circuit fault.

5. An inverter system, characterized in that: The system includes: a three-phase grid-connected inverter, a positive DC bus, a negative DC bus and a bus midpoint arranged on the DC side of the three-phase grid-connected inverter, a positive bus capacitor connected between the positive DC bus and the bus midpoint, a negative bus capacitor connected between the negative DC bus and the bus midpoint, an AC filter capacitor and a three-phase grid-connected relay arranged on the AC side of the three-phase grid-connected inverter, the AC filter capacitor is star-connected and the center point of the star connection is connected to the bus midpoint, and the three-phase grid-connected relay includes a first three-phase grid-connected relay and a second three-phase grid-connected relay connected in series; the system also includes a sampling control unit connected to the three-phase grid-connected inverter and used to control the three-phase grid-connected inverter to generate power in grid connection.

6. The system according to claim 5, characterized in that The three-phase grid-connected inverter also includes a current detection unit, which is connected to the leakage current branch flowing through the three-phase grid-connected relay to collect the leakage current flowing through the three-phase grid-connected relay, and the sampling control unit is connected to the current detection unit to obtain the leakage current.

7. The system according to claim 6, characterized in that The current detection unit includes a leakage current Hall, which is arranged on a leakage current branch flowing through the three-phase grid-connected relay, and the sampling control unit is connected to the leakage current Hall.

8. The system according to any one of claims 5 to 7, characterized in that: The three-phase grid-connected relay includes a first three-phase grid-connected relay and a second three-phase grid-connected relay, the first three-phase grid-connected relay is connected to the AC filter capacitor, the second three-phase grid-connected relay is connected to the first three-phase grid-connected relay, and the sampling control unit is connected to the first three-phase grid-connected relay and the second three-phase grid-connected relay, wherein the second three-phase grid-connected relay is used to access the power grid.

9. The system according to any one of claims 5 to 7, characterized in that: The system further comprises a safety capacitor, one end of which is connected between the three-phase grid-connected relay and the power grid, and the other end of which is grounded.

10. The system according to claim 9, characterized in that The safety capacitor is a Y capacitor.

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