Three-phase grid-connected inverter system and grid-connected switch failure detection method thereof

Abstract

The invention discloses a three-phase grid-connected inverter system and a grid-connected switch failure detection method thereof. The grid-connected switch failure detection method comprises the following steps: controlling one of a first three-phase grid-connected switch and a second three-phase grid-connected switch to be switched on, and controlling the other three-phase grid-connected switch to be switched off; and based on the three-phase current flowing through the three-phase grid-connected switch, determining whether another three-phase grid-connected switch in the off state fails or not. Whether the three-phase grid-connected switch in the off state fails or not is determined through the three-phase current flowing through the three-phase grid-connected switch, the problem that the failure of the grid-connected relay cannot be detected due to the fact that a hardware circuit for multi-path voltage detection is abnormal is avoided, and the detection accuracy is improved.

Description

Technical Field

[0001] The present application relates to the technical field of grid-connected inverters, and in particular to a three-phase grid-connected inverter system and a grid-connected switch failure detection method thereof. Background Art

[0002] In a photovoltaic power generation system, a grid-connected inverter is used to convert the DC power output by the solar panel into AC power and feed it into the grid. According to the relevant safety regulations of the grid-connected inverter, in order to ensure that the grid-connected inverter can be quickly disconnected from the grid when a fault occurs, the grid-connected switch, such as the grid-connected relay, needs a redundant design, that is, two sets of master and slave relays are required. Once the contacts of the grid-connected relay are stuck or short-circuited, it cannot be disconnected in time when the grid-connected inverter fails, resulting in the grid-connected inverter being unable to cut off the connection with the grid, and ultimately causing the fault to expand. For this reason, it is necessary to detect the failure of the grid-connected relay caused by contact sticking or short-circuiting before the grid-connected inverter is connected to the grid.

[0003] The existing method for judging the failure of grid-connected relays is usually based on the difference between the inverter voltage and the grid voltage. This judgment method has many problems. For example, it is impossible to identify the failure of a single grid-connected relay in principle, and when the inverter voltage and the grid voltage sampling circuit are abnormal, it may even cause a two-phase or three-phase relay short circuit to be unable to be identified. In addition, there is another method that collects the voltage value on the voltage divider between the positive DC bus or the negative DC bus and the ground, extracts the AC voltage therein and calculates its effective value, and then determines the size of the AC voltage effective value to confirm whether there is a single-phase relay short circuit. However, this solution requires at least one power grid power frequency cycle to extract the AC voltage component and then calculate the effective value, which takes too long and is not conducive to achieving the rapid detection and protection requirements of single-phase relay short circuits. Summary of the invention

[0004] In view of this, the purpose of the present application is to provide a three-phase grid-connected inverter system and a grid-connected switch failure detection method thereof, so as to avoid the problem that the failure of the grid-connected relay cannot be detected due to an abnormality in the hardware circuit of the multi-channel voltage detection.

[0005] The technical solutions adopted by this application to solve the above technical problems are as follows:

[0006] On one hand, the present application provides a method for detecting failure of a grid-connected switch of a three-phase grid-connected inverter system, wherein the three-phase grid-connected inverter system comprises a three-phase grid-connected inverter, a positive DC bus, a negative DC bus and a bus midpoint arranged on a 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, a filter unit and a three-phase grid-connected switch arranged on an AC side of the three-phase grid-connected inverter, the filter unit comprising a star-connected filter capacitor and a center point of the star connection connected to the bus midpoint, the three-phase grid-connected switch comprising a first three-phase grid-connected switch and a second three-phase grid-connected switch connected in series, and a current sampling module for collecting three-phase currents flowing through the three-phase grid-connected switch;

[0007] The grid-connected switch failure detection method comprises:

[0008] Controlling one of the first three-phase grid-connected switch and the second three-phase grid-connected switch to be closed, and the other three-phase grid-connected switch to be opened;

[0009] Based on the three-phase current flowing through the three-phase grid-connected switch, it is determined whether another three-phase grid-connected switch in an open state fails.

[0010] In one example, determining whether another three-phase grid-connected switch in an off state fails includes:

[0011] If the currents of at least two phases of the three-phase currents flowing through the three-phase grid-connected switch exceed a preset current threshold, it is determined that another three-phase grid-connected switch in the disconnected state fails.

[0012] In one example, determining that another three-phase grid-connected switch in an open state fails includes:

[0013] If the current of two phases of the three-phase current flowing through the three-phase grid-connected switch exceeds the preset current threshold, it is determined that the grid-connected switch of two phases in another three-phase grid-connected switch in the disconnected state fails;

[0014] If the current of each phase of the three-phase current flowing through the three-phase grid-connected switch exceeds the preset current threshold, it is determined that the grid-connected switch of each phase in another three-phase grid-connected switch in the disconnected state has failed.

[0015] In one example, the preset current threshold is greater than a zero drift value of the current sampling module.

[0016] In one example, determining whether another three-phase grid-connected switch in an open state fails based on the three-phase current flowing through the three-phase grid-connected switch includes:

[0017] Based on the absolute value of the instantaneous value of the three-phase current flowing through the three-phase grid-connected switch or the effective value of the three-phase current, it is determined whether another three-phase grid-connected switch in the disconnected state fails.

[0018] In one example, the three-phase grid-connected inverter system further includes a voltage sampling module and a voltage divider, wherein the voltage sampling module is used to collect the voltage across the voltage divider, and the voltage divider is connected between the positive DC bus and the ground or between the negative DC bus and the ground;

[0019] The grid-connected switch failure detection method further includes:

[0020] If the currents of at least two phases of the three-phase currents flowing through the three-phase grid-connected switch do not exceed the preset current threshold, it is determined whether another three-phase grid-connected switch in the disconnected state fails based on the voltage change across the voltage divider.

[0021] In one example, the step of determining again whether another three-phase grid-connected switch in an off state fails includes:

[0022] If the voltage change across the voltage divider exceeds a preset voltage change, it is determined that one phase of the other three-phase grid-connected switch in the disconnected state has failed; otherwise, it is determined that the other three-phase grid-connected switch in the disconnected state has not failed.

[0023] In one example, the voltage sampling module is configured to collect the voltage across the voltage divider during a preset time period.

[0024] In one example, the grid-connected switch failure detection method further includes:

[0025] Switching states of the three-phase grid-connected switches are switched and determining whether one of the three-phase grid-connected switches in an off state fails.

[0026] On the other hand, the present application provides a three-phase grid-connected inverter system, the three-phase grid-connected 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, a filter unit and a three-phase grid-connected switch arranged on the AC side of the three-phase grid-connected inverter, the filter unit includes a star-connected filter capacitor and the center point of the star connection is connected to the bus midpoint, the three-phase grid-connected switch includes a first three-phase grid-connected switch and a second three-phase grid-connected switch connected in series, and a current sampling module for collecting three-phase currents flowing through the three-phase grid-connected switch;

[0027] The three-phase grid-connected inverter system also includes a control unit, which is configured to control one of the first three-phase grid-connected switch and the second three-phase grid-connected switch to be closed and the other three-phase grid-connected switch to be opened; based on the three-phase current flowing through the three-phase grid-connected switch, determine whether the other three-phase grid-connected switch in the disconnected state fails.

[0028] The three-phase grid-connected inverter system and the grid-connected switch failure detection method provided in the embodiment of the present application determine whether the three-phase grid-connected switch in the disconnected state has failed by means of the three-phase current flowing through the three-phase grid-connected switch, thereby avoiding the problem that the failure of the grid-connected relay cannot be detected due to an abnormality in the hardware circuit of the multi-channel voltage detection, and improving the accuracy of the detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 A schematic diagram of a three-phase grid-connected inverter system provided in an embodiment of the present application;

[0030] Figure 2 A schematic diagram of a grid-connected switch failure detection method for a three-phase grid-connected inverter system provided in an embodiment of the present application;

[0031] Figure 3 A schematic diagram of current flow of a three-phase grid-connected inverter system provided in an embodiment of the present application when a three-phase grid-connected switch fails;

[0032] Figure 4 Another schematic diagram of current flow direction of the three-phase grid-connected inverter system provided in an embodiment of the present application when the three-phase grid-connected switch fails;

[0033] Figure 5 Another schematic diagram of current flow in the three-phase grid-connected inverter system provided in an embodiment of the present application when the three-phase grid-connected switch fails.

[0034] The realization of the purpose, functional features and advantages of this application will be further explained in conjunction with embodiments and with reference to the accompanying drawings. DETAILED DESCRIPTION

[0035] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present application clearer and more understandable, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0036] like Figure 1As shown, the three-phase grid-connected inverter system provided in the embodiment of the present application includes a three-phase grid-connected inverter, a positive DC bus BUS+, a negative DC bus BUS- and a bus midpoint O1 arranged on the DC side of the three-phase grid-connected inverter, a positive bus capacitor C1 connected between the positive DC bus BUS+ and the bus midpoint O1, a negative bus capacitor C2 connected between the negative DC bus BUS- and the bus midpoint O1, a filter unit and a three-phase grid-connected switch arranged on the AC side of the three-phase grid-connected inverter, and the filter unit includes a three-phase filter inductor (L1 to L2 in the figure). 3) and a three-phase filter capacitor (shown in Cf1~Cf3 in the figure), the three-phase filter capacitor is star-connected and the center point O2 of the star connection is connected to the bus midpoint O1, the three-phase grid-connected switch includes a first three-phase grid-connected switch S1 and a second three-phase grid-connected switch S2 connected in series, a voltage divider CX connected between the negative DC bus BUS- and the ground PE, a current sampling module for collecting the three-phase current (shown in Ia, Ib, Ic in the figure) flowing through the three-phase grid-connected switch, and a voltage sampling module (not shown in the figure) for collecting the voltage across the voltage divider CX.

[0037] Among them, the topological structure of the three-phase grid-connected inverter is not limited, and the figure shows a three-level topological structure. The first three-phase grid-connected switch S1 and the second three-phase grid-connected switch S2 both include three-phase independent relays, for example, the first three-phase grid-connected switch S1 includes three-phase independent relays K1, K3, K5, and the second three-phase grid-connected switch S2 includes three-phase independent relays K2, K4, K6, and each group of three-phase independent relays is driven together, and the relays between the same phases are connected in series, for example, relay K1 is connected in series with relay K2. The current sampling module is arranged between the filtering unit and the three-phase grid-connected switch.

[0038] It should be noted that the voltage divider CX can be in the form of a capacitor or a resistor, etc., and is not limited thereto. In this embodiment, only a voltage divider in the form of a capacitor is used as an example. In other examples, it is also feasible that the voltage divider CX is connected between the positive DC bus BUS+ and the ground PE.

[0039] In one example, the three-phase grid-connected inverter system further includes a control unit (not shown in the figure), and the control unit can be configured to execute a grid-connected switch failure detection method.

[0040] like Figure 2 As shown, in one example, the grid-connected switch failure detection method includes the following steps:

[0041] S11, controlling one of the first three-phase grid-connected switch and the second three-phase grid-connected switch to be closed, and the other three-phase grid-connected switch to be opened;

[0042] S12: Determine whether another three-phase grid-connected switch in an open state fails based on the three-phase current flowing through the three-phase grid-connected switch.

[0043] In one example, determining whether another three-phase grid-connected switch in an open state fails based on the three-phase current flowing through the three-phase grid-connected switch includes:

[0044] Based on the absolute value of the instantaneous value of the three-phase current flowing through the three-phase grid-connected switch or the effective value of the three-phase current, it is determined whether another three-phase grid-connected switch in the disconnected state fails.

[0045] In the preferred implementation, in order to achieve rapid detection and protection requirements for relay failure, it is recommended to use the absolute value of the instantaneous value of the current of the corresponding phase to determine whether another three-phase grid-connected switch in the disconnected state has failed. Other forms such as effective value can also be used. However, no matter what form it is, the information it represents is that there is current flowing in the grid-connected circuit where the grid-connected switch is located.

[0046] In one example, determining whether another three-phase grid-connected switch in an off state fails includes:

[0047] If the currents of at least two phases of the three-phase currents flowing through the three-phase grid-connected switch exceed a preset current threshold, it is determined that another three-phase grid-connected switch in the disconnected state fails.

[0048] Specifically, if the current of two phases of the three-phase current flowing through the three-phase grid-connected switch exceeds the preset current threshold, it is determined that the grid-connected switch of two phases in another three-phase grid-connected switch in the disconnected state fails;

[0049] If the current of each phase of the three-phase current flowing through the three-phase grid-connected switch exceeds the preset current threshold, it is determined that the grid-connected switch of each phase in another three-phase grid-connected switch in the disconnected state has failed.

[0050] In order to avoid false detection of relay failure and ensure reliable detection in the event of a two-phase or three-phase relay failure, it is recommended that the preset current threshold value be slightly larger than the zero drift value of the current sampling module.

[0051] The following combination Figure 3-Figure 4 To explain:

[0052] The second three-phase grid-connected switch S2 is controlled to be closed and the first three-phase grid-connected switch S1 is controlled to be opened.

[0053] If all three grid-connected switches K1, K3, and K5 in the first three-phase grid-connected switch S1 in the disconnected state are stuck or short-circuited, since the voltage of the filter capacitor Cfx is not of the same frequency but the same phase as the grid voltage, and the bus capacitors C1 and C2 are not soft-started by voltage, current will flow through the three-phase grid-connected switches K2, K4, and K6 in the second three-phase grid-connected switch S2 at the moment of closing, and the following will be formed: Figure 3 The uncontrolled rectifier circuit shown in the figure is specifically a grid-connected switch, a filter inductor, a body diode of the upper bridge arm switch tube of the inverter, positive and negative bus capacitors, and a body diode of the lower bridge arm switch tube of the inverter, and finally flows back to the grid. Under this working condition, if each phase current of the three-phase current collected by the current sampling module is greater than or equal to the preset current threshold, it is determined that the three grid-connected switches K1, K3, and K5 in the first three-phase grid-connected switch S1 that should be in the disconnected state have all failed.

[0054] If two of the three-phase grid-connected switches in the first three-phase grid-connected switch S1 in the disconnected state fail, for example, K1 and K5 fail due to adhesion or short circuit, at the moment when the three grid-connected switches K2, K4, and K6 in the second three-phase grid-connected switch S2 are closed, current will still flow through the grid-connected switches corresponding to phases A and C, and the following will be formed: Figure 4 In this working condition, the current sampling module collects two-phase currents Ia and Ic in the three-phase current greater than or equal to the preset current threshold, so it is determined that the grid-connected switches K1 and K5 in the first three-phase grid-connected switch S1 that should be in the disconnected state have failed.

[0055] In one example, the grid-connected switch failure detection method further includes:

[0056] If the currents of at least two phases of the three-phase currents flowing through the three-phase grid-connected switch do not exceed the preset current threshold, it is determined whether another three-phase grid-connected switch in the disconnected state fails based on the voltage change across the voltage divider.

[0057] By detecting the voltage change of the voltage divider, it is possible to quickly identify whether there is a short-circuit fault in the single-phase relay, thereby solving the problem of long short-circuit detection cycle of single-phase relays in traditional solutions, thereby achieving rapid detection and protection requirements for single-phase relay short circuits.

[0058] In one example, the step of determining again whether another three-phase grid-connected switch in an off state fails includes:

[0059] If the voltage change across the voltage divider exceeds a preset voltage change, it is determined that one phase of the other three-phase grid-connected switch in the disconnected state has failed; otherwise, it is determined that the other three-phase grid-connected switch in the disconnected state has not failed.

[0060] Wherein, the voltage sampling module is configured to collect the voltage across the voltage divider in a preset time period. In order to achieve the rapid detection and protection requirements of single-phase relay short circuit, the preset time period is recommended to be set to 1 / 4 of the power frequency cycle of the power grid. The voltage change can be obtained in the following way: the voltage within the preset time period is divided into N equal parts according to a fixed time interval, and the voltage corresponding to each time node is subtracted from the voltage corresponding to the previous time node to obtain the corresponding voltage change. It can be understood that when the duration of adjacent time nodes is known, it is also feasible to make a judgment based on the corresponding voltage change rate.

[0061] The following combination Figure 5 To explain:

[0062] If one phase of the three grid-connected switches K1, K3, and K5 in the first three-phase grid-connected switch S1 in the disconnected state is stuck or short-circuited, for example, the grid-connected switch K1 is stuck or short-circuited, then an AC current will be emitted from phase A of the power grid, and will pass through the grid-connected switch K2 of the three-phase grid-connected switch S2, the grid-connected switch K1 of the three-phase grid-connected switch S1, the filter capacitor Cf1, the center point O2 of the filter capacitor, the bus midpoint O1, the negative bus capacitor C2, the voltage divider CX, and finally flow into the ground. Figure 5 The AC current loop shown by the dotted arrow. At this time, the voltage at both ends of the voltage divider CX, in addition to the original DC voltage, will also be superimposed with an AC voltage, and this AC voltage cycle is consistent with the grid voltage cycle. It can be understood that the voltage at both ends of the voltage divider CX, after superimposing the AC voltage of the power frequency, will be much higher than the voltage change corresponding to the pure DC voltage when the grid-connected switch K1 is not adhered, because considering that the DC voltage at both ends of the voltage divider CX is related to the component voltage and the insulation impedance to the ground, and the component voltage is related to the component temperature, the DC voltage at both ends of the voltage divider CX will not suddenly change in a short time, so in the absence of a single-phase relay short circuit, the voltage change at both ends of the voltage divider CX is almost 0 in the preset time period recommended in this embodiment, that is, 1 / 4 of the power frequency cycle of the grid. Therefore, it is possible to avoid false detection of relay short circuit failure, and ensure that it can be reliably detected in the event of a single-phase relay short circuit. In general, the preset voltage change is recommended to be slightly larger than the voltage change difference caused by temperature change of the photovoltaic panel during the detection period when the single-phase relay does not fail.

[0063] In one example, the grid-connected switch failure detection method further includes:

[0064] Switching states of the three-phase grid-connected switches are switched and determining whether one of the three-phase grid-connected switches in an off state fails.

[0065] In this example, the switching state of the three-phase grid-connected switch is switched, that is, one of the first three-phase grid-connected switch and the second three-phase grid-connected switch is controlled to be disconnected, and the other three-phase grid-connected switch is controlled to be closed. And firstly, based on the three-phase current flowing through the three-phase grid-connected switch, it is determined whether one of the three-phase grid-connected switches in the disconnected state fails. If the current of at least two phases of the three-phase current flowing through the three-phase grid-connected switch does not exceed the preset current threshold, then based on the voltage change at both ends of the voltage divider, it is determined whether one of the three-phase grid-connected switches in the disconnected state fails. At this time, if one of the three-phase grid-connected switches in the disconnected state has not failed, it is determined that both three-phase grid-connected switches have not failed.

[0066] The preferred embodiments of the present application are described above with reference to the accompanying drawings, but the scope of the present application is not limited thereby. Any modification, equivalent substitution and improvement made by a person skilled in the art without departing from the scope and essence of the present application shall be within the scope of the present application.

Claims

1. A method for detecting failure of a grid-connected switch in a three-phase grid-connected inverter system, characterized in that: The three-phase grid-connected 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, a filter unit and a three-phase grid-connected switch arranged on the AC side of the three-phase grid-connected inverter, the filter unit includes a star-connected filter capacitor and the center point of the star connection is connected to the bus midpoint, the three-phase grid-connected switch includes a first three-phase grid-connected switch and a second three-phase grid-connected switch connected in series, and a current sampling module for collecting three-phase currents flowing through the three-phase grid-connected switch; The grid-connected switch failure detection method comprises: Controlling one of the first three-phase grid-connected switch and the second three-phase grid-connected switch to be closed, and the other three-phase grid-connected switch to be opened; Based on the three-phase current flowing through the three-phase grid-connected switch, it is determined whether another three-phase grid-connected switch in an open state fails.

2. The grid-connected switch failure detection method according to claim 1, characterized in that: The determining whether another three-phase grid-connected switch in the disconnected state fails comprises: If the currents of at least two phases of the three-phase currents flowing through the three-phase grid-connected switch exceed a preset current threshold, it is determined that another three-phase grid-connected switch in the disconnected state fails.

3. The grid-connected switch failure detection method according to claim 2, characterized in that: The determining that another three-phase grid-connected switch in the disconnected state fails comprises: If the current of two phases of the three-phase current flowing through the three-phase grid-connected switch exceeds the preset current threshold, it is determined that the grid-connected switch of two phases in another three-phase grid-connected switch in the disconnected state fails; If the current of each phase of the three-phase current flowing through the three-phase grid-connected switch exceeds the preset current threshold, it is determined that the grid-connected switch of each phase in another three-phase grid-connected switch in the disconnected state has failed.

4. The grid-connected switch failure detection method according to claim 2, characterized in that: The preset current threshold is greater than the zero drift value of the current sampling module.

5. The grid-connected switch failure detection method according to claim 1, characterized in that: The determining whether another three-phase grid-connected switch in an off state fails based on the three-phase current flowing through the three-phase grid-connected switch comprises: Based on the absolute value of the instantaneous value of the three-phase current flowing through the three-phase grid-connected switch or the effective value of the three-phase current, it is determined whether another three-phase grid-connected switch in the disconnected state fails.

6. The grid-connected switch failure detection method according to claim 2, characterized in that: The three-phase grid-connected inverter system further includes a voltage sampling module and a voltage divider, wherein the voltage sampling module is used to collect the voltage across the voltage divider, and the voltage divider is connected between the positive DC bus and the ground or between the negative DC bus and the ground; The grid-connected switch failure detection method further includes: If the currents of at least two phases of the three-phase currents flowing through the three-phase grid-connected switch do not exceed the preset current threshold, it is determined whether another three-phase grid-connected switch in the disconnected state fails based on the voltage change across the voltage divider.

7. The grid-connected switch failure detection method according to claim 6, characterized in that: The re-determining whether another three-phase grid-connected switch in the disconnected state fails comprises: If the voltage change across the voltage divider exceeds a preset voltage change, it is determined that one phase of the other three-phase grid-connected switch in the disconnected state has failed; otherwise, it is determined that the other three-phase grid-connected switch in the disconnected state has not failed.

8. The grid-connected switch failure detection method according to claim 6, characterized in that: The voltage sampling module is configured to collect the voltage across the voltage divider in a preset time period.

9. The grid-connected switch failure detection method according to claim 6, characterized in that: The grid-connected switch failure detection method further includes: Switching states of the three-phase grid-connected switches are switched and determining whether one of the three-phase grid-connected switches in an off state fails.

10. A three-phase grid-connected inverter system, characterized in that: The three-phase grid-connected 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, a filter unit and a three-phase grid-connected switch arranged on the AC side of the three-phase grid-connected inverter, the filter unit includes a star-connected filter capacitor and the center point of the star connection is connected to the bus midpoint, the three-phase grid-connected switch includes a first three-phase grid-connected switch and a second three-phase grid-connected switch connected in series, and a current sampling module for collecting three-phase currents flowing through the three-phase grid-connected switch; The three-phase grid-connected inverter system also includes a control unit, which is configured to control one of the first three-phase grid-connected switch and the second three-phase grid-connected switch to be closed and the other three-phase grid-connected switch to be opened; based on the three-phase current flowing through the three-phase grid-connected switch, determine whether the other three-phase grid-connected switch in the disconnected state fails.

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