Power conversion device and relay failure detection method therefor

Abstract

To provide a power conversion device and a relay failure detection method which can identify whether or not a relay fails before the power conversion device operates.SOLUTION: An inverter circuit outputs alternating current to a U-phase AC terminal and a W-phase AC terminal of a power conversion device, and a controller collects first sampling voltage on both ends of a first relay when difference between voltage of a positive bus capacitor and voltage of a negative bus capacitor is a first voltage threshold or smaller before the power conversion device operates in an off-grid or grid connection system. The controller further performs control so that the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor becomes larger than the first voltage threshold and collects second sampling voltage on both ends of the first relay. The controller further outputs an alarm signal indicating that the first relay is in a failed state on the basis of an absolute value of difference between the first sampling voltage and the second sampling voltage.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This application relates to the field of photovoltaic energy storage technology, and more particularly to power conversion devices and relay fault detection methods therefor. [Background technology]

[0002] When a single-phase hybrid inverter has grid-connected and off-grid functions, it typically includes a DC bus, an inverter circuit, a U-phase relay, a W-phase relay, an O-phase relay, and an O_PE relay. The U-phase relay is connected between the U-phase output terminal of the inverter circuit and the U-phase output terminal of the inverter, the W-phase relay is connected between the W-phase output terminal of the inverter circuit and the W-phase output terminal of the inverter, the O-phase relay is connected between the midpoint of the DC bus and the O-phase output terminal of the inverter, and the O_PE relay is connected between the midpoint of the DC bus and a reference ground. When the single-phase hybrid inverter operates in grid-connected mode or off-grid mode, a controller in the single-phase hybrid inverter controls the U-phase relay, the W-phase relay, the O-phase relay, and the O_PE relay to turn on, transmitting the AC current output by the inverter circuit to the power grid or a load for power supply.

[0003] However, if the O-phase relay fails (for example, if the O-phase relay sticks or opens) before the single-phase hybrid inverter operates in grid-connected or off-grid mode, users or maintenance personnel may receive an electric shock, posing a risk to their safety. Therefore, to ensure the safety of users and maintenance personnel, it is particularly important to know how to identify in advance whether the O-phase relay has failed before the single-phase hybrid inverter operates in grid-connected or off-grid mode. Summary of the Invention [Problem to be solved by the invention]

[0004] The present application provides a power conversion apparatus and a relay fault detection method therefor that can accurately and effectively identify whether a first relay has a fault before the power conversion apparatus operates in an off-grid or grid-connected mode, thereby improving the efficiency of first relay fault detection, ensuring the safety of users and maintenance personnel, and improving the power supply safety of the power conversion apparatus. [Means for solving the problem]

[0005] According to a first aspect, an embodiment of the present application provides a power conversion device. The power conversion device is configured to be connected between a DC power source and a load or a power grid for energy conversion. The power conversion device includes a positive bus capacitor, a negative bus capacitor, an inverter circuit, a first relay, and a controller. When the power conversion device is configured to be connected between the DC power source and the load for energy conversion, the first relay is an off-grid side O-phase relay. When the power conversion device is configured to be connected between the DC power source and the power grid for energy conversion, the first relay is a grid-connected side O-phase relay. The DC end of the power conversion device is configured to connect to a DC power source, the positive bus capacitor and the negative bus capacitor are connected in series and then connected to the DC end of the power conversion device, the DC end of the inverter circuit is connected to the DC end of the power conversion device, the U-phase AC end of the inverter circuit is connected to the U-phase AC end of the power conversion device, the W-phase AC end of the inverter circuit is connected to the W-phase AC end of the power conversion device, the series connection point between the positive bus capacitor and the negative bus capacitor is connected to the O-phase AC end of the power conversion device through a first relay, and the U-phase AC end, W-phase AC end, and O-phase AC end of the power conversion device are all configured to connect to a power grid or a load. Before the power conversion device operates in an off-grid or grid-connected manner, the controller is configured to: cause the inverter circuit to output AC to the U-phase AC end and the W-phase AC end of the power conversion device; and collect a first sampled voltage across the first relay when a difference between a voltage of the positive bus capacitor and a voltage of the negative bus capacitor is less than or equal to a first voltage threshold. It should be understood that the voltages on the positive bus capacitor and the negative bus capacitor are balanced when the difference between the voltages on the positive bus capacitor and the negative bus capacitor is less than or equal to the first voltage threshold. In this case, the first sampled voltage is the sampled voltage of the first relay obtained when the voltages on the positive bus capacitor and the negative bus capacitor are balanced. Furthermore, the controller is further configured to control the difference between the voltages on the positive bus capacitor and the negative bus capacitor to be greater than the first voltage threshold and collect a second sampled voltage across the first relay.It should be understood that when the difference between the voltages of the positive bus capacitor and the negative bus capacitor is greater than the first voltage threshold, the voltages of the positive bus capacitor and the negative bus capacitor are unbalanced, causing a change in the potential of the series connection point, i.e., a change in the potential of one end of the first relay facing the series connection point. This causes a change in the sampled voltage across the first relay. In this case, the second sampled voltage is the sampled voltage of the first relay obtained when the voltages of the positive bus capacitor and the negative bus capacitor are unbalanced. Furthermore, the controller is further configured to output an alarm signal based on the absolute value of the difference between the first sampled voltage and the second sampled voltage. The alarm signal indicates that the first relay is in a fault state.

[0006] In this embodiment of the present application, before the power conversion device operates in an off-grid or grid-connected manner, a first sampled voltage of the first relay is acquired when the voltage of the positive bus capacitor and the voltage of the negative bus capacitor are balanced, and a second sampled voltage of the first relay is acquired when the voltage of the positive bus capacitor and the voltage of the negative bus capacitor are unbalanced. Then, based on the absolute value of the difference between these two sampled voltages, it is determined whether the first relay has failed. This avoids false detections that occur when using a single sampled voltage to determine whether the first relay has failed, thereby accurately and effectively identifying whether the first relay has failed. This improves the efficiency of fault detection of the first relay. It also further prevents users and maintenance personnel from getting electric shocks. This ensures the safety of users and maintenance personnel and further improves the power supply safety of the power conversion device.

[0007] In one possible implementation, when the power conversion device performs stuck-on detection on the first relay, the controller is further configured to send an open command to the first relay before the inverter circuit outputs AC to the U-phase AC terminal and the W-phase AC terminal of the power conversion device and a difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor is equal to or less than a first voltage threshold. The controller is further configured to collect a first sampled voltage across the first relay when the inverter circuit outputs AC to the U-phase AC terminal and the W-phase AC terminal of the power conversion device and a difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor is equal to or less than the first voltage threshold. The controller is further configured to control the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor to be greater than the first voltage threshold and collect a second sampled voltage across the first relay. The controller is further configured to output an alarm signal when the absolute value of the difference is equal to or less than a second voltage threshold. The alarm signal indicates that the first relay is stuck, i.e., the first relay cannot switch from a closed state to an open state. The controller is further configured to determine that the first relay is in a non-fault state when the absolute value of the difference is greater than a second voltage threshold. In this embodiment of the present application, if the first relay is open after the controller sends an open command to the first relay, the absolute value of the difference between the first sampled voltage and the second sampled voltage is large. Conversely, if the first relay is stuck and cannot be opened, both the first sampled voltage and the second sampled voltage are close to zero. In this case, the absolute value of the difference between the first sampled voltage and the second sampled voltage is close to zero. Therefore, when the absolute value of the difference is equal to or less than the second voltage threshold, it is possible to accurately identify that the first relay is stuck. This improves the efficiency of fault detection for the first relay.

[0008] In one possible implementation, when the power conversion device performs closing fault detection on the first relay, the controller is further configured to send a closing command to the first relay before the inverter circuit outputs AC to the U-phase AC terminal and the W-phase AC terminal of the power conversion device and a difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor is equal to or less than a first voltage threshold. The controller is further configured to collect a first sampled voltage across the first relay when the inverter circuit outputs AC to the U-phase AC terminal and the W-phase AC terminal of the power conversion device and a difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor is equal to or less than the first voltage threshold. The controller is further configured to control the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor to be greater than the first voltage threshold and collect a second sampled voltage across the first relay. The controller is further configured to output an alarm signal when the absolute value of the difference is equal to or greater than a second voltage threshold. The alarm signal indicates that the first relay is in a closed fault state, i.e., the first relay cannot switch from an open state to a closed state. The controller determines that the first relay is in a non-fault state when the absolute value of the difference is less than the second voltage threshold. In this embodiment of the present application, after the controller sends a close command to the first relay, if the first relay is closed, both the first sampled voltage and the second sampled voltage are close to zero. In this case, the absolute value of the difference between the first sampled voltage and the second sampled voltage is close to zero. Conversely, if the first relay is not closed and cannot be closed, the absolute value of the difference between the first sampled voltage and the second sampled voltage is large. Therefore, if the absolute value of the difference is equal to or greater than the second voltage threshold, it can be accurately determined that the first relay is in a closed fault state (cannot be closed). This improves the fault detection efficiency of the first relay.

[0009] In one possible implementation, the power converter further includes a balanced bridge circuit. The input terminal of the balanced bridge circuit is connected to the DC terminal of the power converter, and the output terminal of the balanced bridge circuit is connected to the series connection point. The controller is further configured to control the balanced bridge circuit to adjust the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor to be greater than a first voltage threshold. In this embodiment of the present application, the current output to the series connection point can be flexibly adjusted by using the balanced bridge circuit to achieve the goal of the voltage of the positive bus capacitor and the voltage of the negative bus capacitor being unbalanced. The control method is more flexible.

[0010] In one possible implementation, the balanced bridge circuit includes a bridge arm and an inductor, and the bridge arm includes an upper bridge arm switch and a lower bridge arm switch. The upper bridge arm switch and the lower bridge arm switch are connected in series and then connected to the DC end of the power conversion device, and the connection point between the upper bridge arm switch and the lower bridge arm switch is connected to the series connection point via an inductor. The connection point between the upper bridge arm switch and the lower bridge arm switch is the midpoint of the bridge arm. It should be understood that the controller can flexibly adjust the current output from the midpoint of the bridge arm to the series connection point through the inductor by controlling each of the upper bridge arm switch and the lower bridge arm switch to be turned on or off.

[0011] In one possible implementation, the power converter further includes an isolation impedance detection circuit. The isolation impedance detection circuit is connected to the DC end of the power converter. The controller is further configured to control the isolation impedance detection circuit to adjust the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor to be greater than a first voltage threshold, thereby achieving the objective that the voltage of the positive bus capacitor and the voltage of the negative bus capacitor are unbalanced. The control method is more flexible.

[0012] In one possible implementation, the isolation impedance detection circuit includes a first resistor, a second resistor, a third resistor, and a second relay. The first resistor, the second resistor, and the third resistor are connected in series and then connected to a DC end of the power conversion device, one end of the second resistor facing the first resistor is connected to one end of the second relay and a reference ground, and the other end of the second resistor away from the first resistor is connected to the other end of the second relay. The controller is further configured to close the second relay and adjust the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor to be greater than a first voltage threshold. Generally, the positive bus capacitor and the negative bus capacitor are connected in series between the positive DC bus and the negative DC bus. Before the second relay is closed, the first resistor, the second resistor, and the third resistor divide the voltage between the positive DC bus and the negative DC bus. After the second relay is closed, the second resistor is shorted. In this case, the first resistor and the third resistor divide the voltage between the positive DC bus and the negative DC bus. Therefore, before and after the second relay is closed, the voltage across the first resistor and the voltage across the third resistor change. As a result, the potential of the positive DC bus and the potential of the negative DC bus change, achieving the goal of unbalancing the voltages of the positive bus capacitor and the negative bus capacitor.

[0013] In one possible implementation, when the potential of the series connection point is unstable, i.e., when the potential of one end of the first relay facing the series connection point is unstable, the second sampled voltage may be an average value of multiple sampled voltages at both ends of the first relay, thereby improving the accuracy of the second sampled voltage.

[0014] In one possible implementation, the controller is further configured to control a difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor to be equal to or less than a first voltage threshold when the first relay is in a non-fault state, so that the voltage of the positive bus capacitor and the voltage of the negative bus capacitor are balanced, thereby ensuring that the power conversion device stably outputs AC current to the outside and reducing the failure rate of the power conversion device.

[0015] In one possible implementation, the power conversion device further includes a third relay, a fourth relay, and a fifth relay. The third relay is arranged between the U-phase AC end of the inverter circuit and the U-phase AC end of the power conversion device, the fourth relay is arranged between the W-phase AC end of the inverter circuit and the W-phase AC end of the power conversion device, and the fifth relay is arranged between the series connection point and a reference ground. When the U-phase AC end, W-phase AC end, and O-phase AC end of the power conversion device are all configured to connect to a load, the first relay, the third relay, and the fourth relay form an off-grid side relay within the power conversion device. When the U-phase AC end, W-phase AC end, and O-phase AC end of the power conversion device are all configured to connect to a power grid, the first relay, the third relay, and the fourth relay form a grid-connected side relay within the power conversion device. After the power conversion device performs fault detection on the first relay, the controller is further configured to control the first relay, the third relay, the fourth relay, and the fifth relay to all be closed when the first relay is in a non-fault state, thereby enabling a connection between the inverter circuit and the load or the power grid, and causing the power conversion device to output AC current to the load or the power grid. In this embodiment of the present application, when the first relay is identified to be in a non-fault state, an off-grid side relay or a grid-connected side relay in the power conversion device can be controlled to be closed to ensure safe and reliable operation of the power conversion device. This improves the reliability of power supply and power supply safety of the load or the power grid.

[0016] According to a second aspect, an embodiment of the present application provides a relay fault detection method for a power conversion device. In this method, an inverter circuit outputs AC current to a U-phase AC terminal and a W-phase AC terminal of the power conversion device, and when a difference between a voltage of a positive bus capacitor and a voltage of a negative bus capacitor is equal to or less than a first voltage threshold, a controller collects a first sampled voltage across a first relay. The controller may further control the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor to be greater than the first voltage threshold, and collect a second sampled voltage across the first relay. The positive bus capacitor and the negative bus capacitor are connected in series and then connected to a DC terminal of the power conversion device, the DC terminal of the inverter circuit is connected to the DC terminal of the power conversion device, the U-phase AC terminal of the inverter circuit is connected to the U-phase AC terminal of the power conversion device, the W-phase AC terminal of the inverter circuit is connected to the W-phase AC terminal of the power conversion device, and the series connection point between the positive bus capacitor and the negative bus capacitor is connected to the O-phase AC terminal of the power conversion device through the first relay. The controller may further output an alarm signal based on the absolute value of the difference between the first sampled voltage and the second sampled voltage. The alarm signal indicates that the first relay is in a fault state. In this embodiment of the present application, before the power conversion device operates in an off-grid or grid-connected mode, a first sampled voltage of the first relay acquired when the voltage of the positive bus capacitor and the voltage of the negative bus capacitor are balanced and a second sampled voltage of the first relay acquired when the voltage of the positive bus capacitor and the voltage of the negative bus capacitor are unbalanced are separately collected. Then, whether the first relay is faulty is determined based on the absolute value of the difference between the two sampled voltages. This avoids false detection caused when determining whether the first relay is faulty using a single sampled voltage, thereby accurately and effectively identifying whether the first relay is faulty. This improves the efficiency of fault detection of the first relay.

[0017] In one possible implementation, the inverter circuit outputs AC to the U-phase AC terminal and the W-phase AC terminal of the power converter, and the controller may further send an open command to the first relay before the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor is equal to or less than the first voltage threshold. After collecting the first sampled voltage and the second sampled voltage, the controller may further output an alarm signal if the absolute value of the difference is equal to or less than the second voltage threshold. The alarm signal indicates that the first relay is stuck, i.e., the first relay cannot switch from a closed state to an open state. The controller may further determine that the first relay is in a non-fault state if the absolute value of the difference is greater than the second voltage threshold. In this embodiment of the present application, if the first relay is open after the controller sends the open command to the first relay, the absolute value of the difference between the first sampled voltage and the second sampled voltage is large. Conversely, if the first relay is stuck and not open, both the first sampled voltage and the second sampled voltage are close to zero. In this case, the absolute value of the difference between the first sampled voltage and the second sampled voltage is close to 0. Therefore, if the absolute value of the difference is equal to or less than the second voltage threshold, it can be accurately determined that the first relay is stuck. This improves the fault detection efficiency of the first relay.

[0018] In one possible implementation, the inverter circuit outputs AC to the U-phase AC terminal and the W-phase AC terminal of the power conversion device, and the controller may further send a close command to the first relay before the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor is equal to or less than the first voltage threshold. After collecting the first sampled voltage and the second sampled voltage, the controller may further output an alarm signal if the absolute value of the difference is equal to or greater than the second voltage threshold. The alarm signal indicates that the first relay is in a closed fault state, i.e., the first relay cannot switch from an open state to a closed state. The controller may further determine that the first relay is in a non-fault state when the absolute value of the difference is less than the second voltage threshold. In this embodiment of the present application, after the controller sends a close command to the first relay, if the first relay is closed, both the first sampled voltage and the second sampled voltage are close to zero. In this case, the absolute value of the difference between the first sampled voltage and the second sampled voltage is close to zero. Conversely, if the first relay is unable to close, the absolute value of the difference between the first sampled voltage and the second sampled voltage is large. Therefore, when the absolute value of the difference is equal to or greater than the second voltage threshold, it can be accurately determined that the first relay is unable to close. This improves the efficiency of fault detection for the first relay.

[0019] In one possible implementation, the controller can further control the balanced bridge circuit to adjust the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor to be greater than a first voltage threshold. In this case, the voltage of the positive bus capacitor and the voltage of the negative bus capacitor become unbalanced. The input terminal of the balanced bridge circuit is connected to the DC terminal of the power conversion device, and the output terminal of the balanced bridge circuit is connected to the series connection point. In this embodiment of the present application, the current output to the series connection point can be flexibly adjusted by using the balanced bridge circuit, thereby changing the potential of the series connection point and achieving the goal of the voltage of the positive bus capacitor and the voltage of the negative bus capacitor being unbalanced. The control method is more flexible.

[0020] In one possible implementation, the controller may further control the isolation impedance detection circuit to adjust the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor to be greater than a first voltage threshold, thereby realizing the objective of the positive bus capacitor voltage and the negative bus capacitor voltage being unbalanced. The control method is more flexible. The isolation impedance detection circuit is connected to the DC end of the power conversion device.

[0021] In one possible implementation, the controller may further control the second relay to close to adjust the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor to be greater than a first voltage threshold. When the isolation impedance detection circuit includes a first resistor, a second resistor, a third resistor, and a second relay, the first resistor, the second resistor, and the third resistor are connected in series and then connected to the DC end of the power converter, one end of the second resistor facing the first resistor is connected to one end of the second relay and a reference ground, and the other end of the second resistor away from the first resistor is connected to the other end of the second relay. It should be understood that before the second relay is closed, the first resistor, the second resistor, and the third resistor divide the voltage between the positive DC bus and the negative DC bus. After the second relay is closed, the second resistor is short-circuited. In this case, the first resistor and the third resistor divide the voltage between the positive DC bus and the negative DC bus, and therefore the voltage across the first resistor and the voltage across the third resistor change before and after the second relay is closed, thereby changing the potential of the positive DC bus and the potential of the negative DC bus, thereby achieving the purpose of unbalancing the voltages of the positive bus capacitor and the negative bus capacitor.

[0022] In one possible implementation, when the potential of the series connection point is unstable, i.e., when the potential of one end of the first relay facing the series connection point is unstable, the second sampled voltage may be an average value of multiple sampled voltages at both ends of the first relay, thereby improving the accuracy of the second sampled voltage.

[0023] In one possible implementation, when the first relay is in a non-fault state, the controller controls the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor to be equal to or less than a first voltage threshold, so that the voltage of the positive bus capacitor and the voltage of the negative bus capacitor are balanced, thereby ensuring that the power conversion device can stably output AC current to the outside and reducing the failure rate of the power conversion device.

[0024] It should be understood that the implementations and beneficial effects of the aforementioned aspects of the present application may be referenced to one another. [Brief explanation of the drawings]

[0025] [Figure 1] 1 is a diagram of the structure of a power generation system according to an embodiment of the present application; [Figure 2] 1 is a diagram of a power converter structure according to an embodiment of the present application; [Figure 3] 1 is a diagram of an application scenario of a power converter in a parallel connection scenario according to an embodiment of the present application; FIG. [Figure 4] 1 is a schematic diagram of a circuit of a power converter according to an embodiment of the present application; [Figure 5] FIG. 2 is a schematic diagram of another circuit of a power converter according to an embodiment of the present application. [Figure 6] FIG. 2 is a schematic diagram of another circuit of a power converter according to an embodiment of the present application. [Figure 7] FIG. 1 is a circuit schematic diagram when a power converter includes only an isolation impedance detection circuit, according to an embodiment of the present application. [Figure 8A] FIG. 2 is a schematic diagram of a circuit where a power converter has both off-grid and grid-connected operation capabilities, according to an embodiment of the present application. [Figure 8B] FIG. 10 is a schematic diagram of another circuit where the power converter has both off-grid and grid-connected operation capabilities, according to an embodiment of the present application. [Figure 9A]FIG. 10 is a schematic diagram of another circuit where the power converter has both off-grid and grid-connected operation capabilities, according to an embodiment of the present application. [Figure 9B] FIG. 10 is a schematic diagram of another circuit where the power converter has both off-grid and grid-connected operation capabilities, according to an embodiment of the present application. [Figure 10] 1 is a schematic flowchart of a relay fault detection method for a power converter, according to an embodiment of the present application; [Figure 11] FIG. 10 is a diagram of a procedure in which the controller performs stuck detection on the first relay by using a balanced bridge circuit, according to an embodiment of the present application. [Figure 12] FIG. 10 is a diagram of a procedure in which the controller performs close fault detection on the first relay by using a balanced bridge circuit, according to an embodiment of the present application. [Figure 13] FIG. 10 is a diagram of a procedure in which a controller performs stuck detection of a first relay by using an insulation impedance detection circuit, according to an embodiment of the present application. [Figure 14] FIG. 10 is a diagram of a procedure for the controller to perform closing fault detection on the first relay by using an insulation impedance detection circuit, according to an embodiment of the present application. DETAILED DESCRIPTION OF THE INVENTION

[0026] The following clearly and completely describes the technical solutions in the embodiments of the present application with reference to the accompanying drawings in the embodiments of the present application. It is clear that the described embodiments are only a part of the embodiments of the present application, and are not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present application without creative efforts shall fall within the protection scope of the present application.

[0027] The implementation of the technical solutions of the present application is further described in detail below with reference to the accompanying drawings.

[0028] 1 is a diagram of a structure of a power generation system according to an embodiment of the present application. As shown in FIG. 1, the power generation system includes a power conversion device 11. A DC end of the power conversion device 11 is configured to connect to a photovoltaic string 10, an off-grid side AC end 111 of the power conversion device 11 is configured to connect to a load 12, and a grid-connected side AC end 112 of the power conversion device 11 is configured to connect to a power grid 13.

[0029] The photovoltaic string 10 is configured to absorb solar energy, convert the solar energy into electrical energy, and output direct current to the power converter 11 .

[0030] The power converter 11 includes an inverter circuit, a grid-connected-side relay, and an off-grid-side relay. The DC end of the inverter circuit is connected to the DC end of the power converter 11, the AC end of the inverter circuit is connected to the off-grid-side AC end 111 of the power converter 11 through the off-grid-side relay, and the AC end of the inverter circuit is further connected to the grid-connected-side AC end 112 of the power converter 11 through the grid-connected-side relay. The inverter circuit is configured to convert the DC current output by the photovoltaic string 10 into AC current. When the power converter 11 is in an off-grid operation mode, a controller in the power converter 11 is configured to control the off-grid-side relay to close, thereby enabling a connection between the AC end of the inverter circuit and the off-grid-side AC end 111 of the power converter 11 and transmitting the AC current output by the inverter circuit to the load 12 for power supply. The off-grid-side relay includes an off-grid-side U-phase relay, an off-grid-side W-phase relay, and an off-grid-side O-phase relay. Optionally, the off-grid-side relays may not include an off-grid-side O-phase relay. When the power conversion device 11 is in a grid-connected operation mode, a controller within the power conversion device 11 is configured to control the grid-connected-side relays to close, thereby enabling a connection between the AC end of the inverter circuit and the grid-connected-side AC end 112 of the power conversion device 11 and transmitting the AC output by the inverter circuit to the power grid 13 for power supply. The grid-connected-side relays include a grid-connected-side U-phase relay, a grid-connected-side W-phase relay, and a grid-connected-side O-phase relay. Optionally, the grid-connected-side relays may not include the grid-connected-side O-phase relay.

[0031] Before controlling the off-grid-side relay or the grid-connected relay to close, fault detection must be performed on the off-grid-side relay or the grid-connected relay in advance. An example in which fault detection is performed on the off-grid-side O-phase relay or the grid-connected O-phase relay is used for explanation. Before the power conversion device 11 operates in the off-grid mode or the grid-connected mode, the controller in the power conversion device 11 is configured to collect a first sampled voltage across the off-grid-side O-phase relay or the grid-connected O-phase relay when the inverter circuit outputs AC to the off-grid-side AC end 111 or the grid-connected AC end 112 of the power conversion device 11 and a difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor is equal to or less than a first voltage threshold. The controller is further configured to control the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor to be greater than the first voltage threshold and collect a second sampled voltage across the off-grid-side O-phase relay or the grid-connected O-phase relay. The controller is further configured to output an alarm signal based on the absolute value of the difference between the first sampled voltage and the second sampled voltage. The alarm signal indicates that the off-grid-side O-phase relay or the grid-connected O-phase relay is in a fault state. In this embodiment of the present application, by quickly identifying whether the off-grid-side O-phase relay or the grid-connected O-phase relay is faulty before the off-grid-side relay or the grid-connected O-phase relay is controlled to close, the reliability and safety of the off-grid or grid-connected operation of the power conversion device 11 can be ensured, and electric shock to users or maintenance personnel of the power generation system can be further prevented. This ensures the safety of users and maintenance personnel and further improves the power supply safety of the load 12 and the power grid 13.

[0032] As shown in FIG. 1 , the power conversion device 11 further includes a direct current (DC) / DC conversion circuit. The DC / DC conversion circuit is connected between the DC end of the inverter circuit and the DC end of the power conversion device 11, and the DC / DC conversion circuit is connected to the DC end of the inverter circuit through a positive DC bus and a negative DC bus. The DC / DC conversion circuit is configured to convert the DC output by the photovoltaic string 10 and output the DC to the inverter circuit through the positive DC bus and the negative DC bus. When the power generation system is a photovoltaic energy storage system, the power generation system further includes an energy storage device 14, which is connected to the positive DC bus and the negative DC bus. The energy storage device 14 includes, for example, an energy storage battery. Optionally, the energy storage device 14 includes an energy storage battery and an energy storage converter, and the energy storage converter is a DC / DC converter. When there is light, the DC / DC conversion circuit is further configured to output DC to the energy storage device 14 for charging the energy storage device 14. When there is no light (e.g., at night or on a cloudy day), the power conversion device 11 is further configured to receive the AC current output by the power grid 13 when the grid-connected relay is closed, and output a DC current to the energy storage device 14 for charging based on the AC current. When there is no light, the energy storage device 14 is further configured to output a DC current to the power conversion device 11 to ensure that the power conversion device 11 can still normally supply power to the load 12 or the power grid 13 when there is no light. This improves the power supply efficiency of the power generation system.

[0033] Hereinafter, examples of the power converter provided in the present application and the working principles of the power converter will be described with reference to FIGS. 2 to 9B.

[0034] 2 is a diagram of a structure of a power conversion device according to an embodiment of the present application. As shown in FIG. 2, the power conversion device 2 is configured to be connected between a DC power source 3 and a load 41 for energy conversion. Specifically, the power conversion device 2 is configured to convert a DC output from the DC power source 3 into an AC current to supply power to the load 41. For example, the DC power source 3 may be at least one of a solar cell string, a storage battery, and another DC power source.

[0035] The power conversion device 2 includes a positive bus capacitor C1, a negative bus capacitor C2, an inverter circuit 21, a first off-grid side relay K_o1, and a controller 22. The DC terminal of the power conversion device 2 is configured to be connected to a DC power source 3, and the positive bus capacitor C1 and the negative bus capacitor C2 are connected in series and then connected to the DC terminal of the power conversion device 2. Specifically, the DC terminal of the power conversion device 2 includes a DC terminal dc11 and a DC terminal dc12, and the DC terminals dc11 and dc12 of the power conversion device 2 are configured to be connected to both ends of the DC power source 3. The positive bus capacitor C1 and the negative bus capacitor C2 are connected in series and then connected between the DC terminal dc11 and the DC terminal dc12 of the power converter 2; that is, the positive bus capacitor C1 and the negative bus capacitor C2 are connected in series and then connected between the DC bus BUS+ and the negative DC bus BUS-. The DC terminal of the inverter circuit 21 is connected to the DC terminal of the power converter 2. Specifically, the DC terminal of the inverter circuit 21 includes a DC terminal dc21 and a DC terminal dc22. The DC terminal dc21 of the inverter circuit 21 is connected to the DC terminal dc11 of the power converter 2 through the DC bus BUS+, and the DC terminal dc22 of the inverter circuit 21 is connected to the DC terminal dc12 of the power converter 2 through the negative DC bus BUS-. The U-phase AC terminal ac21 of the inverter circuit 21 is connected to the off-grid U-phase AC terminal ac11 of the power conversion device 2, and the W-phase AC terminal ac22 of the inverter circuit 21 is connected to the off-grid W-phase AC terminal ac12 of the power conversion device 2. A series connection point a between the positive bus capacitor C1 and the negative bus capacitor C2 is connected to the off-grid O-phase AC terminal ac13 of the power conversion device 2 through an off-grid first relay K_o1. The series connection point a may also be referred to as a bus midpoint between the positive bus capacitor C1 and the negative bus capacitor C2. The off-grid U-phase AC terminal ac11, the off-grid W-phase AC terminal ac12, and the off-grid O-phase AC terminal ac13 of the power conversion device 2 are all connected to a load 41.

[0036] Before the power conversion device 2 operates in an off-grid manner, the controller 22 is configured to cause the inverter circuit 21 to output AC to the off-grid-side U-phase AC terminal ac11 and the off-grid-side W-phase AC terminal ac12 of the power conversion device 2, and to collect a first sampled voltage across the first off-grid-side relay K_o1 when a difference between the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 is equal to or less than a first voltage threshold. It should be understood that when a difference between the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 is equal to or less than the first voltage threshold, the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 are balanced. In this case, the first sampled voltage is the sampled voltage of the first off-grid-side relay K_o1 when the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 are balanced. Furthermore, the controller 22 is further configured to control the difference between the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 to be greater than a first voltage threshold and collect a second sampled voltage across the first off-grid-side relay K_o1. It should be understood that when the difference between the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 is greater than the first voltage threshold, the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 are unbalanced. In this case, the potential of the series connection point a changes, i.e., the potential of one end of the first off-grid-side relay K_o1 facing the series connection point a changes. This changes the sampled voltage across the first off-grid-side relay K_o1. In this case, the second sampled voltage is the sampled voltage of the first off-grid-side relay K_o1 obtained when the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 are unbalanced. The controller 22 is further configured to output an alarm signal based on the absolute value of the difference between the first sampled voltage and the second sampled voltage, and the alarm signal indicates that the off-grid-side first relay K_o1 is in a fault state, the fault state including a stuck state or a closed fault state.

[0037] In this embodiment of the present application, before the power conversion device 2 operates in an off-grid mode, a first sampled voltage of the off-grid-side first relay K_o1 obtained when the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 are balanced and a second sampled voltage of the off-grid-side first relay K_o1 obtained when the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 are unbalanced are separately collected. Then, based on the absolute value of the difference between the two sampled voltages, it is determined whether the off-grid-side first relay K_o1 has a fault. This avoids false detections that would occur if a single sampled voltage were used to determine whether the off-grid-side first relay K_o1 has a fault, and accurately and effectively identifies whether the off-grid-side first relay K_o1 has a fault. This improves the efficiency of fault detection for the off-grid-side first relay K_o1. It also further prevents users and maintenance personnel of the power conversion device 2 from getting electric shock. This ensures the safety of users and maintenance personnel, and further improves the safety of the power supply of the power conversion device 2.

[0038] It can be understood that the potential of the series connection point a is determined based on the potential of the positive DC bus BUS+ and the potential of the negative DC bus BUS-. For example, when the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 are balanced, the potential of the series connection point a may or may not be 0. When the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 are balanced, the voltage VUO between the off-grid side U-phase AC terminal ac11 and the off-grid side O-phase AC terminal ac13 and the voltage VWO between the off-grid side W-phase AC terminal ac12 and the off-grid side O-phase AC terminal ac13 are balanced. When the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 are unbalanced, the voltages VUO and VWO become unbalanced.

[0039] In this embodiment of the present application, the power conversion device 2 may be one of a single-phase photovoltaic inverter, a single-phase hybrid inverter, a single-phase photovoltaic energy storage hybrid inverter, an inverter with an O phase or a split phase, an uninterruptible power supply (UPS) with an O phase or a split phase, a single-phase inverter with a balanced bridge circuit, and a microinverter. The O phase or the split phase is the phase in which the off-grid-side O-phase AC terminal ac13 of the power conversion device 2 is located. Therefore, the off-grid-side first relay K_o1 may also be referred to as the off-grid-side O-phase relay. When the power conversion device 2 is a UPS with an O phase or a split phase, the power conversion device 2 further includes a rectifier circuit, a filter circuit, a bypass switch, etc. for performing alternating current (AC) / direct current (DC) conversion. In this case, the DC power source 3 may be disposed inside the power conversion device 2, and the DC power source may be a storage battery.

[0040] In some possible implementations, the power converter 2 may be used in the power generation system shown in Fig. 1. Optionally, the power converter 2 may further be used in a power filter or a reactive power compensator. The actual application scenario of the power converter 2 is not limited in this embodiment of the present application.

[0041] FIG. 3 is a diagram of an application scenario of a power converter in a parallel connection scenario according to an embodiment of the present application. As shown in FIG. 3, a power converter 5 is configured to be connected between a DC power source 6 and a load 41 for energy conversion. The AC terminals of the power converter 5 and the AC terminals of the power converter 2 are connected in parallel and then connected to the load 41. Specifically, the off-grid-side U-phase AC terminal ac31 of the power converter 5 is connected to the off-grid-side U-phase AC terminal ac11 of the power converter 2, the off-grid-side W-phase AC terminal ac32 of the power converter 5 is connected to the off-grid-side W-phase AC terminal ac12 of the power converter 2, and the off-grid-side O-phase AC terminal ac33 of the power converter 5 is connected to the off-grid-side O-phase AC terminal ac13 of the power converter 2. It can be seen that the potential of the right end point b of the off-grid-side first relay K_o1 is the same as the potential of the off-grid-side O-phase AC terminal ac33 of the power converter 5. When the potential at the right end point b of the off-grid-side first relay K_o1 is the same as the potential at the series connection point a, the sampled voltage across the off-grid-side first relay K_o1 is zero.

[0042] For the purpose of explanation, an example in which fault detection is performed on the off-grid-side first relay K_o1 using a single sampling voltage (e.g., a first sampling voltage) will be used. When the off-grid-side first relay K_o1 is not stuck and is normally open, the first sampling voltage is 0. However, when the off-grid-side first relay K_o1 is stuck, the potential of the right end point b of the off-grid-side first relay K_o1 is the same as the potential of the series connection point a, and the first sampling voltage remains 0. For this reason, the controller 22 may directly output an alarm signal based on the first sampling voltage, which may result in the off-grid-side first relay K_o1 being erroneously detected as stuck. For example, the off-grid-side first relay K_o1, which is normally open, may be erroneously detected as stuck. This reduces the efficiency of fault detection for the off-grid-side first relay K_o1.

[0043] In this embodiment of the present application, after collecting the first sampled voltage, the controller 22 further controls the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 to be unbalanced, and collects a second sampled voltage across the first off-grid-side relay K_o1. It should be understood that if the first off-grid-side relay K_o1 is normally open, the second sampled voltage is greater than zero, and if the first off-grid-side relay K_o1 is stuck, the second sampled voltage is zero. Therefore, when the controller 22 performs fault detection based on the absolute value of the difference between the first sampled voltage and the second sampled voltage, it is possible to avoid erroneous detection resulting from determining that the first off-grid-side relay K_o1 is faulty using a single sampled voltage. This improves the efficiency of fault detection for the first off-grid-side relay K_o1.

[0044] In some possible implementations, when the power conversion device 2 performs stuck detection on the off-grid-side first relay K_o1, the controller 22 is further configured to send an open command to the off-grid-side first relay K_o1 before the inverter circuit 21 outputs AC to the off-grid-side U-phase AC terminal ac11 and the off-grid-side W-phase AC terminal ac12 of the power conversion device 2 and the difference between the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 becomes equal to or less than a first voltage threshold. The open command is used to control the off-grid-side first relay K_o1 to open. When the power conversion device 2 performs closing fault detection on the off-grid-side first relay K_o1, the controller 22 is further configured to: cause the inverter circuit 21 to output AC to the off-grid-side U-phase AC terminal ac11 and the off-grid-side W-phase AC terminal ac12 of the power conversion device 2; and deliver a close command to the off-grid-side first relay K_o1 before a difference between a voltage of the positive bus capacitor C1 and a voltage of the negative bus capacitor C2 becomes equal to or less than a first voltage threshold; the close command is used to control the off-grid-side first relay K_o1 to close. In this embodiment of the present application, the on / off command (e.g., an open command or a close command) of the off-grid-side first relay K_o1 may be delivered flexibly, thereby meeting the requirement of performing stuck detection or closing fault detection on the off-grid-side first relay K_o1 and providing greater control flexibility.

[0045] 4 is a schematic diagram of a circuit of a power conversion apparatus according to an embodiment of the present application. As shown in FIG. 4, the inverter circuit 21 shown in FIG. 2 includes a switch Q1, a switch Q2, a switch Q3, a switch Q4, an inductor L1, an inductor L2, and a capacitor C. The collector of the switch Q1 and the collector of the switch Q3 are both connected to the DC end dc21 of the inverter circuit 21, the emitter of the switch Q1 is connected to the collector of the switch Q2 and one end of the inductor L1, the emitter of the switch Q3 is connected to the collector of the switch Q4 and one end of the inductor L2, the emitter of the switch Q2 and the emitter of the switch Q4 are both connected to the DC end dc22 of the inverter circuit 21, the other end of the inductor L1 and one end of the capacitor C are both connected to the U-phase AC end ac21 of the inverter circuit 21, and the other end of the inductor L2 and the other end of the capacitor C are both connected to the W-phase AC end ac22 of the inverter circuit 21. It should be understood that the circuit topology of the inverter circuit 21 shown in FIG. 4 is merely an example, and the specific circuit topology of the inverter circuit 21 is not limited to this application. After controlling the off-grid-side first relay K_o1 to be open or closed, the controller 22 is configured to control each switch among the switches Q1, Q2, Q3, and Q4 to be on or off using a PWM driver gating signal transmission method, so that the inverter circuit 21 outputs AC current to the off-grid-side U-phase AC terminal ac11 and the off-grid-side W-phase AC terminal ac12 of the power conversion device 2. The voltage amplitude V0 of the AC current is smaller than a safe voltage. For example, the safe voltage is 36 V. In this case, the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 are balanced. Furthermore, the controller 22 is further configured to collect a first sampled voltage across the off-grid-side first relay K_o1 by using a voltage sampling circuit. The voltage sampling circuit is connected in parallel across the off-grid-side first relay K_o1, and the voltage sampling circuit may be disposed inside or outside the controller 22. This is not a limitation in this specification.

[0046] As shown in FIG. 4, the power converter 2 shown in FIG. 2 further includes a balanced bridge circuit 20. The input terminal of the balanced bridge circuit 20 is connected to the DC terminal of the power converter 2. Specifically, the input terminal of the balanced bridge circuit 20 includes an input terminal in11 and an input terminal in12. The input terminal in11 of the balanced bridge circuit 20 is connected to the DC terminal dc11 of the power converter 2 through the positive DC bus BUS+, and the input terminal in12 of the balanced bridge circuit 20 is connected to the DC terminal dc12 of the power converter 2 through the negative DC bus BUS-. The output terminal out1 of the balanced bridge circuit 20 is connected to a series connection point a. The series connection point a may also be referred to as a bus midpoint between the positive bus capacitor C1 and the negative bus capacitor C2. After collecting the first sampled voltage, the controller 22 is further configured to control the balanced bridge circuit 20 to adjust the difference between the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 to be greater than a first voltage threshold, and to collect a second sampled voltage across the off-grid side first relay K_o1 by using the voltage sampling circuit.

[0047] In a specific implementation, the balanced bridge circuit 20 includes a bridge arm 201 and an inductor L0. The bridge arm 201 includes an upper bridge arm switch Q5 and a lower bridge arm switch Q6. The upper bridge arm switch Q5 and the lower bridge arm switch Q6 are connected in series and then connected to the DC end of the power converter 2. The connection point between the upper bridge arm switch Q5 and the lower bridge arm switch Q6 is connected to a series connection point a through the inductor L0. The connection point between the upper bridge arm switch Q5 and the lower bridge arm switch Q6 is sometimes referred to as the midpoint of the bridge arm 201. It should be understood that one end of the upper bridge arm switch Q5, remote from the lower bridge arm switch Q6, is used as the input end in11 of the balanced bridge circuit 20 and is connected to the DC end dc11 of the power converter 2, and one end of the lower bridge arm switch Q6, remote from the upper bridge arm switch Q5, is used as the input end in12 of the balanced bridge circuit 20 and is connected to the DC end dc12 of the power converter 2. One end of the inductor L0, remote from the midpoint of the bridge arm 201, is used as the output end out1 of the balanced bridge circuit 20 and is connected to the series connection point a. Note that the balanced bridge circuit 20 shown in FIG. 4 is merely an example, and other circuit topologies having the function of unbalancing the voltages of the positive bus capacitor C1 and the negative bus capacitor C2 may be used. This is not a limitation of the present application.

[0048] After collecting the first sampled voltage, the controller 22 is further configured to generate drive signals for the upper bridge arm switch Q5 and the lower bridge arm switch Q6 based on the target current I0, and control the upper bridge arm switch Q5 and the lower bridge arm switch Q6 to be on or off based on the drive signals for the upper bridge arm switch Q5 and the lower bridge arm switch Q6, respectively, so that the bridge 201 outputs the target current I0 to the series connection point a through the inductor L0. The drive signals include a switch duty cycle, a switch on time, and a dead time, and the dead time is the time during which the upper bridge arm switch Q5 and the lower bridge arm switch Q6 are simultaneously in the off state. When the upper bridge arm switch Q5 is on, the lower bridge arm switch Q6 is off, storing energy for inductor L0. When the upper bridge arm switch Q5 is off, the lower bridge arm switch Q6 is on, outputting the target current I0 stored in inductor L0 to series connection point a, causing the voltages of the positive bus capacitor C1 and the negative bus capacitor C2 to become unbalanced. The difference Vthres between the voltages of the positive bus capacitor C1 and the negative bus capacitor C2 is positively correlated with the target current I0. That is, as the difference Vthres increases, the target current I0 increases, and as the difference Vthres decreases, the target current I0 decreases.

[0049] In this embodiment of the present application, the current output to the series connection point a can be flexibly adjusted by using the balanced bridge circuit 20 to change the value of the difference Vthres between the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2, so as to achieve the purpose of the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 being unbalanced. The control method is more flexible.

[0050] 4, the target current I0 may be collected by a current sampling device located internally or externally to the controller 22. The current sampling device may be a current sensor (e.g., an HCT series current sensor), a current transformer, a Hall sensor, a Rogowski coil, or another device. For example, the current sensor may be a high current test (HCT) current sensor that detects the current-withstanding capability of the balanced bridge circuit 20 and ensures safe and reliable operation of the balanced bridge circuit 20.

[0051] FIG. 5 is a schematic diagram of another circuit of a power converter according to an embodiment of the present application. As shown in FIG. 5, the power converter 2 shown in FIG. 4 further includes an insulation impedance detection circuit 23. The insulation impedance detection circuit 23 is connected to the DC end of the power converter 2. Specifically, a first connection end of the insulation impedance detection circuit 23 is connected to the DC end dc11 of the power converter 2 through the positive DC bus BUS+, and a second connection end of the insulation impedance detection circuit 23 is connected to the DC end dc12 of the power converter 2 through the negative DC bus BUS-. The controller 22 is further configured to control the insulation impedance detection circuit 23 to adjust the difference between the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 to be greater than a first voltage threshold, thereby achieving the objective of the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 being unbalanced. This provides a more flexible control method.

[0052] In a specific implementation, as shown in FIG. 6, the insulation impedance detection circuit 23 shown in FIG. 5 further includes a first resistor R1, a second resistor R2, a third resistor R3, and a second relay K_iso. The first resistor R1, the second resistor R2, and the third resistor R3 are connected in series and then connected to the DC end of the power converter 2. That is, one end of the series-connected first resistor R1, the second resistor R2, and the third resistor R3 serves as a first connection end of the insulation impedance detection circuit 23 and is connected to the DC end dc11 of the power converter 2. The other end of the series-connected first resistor R1, the second resistor R2, and the third resistor R3 serves as a second connection end of the insulation impedance detection circuit 23 and is connected to the DC end dc12 of the power converter 2. One end of the second resistor R2 facing the first resistor R1 is connected to one end of the second relay K_iso and the reference ground PE, and the other end of the second resistor R2 away from the first resistor R1 is connected to the other end of the second relay K_iso. It should be understood that the circuit topology of the insulation impedance detection circuit 23 shown in FIG. 6 is merely an example, and that another circuit topology having the function of unbalancing the voltages of the positive bus capacitor C1 and the negative bus capacitor C2 may be used. This is not a limitation of this specification. Before the first sampled voltage is acquired, the second relay K_iso is in an open state.

[0053] After collecting the first sampled voltage, the controller 22 is further configured to control the second relay K_iso to close to adjust the difference between the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 to be greater than a first voltage threshold. Specifically, the controller 22 is further configured to send a close command to the second relay K_iso. The second relay K_iso is configured to receive the close command and is stably closed after a response time t1. When the second relay K_iso is stably closed, the difference between the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 is greater than the first voltage threshold. Furthermore, the controller 22 is further configured to collect multiple sampled voltages across the off-grid-side first relay K_o1 during a time window t2 after the second relay K_iso is stably closed, and use the average value of the multiple sampled voltages as a second sampled voltage. The multiple sampled voltages are voltages collected by the controller 22 at different times within the time window t2. It should be understood that during time window t2, the potential at the series connection point a is unstable, i.e., the potential at one end of the off-grid-side first relay K_o1 facing the series connection point a is unstable. In this case, the second sampled voltage is the average value of multiple sampled voltages at both ends of the off-grid-side first relay K_o1. This can improve the accuracy of the second sampled voltage.

[0054] In this embodiment of the present application, before the second relay K_iso is closed, the first resistor R1, the second resistor R2, and the third resistor R3 divide the voltage between the positive bus BUS+ and the negative DC bus BUS-. After the second relay K_iso is closed, the second resistor R2 is shorted. In this case, the first resistor R1 and the third resistor R3 divide the voltage between the positive bus BUS+ and the negative DC bus BUS-. Therefore, before and after the second relay K_iso is closed, the voltages across the first resistor R1 and the third resistor R3 change, thereby changing the potential of the positive bus BUS+ and the negative DC bus BUS-, thereby achieving the goal of unbalancing the voltages across the positive bus capacitor C1 and the negative bus capacitor C2.

[0055] In some possible implementations, the power conversion device 2 may include at least one of a balanced bridge circuit 20 and an insulation impedance detection circuit 23. For example, when the power conversion device 2 includes the balanced bridge circuit 20, the circuit topology of the power conversion device 2 is shown in FIG. 4. When the power conversion device 2 includes the balanced bridge circuit 20 and the insulation impedance detection circuit 23, the circuit topology of the power conversion device 2 is shown in FIGS. 5 and 6. In this case, the controller 22 has a higher priority in controlling the balanced bridge circuit 20 to unbalance the voltages of the positive bus capacitor C1 and the negative bus capacitor C2 than in controlling the insulation impedance detection circuit 23 to unbalance the voltages of the positive bus capacitor C1 and the negative bus capacitor C2. When the power conversion device 2 includes the insulation impedance detection circuit 23, the circuit topology of the power conversion device 2 is shown in FIG. 7. It should be understood that the circuit structure and operating principle of the insulation impedance detection circuit 23 in FIG. 7 are referred to the embodiment corresponding to FIG. 6. Details will not be described again in this specification.

[0056] When the power conversion device 2 performs stuck detection on the off-grid-side first relay K_o1 and acquires the first sampled voltage V11 and the second sampled voltage V21 or V23, the absolute value of the difference can be expressed as ΔV11=abs(V21-V11) or ΔV13=abs(V23-V11). V23 is the average value of multiple sampled voltages across the off-grid-side first relay K_o1. The controller 22 is further configured to output an alarm signal when the absolute value of the difference ΔV11 or ΔV13 is equal to or less than a second voltage threshold. The alarm signal indicates that the off-grid-side first relay K_o1 is stuck, i.e., the off-grid-side first relay K_o1 cannot switch from a closed state to an open state. In this case, the off-grid-side first relay K_o1 is short-circuited. Generally, the second voltage threshold is smaller than the difference Vthres. For example, the second voltage threshold is 10 V. The controller 22 is further configured to determine that the first off-grid-side relay K_o1 is in a non-fault state if the absolute value of the difference ΔV11 or ΔV13 is greater than a second voltage threshold. In this case, the first off-grid-side relay K_o1 is switched from a closed state to an open state. In this embodiment of the present application, if the first off-grid-side relay K_o1 is open after the controller 22 sends an open command to the first off-grid-side relay K_o1, the absolute value ΔV11 or ΔV13 of the difference between the first sampled voltage and the second sampled voltage is large. Conversely, if the first off-grid-side relay K_o1 is stuck and cannot be opened, both the first sampled voltage and the second sampled voltage are close to zero. In this case, the absolute value ΔV11 or ΔV13 of the difference between the first sampled voltage and the second sampled voltage is close to zero. Therefore, when the absolute value of the difference ΔV11 or ΔV13 is equal to or less than the second voltage threshold, it is possible to accurately identify whether the off-grid-side first relay K_o1 is stuck, thereby improving the efficiency of fault detection for the off-grid-side first relay K_o1.

[0057] When the power conversion device 2 performs closing fault detection for the off-grid-side first relay K_o1 and acquires a first sampled voltage V12 and a second sampled voltage V22 or V24, the absolute value of the difference can be expressed as ΔV12 = abs(V22 - V12) or ΔV14 = abs(V24 - V12), where V24 is the average value of the multiple sampled voltages across the off-grid-side first relay K_o1. The controller 22 is further configured to output an alarm signal when the absolute value of the difference ΔV12 or ΔV14 is equal to or greater than a second voltage threshold. The alarm signal indicates that the off-grid-side first relay K_o1 is in an open state, i.e., the off-grid-side first relay K_o1 cannot switch from an open state to a closed state. In this case, the off-grid-side first relay K_o1 is open. The controller 22 is further configured to determine that the first off-grid-side relay K_o1 is in a non-fault state if the absolute value of the difference ΔV12 or ΔV14 is less than a second voltage threshold. In this case, the first off-grid-side relay K_o1 is switched from an open state to a closed state. In this embodiment of the present application, after the controller 22 sends a close command to the first off-grid-side relay K_o1, if the first off-grid-side relay K_o1 is closed, both the first sampled voltage and the second sampled voltage are close to zero. In this case, the absolute value ΔV12 or ΔV14 of the difference between the first sampled voltage and the second sampled voltage is close to zero. Conversely, if the first off-grid-side relay K_o1 is not closed and cannot be closed, the absolute value ΔV12 or ΔV14 of the difference between the first sampled voltage and the second sampled voltage is large. Therefore, when the absolute value of the difference ΔV12 or ΔV14 is equal to or greater than the second voltage threshold, a closing fault of the off-grid-side first relay K_o1 can be accurately identified, thereby improving the efficiency of fault detection of the off-grid-side first relay K_o1.

[0058] After the stuck-on detection or closed fault detection is performed on the off-grid-side first relay K_o1, the controller 22 is further configured to control the difference between the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 to be equal to or less than a first voltage threshold when the off-grid-side first relay K_o1 is in a non-fault state. In this case, the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 are balanced. In a specific implementation, when the balanced bridge circuit 20 is used to unbalance the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2, the controller 22 is further configured to control the upper bridge arm switch Q5 and the lower bridge arm switch Q6 to be open when the off-grid-side first relay K_o1 is in a non-fault state, so that the difference between the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 is equal to or less than the first voltage threshold. Optionally, when the insulation impedance detection circuit 23 is used to unbalance the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2, the controller 22 is further configured to control the second relay K_iso to be open when the first off-grid-side relay K_o1 is in a non-fault state, so that the difference between the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 is equal to or less than a first voltage threshold. In this embodiment of the present application, when the first off-grid-side relay K_o1 is in a non-fault state, the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 can be controlled to be balanced. This ensures that the power conversion device 2 stably outputs AC current to the outside and reduces the occurrence of a fault in the power conversion device 2.

[0059] In some possible implementations, as shown in FIGS. 4 to 7 , the power conversion device 2 further includes an off-grid-side third relay K_u1, an off-grid-side fourth relay K_w1, and an off-grid-side fifth relay K_ope1. The off-grid-side third relay K_u1 is also referred to as the off-grid-side U-phase relay, and the off-grid-side fourth relay K_w1 is also referred to as the off-grid-side W-phase relay. The off-grid-side third relay K_u1 is disposed between the U-phase AC terminal ac21 of the inverter circuit 21 and the off-grid-side U-phase AC terminal ac11 of the power conversion device 2, the off-grid-side fourth relay K_w1 is disposed between the W-phase AC terminal ac22 of the inverter circuit 21 and the off-grid-side W-phase AC terminal ac12 of the power conversion device 2, and the off-grid-side fifth relay K_ope1 is disposed between the series connection point a and a reference ground (protective earth, PE). For example, the reference ground PE may be the ground or the housing of the power conversion device 2. In this case, the off-grid first relay K_o1, the off-grid third relay K_u1, and the off-grid fourth relay K_w1 form off-grid side relays in the power conversion device 2.

[0060] If the off-grid-side fifth relay K_ope1 is closed before the power conversion device 2 detects a stuck or closed fault in the off-grid-side first relay K_o1, the potential at the series connection point a is pulled down to 0. In this case, the absolute value of the difference between the first sampled voltage and one of the second sampled voltages or the average value is close to 0. Therefore, it is not possible to determine whether the off-grid-side first relay K_o1 is faulty based on the absolute value of the difference. Therefore, the off-grid-side fifth relay K_ope1 is in an open state. The off-grid-side third relay K_u1 and the off-grid-side fourth relay K_w1 may be in an on state or a closed state. This is not a limitation in this specification. Generally, before the power conversion device 2 operates in an off-grid mode, fault detection must be performed on the off-grid-side third relay K_u1 and the off-grid-side fourth relay K_w1. Therefore, voltage sampling circuits are connected in parallel to both ends of the off-grid-side third relay K_u1 and both ends of the off-grid-side fourth relay K_w1. When both the off-grid side third relay K_u1 and the off-grid side fourth relay K_w1 are in the open state, the AC output by the inverter circuit 21 can be transmitted to the off-grid side U-phase AC terminal ac11 and the off-grid side W-phase AC terminal ac12 of the power conversion device 2 through a voltage sampling circuit connected in parallel across both ends of the off-grid side third relay K_u1 and a voltage sampling circuit connected in parallel across both ends of the fourth relay K_w1.

[0061] After the power conversion device 2 performs stuck detection or closed fault detection for the off-grid-side first relay K_o1, the controller 22 is further configured to control the off-grid-side first relay K_o1, the off-grid-side third relay K_u1, the off-grid-side fourth relay K_w1, and the off-grid-side fifth relay K_ope1 to all be on when the off-grid-side first relay K_o1 is in a non-fault state, thereby enabling a connection between the inverter circuit 21 and the load 41, so that the power conversion device 2 outputs AC current to the load 41 for power supply. In this embodiment of the present application, when the off-grid-side first relay K_o1 is identified to be in a non-fault state, the off-grid-side relay inside the power conversion device 2 can be controlled to be closed, ensuring safe and reliable operation of the power conversion device 2. This improves the power supply reliability and power supply safety of the load 41.

[0062] 8A is a schematic diagram of a circuit when a power conversion device has both off-grid operation function and grid-connected operation function according to an embodiment of the present application. As shown in FIG. 8A, the power conversion device 2 is further configured to be connected between a DC power source 3 and a power grid 42 for energy conversion. Specifically, the power conversion device 2 is configured to convert DC output from the DC power source 3 into AC and supply power to the power grid 42. The power conversion device 2 shown in FIG. 6 further includes a grid-connected third relay K_u2, a grid-connected fourth relay K_w2, a capacitor C3, and a capacitor C4. The grid-connected third relay K_u2 and the grid-connected fourth relay K_w2 may form a grid-connected side relay inside the power conversion device 2. The grid-connected-side third relay K_u2 is arranged between the U-phase AC terminal ac21 and the grid-connected-side U-phase AC terminal ac14 of the inverter circuit 21, and the grid-connected-side fourth relay K_w2 is arranged between the W-phase AC terminal ac22 and the grid-connected-side W-phase AC terminal ac15 of the inverter circuit 21. The grid-connected-side third relay K_u2 is also called the grid-connected-side U-phase relay, and the grid-connected-side fourth relay K_w2 is also called the grid-connected-side W-phase relay. Capacitors C3 and C4 are connected in series and connected between the grid-connected-side U-phase AC terminal ac14 and the grid-connected-side W-phase AC terminal ac15, and the series connection point between capacitors C3 and C4 is connected to the grid-connected-side O-phase AC terminal ac16 of the power conversion device 2. When the power conversion device 2 operates in a grid-connected mode, the controller 22 is further configured to control both the grid-connected side third relay K_u2 and the grid-connected side fourth relay K_w2 to close, thereby enabling a connection between the inverter circuit 21 and the power grid 42, so that the AC current output by the inverter circuit 21 is filtered by the capacitor C3 and the capacitor C4, and then supplies power to the power grid 42.

[0063] When the grid-connected side relays in the power conversion device 2 shown in FIG. 8A further include a grid-connected side O-phase relay, the circuit topology of the power conversion device 2 may be as shown in FIG. 8B. The power conversion device 2 shown in FIG. 8A further includes a grid-connected side first relay K_o2 and a grid-connected side fifth relay K_ope2. The grid-connected side first relay K_o2 is connected between the series connection point a and the grid-connected side O-phase AC terminal ac16, and the grid-connected side fifth relay K_ope2 is connected between the series connection point a and the reference ground PE. The grid-connected side first relay K_o2 is also called a grid-connected side O-phase relay, and the grid-connected side first relay K_o2 is also called a grid-connected side neutral ground relay. In this case, the grid-connected side first relay K_o2, the grid-connected side third relay K_u2, and the grid-connected side fourth relay K_w2 may constitute grid-connected side relays within the power conversion device 2.

[0064] Performing fault detection on the grid-connected first relay K_o2 is used as an example for explanation. Before the power conversion device 2 operates in the grid-connected mode, the controller 22 is configured to: cause the inverter circuit 21 to output AC to the grid-connected U-phase AC terminal ac14 and the grid-connected W-phase AC terminal ac15 of the power conversion device 2; and collect a first sampled voltage across the grid-connected first relay K_o2 when a difference between the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 is equal to or less than a first voltage threshold. The controller 22 is further configured to control the balanced bridge circuit 20 or the insulation impedance detection circuit 23 to adjust the difference between the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2 to be greater than the first voltage threshold; and collect a second sampled voltage across the grid-connected first relay K_o2. The controller 22 is further configured to output an alarm signal based on the absolute value of the difference between the first sampled voltage and the second sampled voltage. The alarm signal indicates that the grid-connected first relay K_o2 is in a fault state. In this embodiment of the present application, the efficiency of detecting a fault in the grid-connected first relay K_o2 can be improved, and the user or maintenance personnel of the power conversion device 2 can be prevented from being electric shock. This ensures the safety of the user or maintenance personnel and further improves the power supply safety of the power conversion device 2.

[0065] Before performing fault detection on the grid-connected first relay K_o2, the grid-connected third relay K_u2 and the grid-connected fourth relay K_w2 may each be in an on state or a closed state. This is not a limitation in the present specification. The grid-connected fifth relay K_ope2 is in an open state. After fault detection is performed on the grid-connected first relay K_o2, the controller 22 is further configured to control the grid-connected first relay K_o2, the grid-connected third relay K_u2, the grid-connected fourth relay K_w2, and the grid-connected fifth relay K_ope2 to all be closed when the grid-connected first relay K_o2 is in a non-fault state, thereby enabling a connection between the inverter circuit 21 and the power grid 42, so that the AC current output by the inverter circuit 21 is filtered by the capacitor C3 and the capacitor C4 and then supplies power to the power grid 42. In the embodiment of the present application, when the grid-connected first relay K_o2 is identified as being in a non-fault state, the grid-connected relay in the power conversion device 2 can be controlled to close, which improves the reliability of power supply and power supply safety of the power grid 42.

[0066] It should be understood that for specific operations and corresponding beneficial effects of the fault detection performed by the controller 22 on the grid-connected-side first relay K_o2, please refer to the description and beneficial effects of the fault detection performed by the controller 22 on the off-grid-side first relay K_o1 in the embodiments corresponding to Figures 2 to 6. The details will not be described again in this specification.

[0067] FIG. 9A is a schematic diagram of another circuit when a power conversion device has both off-grid operation and grid-connected operation functions according to an embodiment of the present application. As shown in FIG. 9A , the power conversion device 2 shown in FIG. 7 further includes a grid-connected third relay K_u2, a grid-connected fourth relay K_w2, a capacitor C3, and a capacitor C4. For the connection relationships and control methods between the components, please refer to the embodiment corresponding to FIG. 8A . Details will not be described again herein. When the grid-connected relay in the power conversion device 2 shown in FIG. 9A further includes a grid-connected O-phase relay, the circuit topology of the power conversion device 2 may be as shown in FIG. 9B . The power conversion device shown in FIG. 9A further includes a grid-connected first relay K_o2 and a grid-connected fifth relay K_ope2. For the connection relationships and control methods between the components, please refer to the embodiment corresponding to FIG. 8B . Details will not be described again herein.

[0068] In the power conversion device 2 provided in the present application, before the power conversion device 2 operates in an off-grid or grid-connected mode, the balancing bridge circuit 20 or the insulation impedance detection circuit 23 is controlled to unbalance the voltage of the positive bus capacitor C1 and the voltage of the negative bus capacitor C2. Then, based on the absolute value of the difference between the sampled voltages of the off-grid-side first relay K_o1 or the grid-connected first relay K_o2 before and after balancing, it is determined whether the off-grid-side first relay K_o1 or the grid-connected first relay K_o2 has a fault. This avoids false detections that would occur if a single sampled voltage were used to determine that the off-grid-side first relay K_o1 or the grid-connected first relay K_o2 has a fault, improving the efficiency of fault detection for the off-grid-side first relay K_o1 or the grid-connected first relay K_o2. This fault detection method is more flexible.

[0069] 10 is a schematic flowchart of a relay fault detection method for a power conversion device according to an embodiment of the present application. The relay fault detection method is generally performed by a controller disposed inside or outside the power conversion device. Specifically, as shown in FIG. 10, the relay fault detection method includes the following steps S11 to S13.

[0070] Step S11: When the inverter circuit outputs AC to the U-phase AC terminal and the W-phase AC terminal of the power conversion device, and the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor is less than or equal to a first voltage threshold, the controller collects a first sampled voltage across the first relay.

[0071] Step S12: The controller may further control the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor to be greater than a first voltage threshold, and collect a second sampled voltage across the first relay.

[0072] In step S12, the positive bus capacitor and the negative bus capacitor are connected in series and connected to the DC end of the power converter, the DC end of the inverter circuit is connected to the DC end of the power converter, the U-phase AC end of the inverter circuit is connected to the U-phase AC end of the power converter, the W-phase AC end of the inverter circuit is connected to the W-phase AC end of the power converter, and the series connection point between the positive bus capacitor and the negative bus capacitor is connected to the O-phase AC end of the power converter through a first relay.

[0073] Step S13: The controller may further output an alarm signal based on the absolute value of the difference between the first sampled voltage and the second sampled voltage.

[0074] In step S13, the alarm signal indicates that the first relay is in a fault condition.

[0075] 11 is a diagram of an operation procedure in which a controller performs stuck detection of the first relay by using a balanced bridge circuit according to an embodiment of the present application. As shown in FIG. 11, the operation procedure includes the following steps S101 to S106.

[0076] Step S101: The inverter circuit outputs AC to the U-phase AC terminal and the W-phase AC terminal of the power conversion device, and before the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor becomes less than or equal to the first voltage threshold, the controller sends an open command to the first relay.

[0077] Step S102: When the inverter circuit outputs AC to the U-phase AC terminal and the W-phase AC terminal of the power conversion device, and the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor is less than or equal to a first voltage threshold, the controller collects a first sampling voltage V11 across the first relay.

[0078] Step S103: The controller may further control the balanced bridge circuit to adjust the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor to be greater than a first voltage threshold, and collect a second sampled voltage V21 across the first relay.

[0079] In step S103, the input terminal of the balanced bridge circuit is connected to the DC terminal of the power conversion device, and the output terminal of the balanced bridge circuit is connected to the series connection point.

[0080] Step S104: The absolute value of the difference ΔV11=abs(V21−V11) is equal to or less than the second voltage threshold.

[0081] If step S104 is true, the controller executes step S105. If step S104 is not true, that is, if the absolute value of the difference ΔV11=abs(V21-V11) is greater than the second voltage threshold, the controller executes step S106.

[0082] Step S105: The controller outputs an alarm signal.

[0083] In step S105, the alarm signal indicates that the first relay is stuck.

[0084] Step S106: The controller determines that the first relay is in a non-fault state.

[0085] In this embodiment of the present application, after the controller sends an open command to the first relay, if the first relay is open, the absolute value ΔV11 of the difference between the first sampled voltage V11 and the second sampled voltage V21 is large. Conversely, if the first relay is stuck and not open, both the first sampled voltage V11 and the second sampled voltage V21 are close to zero. In this case, the absolute value ΔV11 of the difference between the first sampled voltage V11 and the second sampled voltage V21 is close to zero. Therefore, if the absolute value ΔV11 of the difference is equal to or less than the second voltage threshold, it can be accurately identified that the first relay is stuck. This improves the efficiency of fault detection for the first relay.

[0086] 12 is a diagram of an operation procedure in which the controller performs on-fault detection for the first relay by using a balanced bridge circuit according to an embodiment of the present application. As shown in FIG. 12, the operation procedure includes the following steps S201 to S206.

[0087] Step S201: The inverter circuit outputs AC to the U-phase AC terminal and the W-phase AC terminal of the power conversion device, and before the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor becomes less than or equal to a first voltage threshold, the controller sends a close command to the first relay.

[0088] Step S202: When the inverter circuit outputs AC to the U-phase AC terminal and the W-phase AC terminal of the power conversion device, and the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor is less than or equal to a first voltage threshold, the controller collects a first sampled voltage V12 across the first relay.

[0089] Step S203: The controller may further control the balanced bridge circuit to adjust the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor to be greater than a first voltage threshold, and collect a second sampled voltage V22 across the first relay.

[0090] Step S204: The absolute value of the difference ΔV12=abs(V22−V12) is equal to or greater than the second voltage threshold.

[0091] If step S204 is true, the controller executes step S205. If step S204 is not true, that is, if the absolute value of the difference ΔV12=abs(V22-V12) is less than the second voltage threshold, the controller executes step S206.

[0092] Step S205: The controller outputs an alarm signal.

[0093] In step S205, the alarm signal indicates that the first relay is in a closed fault state.

[0094] Step S206: The controller determines that the first relay is in a non-fault state.

[0095] In this embodiment of the present application, after the controller sends a close command to the first relay, if the first relay is closed, both the first sampled voltage V12 and the second sampled voltage V22 are close to 0. In this case, the absolute value ΔV12 of the difference between the first sampled voltage V12 and the second sampled voltage V22 is close to 0. Conversely, if the first relay fails to close and cannot be closed, the absolute value ΔV12 of the difference between the first sampled voltage V12 and the second sampled voltage V22 is large. Therefore, if the absolute value ΔV12 of the difference is equal to or greater than the second voltage threshold, it is possible to accurately identify that the first relay has a closing fault. This improves the efficiency of fault detection for the first relay.

[0096] 13 is a diagram of an operation procedure in which the controller performs stuck detection on the first relay by using an insulation impedance detection circuit according to an embodiment of the present application. As shown in FIG. 13, the operation procedure diagram includes the following steps S301 to S306.

[0097] Step S301: The inverter circuit outputs AC to the U-phase AC terminal and the W-phase AC terminal of the power conversion device, and before the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor becomes less than or equal to the first voltage threshold, the controller sends an open command to the first relay.

[0098] Step S302: When the inverter circuit outputs AC to the U-phase AC terminal and the W-phase AC terminal of the power conversion device, and the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor is less than or equal to a first voltage threshold, the controller collects a first sampled voltage V11 across the first relay.

[0099] Step S303: The controller may further control the insulation impedance detection circuit to adjust the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor to be greater than a first voltage threshold, and collect an average value of multiple sampled voltages across the first relay as a second sampled voltage V23.

[0100] In a specific implementation, the controller may further control the second relay to close to adjust the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor to be greater than the first voltage threshold. When the isolation impedance detection circuit includes a first resistor, a second resistor, a third resistor, and a second relay, the first resistor, the second resistor, and the third resistor are connected in series and then connected to the DC end of the power conversion device, one end of the second resistor facing the first resistor is connected to one end of the second relay and the reference ground, and the other end of the second resistor away from the first resistor is connected to the other end of the second relay. It should be understood that before the second relay is closed, the first resistor, the second resistor, and the third resistor divide the voltage between the positive DC bus and the negative DC bus. After the second relay is closed, the second resistor is short-circuited. In this case, the first resistor and the third resistor divide the voltage between the positive bus and the negative DC bus. Therefore, before and after the second relay is closed, the voltage across the first resistor and the voltage across the third resistor change, thereby changing the potential of the positive bus and the negative DC bus, thereby achieving the purpose of unbalancing the voltages of the positive bus capacitor and the negative bus capacitor.

[0101] It can be seen that during the time window after the second relay is stably closed, the potential at the series connection point is unstable, i.e., the potential at one end of the first relay facing the series connection point is unstable. In this case, the second sampled voltage is the average value of multiple sampled voltages at both ends of the first relay. This improves the accuracy of the second sampled voltage. The multiple sampled voltages are voltages collected by the controller at different times within the time window.

[0102] Step S304: The absolute value of the difference ΔV13=abs(V23−V11) is equal to or less than the second voltage threshold.

[0103] If step S304 is true, the controller executes step S305. If step S304 is not true, that is, if the absolute value of the difference ΔV13=abs(V23-V11) is greater than the second voltage threshold, the controller executes step S306.

[0104] Step S305: The controller outputs an alarm signal.

[0105] In step S304, the alarm signal indicates that the first relay is stuck.

[0106] Step S306: The controller determines that the first relay is in a non-fault state.

[0107] In this embodiment of the present application, if the first relay is open after the controller sends an open command to the first relay, the absolute value ΔV13 of the difference between the first sampled voltage V11 and the second sampled voltage V23 is large. Conversely, if the first relay is stuck and not open, both the first sampled voltage V11 and the second sampled voltage V23 are close to zero. In this case, the absolute value ΔV13 of the difference between the first sampled voltage V11 and the second sampled voltage V23 is close to zero. Therefore, if the absolute value ΔV13 of the difference is equal to or less than the second voltage threshold, it can be accurately determined that the first relay is stuck. This improves the efficiency of fault detection for the first relay.

[0108] 14 is a diagram of an operation procedure in which the controller performs on-fault detection in the first relay by using an insulation impedance detection circuit according to an embodiment of the present application. As shown in FIG. 14, the operation procedure diagram includes the following steps S401 to S406.

[0109] Step S401: The inverter circuit outputs AC to the U-phase AC terminal and the W-phase AC terminal of the power conversion device, and before the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor becomes less than or equal to the first voltage threshold, the controller sends a close command to the first relay.

[0110] Step S402: When the inverter circuit outputs AC to the U-phase AC terminal and the W-phase AC terminal of the power conversion device, and the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor is less than or equal to a first voltage threshold, the controller collects a first sampled voltage V12 across the first relay.

[0111] Step S403: The controller may further control the insulation impedance detection circuit to adjust the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor to be greater than a first voltage threshold, and collect an average value of multiple sampled voltages across the first relay as a second sampled voltage V24.

[0112] Step S404: The absolute value of the difference ΔV14=abs(V24−V12) is equal to or greater than the second voltage threshold.

[0113] If step S404 is true, the controller executes step S405. If step S404 is not true, that is, if the absolute value of the difference ΔV14=abs(V24-V12) is less than the second voltage threshold, the controller executes step S406.

[0114] Step S405: The controller outputs an alarm signal.

[0115] In step S405, the alarm signal indicates that the first relay is in a closed fault state.

[0116] Step S406: The controller determines that the first relay is in a non-fault state.

[0117] In this embodiment of the present application, after the controller sends a close command to the first relay, if the first relay is closed, both the first sampled voltage V12 and the second sampled voltage V24 are close to zero. In this case, the absolute value ΔV14 of the difference between the first sampled voltage V12 and the second sampled voltage V24 is close to zero. Conversely, if the first relay fails to close and cannot be closed, the absolute value ΔV14 of the difference between the first sampled voltage V12 and the second sampled voltage V24 is large. Therefore, if the absolute value ΔV14 of the difference is equal to or greater than the second voltage threshold, it is possible to accurately identify that the first relay has a closing fault. This improves the efficiency of fault detection for the first relay.

[0118] In some possible implementations, after performing fault detection on the first relay, when the first relay is in a non-fault state, the controller controls the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor to be equal to or less than a first voltage threshold, so that the voltage of the positive bus capacitor and the voltage of the negative bus capacitor are balanced, thereby ensuring that the power conversion device can stably output AC current to the outside and reducing the fault occurrence rate of the power conversion device.

[0119] For more specific implementations of the relay fault detection method for a power converter provided herein, and the beneficial effects corresponding to those operations, please refer to the implementations performed by the controller of the power converter and the operating principles of the power converter, as well as the beneficial effects corresponding to those implementations, shown in Figures 2 to 9B. The details will not be described again in this specification.

[0120] In the relay fault detection method provided herein, before the power conversion device operates in an off-grid or grid-connected mode, the balancing bridge circuit or the insulation impedance detection circuit is controlled to unbalance the voltage of the positive bus capacitor and the voltage of the negative bus capacitor. Then, whether the first relay is faulty is determined based on the absolute value of the difference between the sampled voltages of the first relay before and after balancing. This avoids false detections that occur when a single sampled voltage is used to determine the first relay, improving the fault detection efficiency of the first relay and providing a more flexible fault detection method.

[0121] The above description is merely a specific implementation of the present application and is not intended to limit the scope of protection of the present application. Any modifications or replacements that can be easily conceived by those skilled in the art within the technical scope disclosed in the present application shall fall within the scope of protection of the present application. Therefore, the scope of protection of the present application shall be subject to the scope of protection of the claims.

Claims

1. 1. A power converter configured for connection between a DC power source and a load or a power grid for energy conversion, the power converter comprising: a positive bus capacitor; a negative bus capacitor; an inverter circuit; a first relay; and a controller; a DC end of the power converter configured to connect to the DC power source, the positive bus capacitor and the negative bus capacitor are connected in series and then connected to the DC end of the power converter, the DC end of the inverter circuit is connected to the DC end of the power converter, a U-phase AC end of the inverter circuit is connected to a U-phase AC end of the power converter, a W-phase AC end of the inverter circuit is connected to a W-phase AC end of the power converter, a series connection point between the positive bus capacitor and the negative bus capacitor is connected to an O-phase AC end of the power converter through the first relay, and the U-phase AC end, the W-phase AC end, and the O-phase AC end of the power converter are all configured to connect to the power grid or the load; The controller is configured to: collect a first sampled voltage across the first relay when the inverter circuit outputs AC to the U-phase AC terminal and the W-phase AC terminal of the power conversion device and a difference between a voltage of the positive bus capacitor and a voltage of the negative bus capacitor is less than or equal to a first voltage threshold; and collect a second sampled voltage across the first relay by controlling the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor to be greater than the first voltage threshold; The controller is further configured to output an alarm signal based on an absolute value of a difference between the first sampled voltage and the second sampled voltage, the alarm signal indicating that the first relay is in a fault condition. Power conversion device.

2. the fault condition comprises a stuck condition; The controller: sending an open command to the first relay before the inverter circuit outputs the AC current to the U-phase AC terminal and the W-phase AC terminal of the power conversion device and the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor is less than or equal to the first voltage threshold; When the inverter circuit outputs the AC current to the U-phase AC terminal and the W-phase AC terminal of the power conversion device and the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor is less than or equal to the first voltage threshold, collecting the first sampled voltage across the first relay; controlling the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor to be greater than the first voltage threshold, and collecting the second sampled voltage across the first relay; outputting the alarm signal when the absolute value of the difference between the first sampled voltage and the second sampled voltage is less than or equal to a second voltage threshold; wherein the alarm signal indicates that the first relay is in the stuck state. The power conversion device according to claim 1 .

3. the fault condition comprises a close fault condition; The controller: sending a close command to the first relay before the inverter circuit outputs the AC current to the U-phase AC terminal and the W-phase AC terminal of the power conversion device and the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor is less than or equal to the first voltage threshold; When the inverter circuit outputs the AC current to the U-phase AC terminal and the W-phase AC terminal of the power conversion device and the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor is less than or equal to the first voltage threshold, collecting the first sampled voltage across the first relay; controlling the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor to be greater than the first voltage threshold, and collecting the second sampled voltage across the first relay; outputting the alarm signal when the absolute value of the difference between the first sampled voltage and the second sampled voltage is greater than or equal to a second voltage threshold; and the alarm signal indicates that the first relay is in the closed fault state. The power conversion device according to claim 1 .

4. The power conversion device further includes a balanced bridge circuit, an input terminal of the balanced bridge circuit being connected to the DC terminal of the power conversion device, and an output terminal of the balanced bridge circuit being connected to the series connection point. the controller is further configured to control the balanced bridge circuit to adjust the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor to be greater than the first voltage threshold. The power conversion device according to any one of claims 1 to 3.

5. the balanced bridge circuit includes a bridge arm and an inductor, the bridge arm including an upper bridge arm switch and a lower bridge arm switch; the upper bridge arm switch and the lower bridge arm switch are connected in series and then connected to the DC end of the power conversion device, and a connection point between the upper bridge arm switch and the lower bridge arm switch is connected to the series connection point through the inductor; The power conversion device according to claim 4.

6. the power conversion device further includes an insulation impedance detection circuit, the insulation impedance detection circuit being connected to the DC end of the power conversion device; the controller is further configured to control the isolation impedance detection circuit to adjust the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor to be greater than the first voltage threshold. The power conversion device according to claim 1 .

7. the insulation impedance detection circuit includes a first resistor, a second resistor, a third resistor, and a second relay, the first resistor, the second resistor, and the third resistor being connected in series and then connected to the DC end of the power conversion device, one end of the second resistor facing the first resistor being connected to one end of the second relay and a reference ground, and the other end of the second resistor remote from the first resistor being connected to the other end of the second relay; the controller is further configured to control the second relay to close to regulate the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor to be greater than the first voltage threshold. The power conversion device according to claim 6.

8. The power conversion device according to claim 6 or 7, wherein the second sampled voltage is an average value of a plurality of sampled voltages at both ends of the first relay.

9. 2. The power conversion apparatus of claim 1, wherein the controller is further configured to control the difference between the voltage on the positive bus capacitor and the voltage on the negative bus capacitor to be less than or equal to the first voltage threshold when the first relay is in a non-fault state.

10. the power conversion device further includes a third relay, a fourth relay, and a fifth relay, the third relay being arranged between the U-phase AC end of the inverter circuit and the U-phase AC end of the power conversion device, the fourth relay being arranged between the W-phase AC end of the inverter circuit and the W-phase AC end of the power conversion device, and the fifth relay being arranged between the series connection point and a reference ground; the controller is further configured to control the first relay, the third relay, the fourth relay, and the fifth relay to all be closed when the first relay is in the non-fault state, thereby causing the power conversion device to output alternating current to the load or the power grid. The power conversion device according to claim 1 .

11. 1. A method for relay fault detection for a power converter, the method comprising: an inverter circuit outputs AC current to a U-phase AC terminal and a W-phase AC terminal of the power conversion device, and when a difference between a voltage of a positive bus capacitor and a voltage of a negative bus capacitor is less than or equal to a first voltage threshold, collecting a first sampled voltage across a first relay; collecting a second sampled voltage across the first relay by controlling the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor to be greater than the first voltage threshold, wherein the positive bus capacitor and the negative bus capacitor are connected in series and then connected to a DC end of the power converter, a DC end of the inverter circuit is connected to the DC end of the power converter, a U-phase AC end of the inverter circuit is connected to the U-phase AC end of the power converter, a W-phase AC end of the inverter circuit is connected to the W-phase AC end of the power converter, and a series connection point between the positive bus capacitor and the negative bus capacitor is connected to an O-phase AC end of the power converter through the first relay; outputting an alarm signal based on an absolute value of a difference between the first sampled voltage and the second sampled voltage, the alarm signal indicating that the first relay is in a fault condition. Relay fault detection method.

12. the fault condition comprises a stuck condition; Before the inverter circuit outputs the AC to the U-phase AC terminal and the W-phase AC terminal of the power converter and the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor is less than or equal to the first voltage threshold, the method further includes: sending an open command to the first relay; The step of outputting an alarm signal based on an absolute value of a difference between the first sampled voltage and the second sampled voltage comprises: outputting the alarm signal when the absolute value of the difference between the first sampled voltage and the second sampled voltage is less than or equal to a second voltage threshold, the alarm signal indicating that the first relay is in the stuck state. The method for detecting a relay fault according to claim 11.

13. the fault condition comprises a close fault condition; Before the inverter circuit outputs the AC to the U-phase AC terminal and the W-phase AC terminal of the power converter and the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor is less than or equal to the first voltage threshold, the method further includes: sending a close command to the first relay; The step of outputting an alarm signal based on an absolute value of a difference between the first sampled voltage and the second sampled voltage comprises: outputting the alarm signal when the absolute value of the difference between the first sampled voltage and the second sampled voltage is equal to or greater than a second voltage threshold, the alarm signal indicating that the first relay is in the closed fault state; The method for detecting a relay fault according to claim 11.

14. Controlling the difference between the voltage on the positive bus capacitor and the voltage on the negative bus capacitor to be greater than the first voltage threshold includes: controlling a balanced bridge circuit to adjust the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor to be greater than the first voltage threshold, an input terminal of the balanced bridge circuit being connected to the DC terminal of the power converter and an output terminal of the balanced bridge circuit being connected to the series connection point; 14. A method for detecting a relay fault according to any one of claims 11 to 13.

15. Controlling the difference between the voltage on the positive bus capacitor and the voltage on the negative bus capacitor to be greater than the first voltage threshold includes: controlling an isolation impedance detection circuit to adjust the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor to be greater than the first voltage threshold, the isolation impedance detection circuit being connected to the DC end of the power conversion device; 14. A method for detecting a relay fault according to any one of claims 11 to 13.

16. controlling an isolation impedance detection circuit to adjust the difference between the voltage on the positive bus capacitor and the voltage on the negative bus capacitor to be greater than the first voltage threshold; and controlling a second relay to close, thereby adjusting the difference between the voltage of the positive bus capacitor and the voltage of the negative bus capacitor to be greater than the first voltage threshold; the isolation impedance detection circuit includes a first resistor, a second resistor, a third resistor, and the second relay, the first resistor, the second resistor, and the third resistor being connected in series and then connected to the DC end of the power conversion device, one end of the second resistor facing the first resistor being connected to one end of the second relay and a reference ground, and the other end of the second resistor remote from the first resistor being connected to the other end of the second relay.

16. The method of claim 15, wherein the relay fault detection method is

17. 16. The method of claim 15, wherein the second sampled voltage is an average value of a plurality of sampled voltages across the first relay.

18. The method comprises:

12. The method of claim 11, comprising regulating the difference between the voltage on the positive bus capacitor and the voltage on the negative bus capacitor to be less than or equal to the first voltage threshold when the first relay is in a non-fault condition.

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