Grid-connected inverter and photovoltaic inverter system

Abstract

The present application provides a grid-connected inverter and a photovoltaic inverter system, wherein the grid-connected inverter comprises: an inverter circuit, at least two phase line branches, at least two first capacitors and a control circuit. The input end of the inverter circuit is used to connect a DC power supply, and the output end of the inverter circuit is connected to the power grid through the phase line branch. Each phase line branch comprises a first relay and a second relay connected in series. The first end of each first capacitor is connected between the output end of the inverter circuit and the corresponding phase line branch, and the second end of the first capacitor is grounded. The control circuit connects the first relay and the second relay, and the control circuit is used to control the first relay and / or the second relay to close before the grid-connected inverter is connected to the grid, obtain the capacitor voltage of the first capacitor and the phase voltage of the power grid, and confirm the fault detection results of the first relay and the second relay according to the capacitor voltage and the phase voltage. Thus, it is possible to detect whether the relay is faulty in a timely manner, thereby improving the accuracy of relay failure detection.

Description

Technical Field

[0001] The present application belongs to the technical field of power systems, and in particular relates to a grid-connected inverter and a photovoltaic inverter system. Background Art

[0002] In photovoltaic power generation systems, grid-connected inverters are generally required to convert the DC power generated by solar panels into AC power that can be connected to the AC power grid. During the conversion process, the DC power output by the solar panels needs to be input into the grid-connected inverter to invert the DC power into AC power that can be used by the grid. In order to reduce the losses caused by the operation of the grid-connected inverter itself, the grid-connected inverter usually uses a non-isolated inverter. The solar panels will generate leakage current through the non-isolated inverter and transmit it to the grid, which will affect the performance of the grid. Utility Model Content

[0003] The embodiments of the present application provide a grid-connected inverter and a photovoltaic inverter system, which can allow leakage current to flow into the ground through a first capacitor, thereby reducing the impact of the leakage current.

[0004] In a first aspect, an embodiment of the present application provides a grid-connected inverter, comprising an inverter circuit, at least two phase line branches and at least two first capacitors and a control circuit, wherein the input end of the inverter circuit is used to connect a DC power supply, and the output end of the inverter circuit is connected to a power grid through the phase line branch; each phase line branch comprises a first relay and a second relay connected in series; the first end of each first capacitor is connected between the output end of the inverter circuit and the corresponding phase line branch, and the second end of the first capacitor is grounded; the control circuit connects the first relay and the second relay, and the control circuit is used to control the first relay and / or the second relay to close before the grid-connected inverter is connected to the grid, obtain the capacitor voltage of the first capacitor and the phase voltage of the power grid, and confirm the fault detection results of the first relay and the second relay according to the capacitor voltage and the phase voltage.

[0005] In one possible implementation, the control circuit is used to: before the grid-connected inverter is connected to the grid, control one of the first relay and the second relay to be disconnected and the other to be closed; obtain the capacitor voltage of the first capacitor and the phase voltage of the grid; when the voltage difference between the capacitor voltage and the corresponding phase voltage is less than a preset voltage threshold, confirm that the currently disconnected relay has a sticking fault; when the voltage difference is greater than the preset voltage threshold, confirm that the currently disconnected relay does not have a sticking fault.

[0006] In one possible implementation, the control circuit is used to: control the first relay and the second relay to be closed before the grid-connected inverter is connected to the grid; obtain the capacitor voltage of the first capacitor and the phase voltage of the grid; when the voltage difference between the capacitor voltage and the corresponding phase voltage is less than a preset voltage threshold, confirm that the currently disconnected relay does not have an open circuit fault; when the voltage difference is greater than the preset voltage threshold, confirm that at least one of the first relay and the second relay has an open circuit fault.

[0007] In a possible implementation, the grid-connected inverter further includes a DC bus and two bus capacitors, and the input end of the inverter circuit is connected to a DC power source through the DC bus; and the two bus capacitors are connected in series to the DC bus.

[0008] In one possible implementation, the grid-connected inverter further includes at least two second capacitors, a first end of each second capacitor is correspondingly connected between the output end of the inverter circuit and the phase line branch, and a second end of each second capacitor is connected to the connection midpoint of the two bus capacitors.

[0009] In one possible implementation, the phase line branch also includes a first inductor, a first end of the first inductor is connected to the output end of the inverter circuit, a second end of the first inductor is correspondingly connected to the first end of the first relay, a second end of the first relay is connected to the first end of the second relay, and a second end of the second relay is connected to the power grid.

[0010] In a possible implementation, the phase line branch further includes a second inductor, and the second end of the second relay is connected to the power grid through the corresponding second inductor.

[0011] In a possible implementation, the grid-connected inverter further includes a current detection circuit connected between each phase line branch and the grid, and the current detection circuit is used to detect the current of the phase line branch.

[0012] In a possible implementation, the phase line branches include three, the inverter circuit includes three output terminals, and the three output terminals of the inverter circuit are respectively connected to a neutral terminal and two live terminals of a power grid through the three phase line branches.

[0013] In a second aspect, an embodiment of the present application provides a photovoltaic inverter system, including a photovoltaic module and a grid-connected inverter as described above; the photovoltaic module is connected to the input end of the inverter circuit.

[0014] The grid-connected inverter provided by the embodiment of the present application includes but is not limited to the following technical effects:

[0015] By connecting the first end of each first capacitor between the output end of the inverter circuit and the corresponding phase line branch, and grounding the second end of the first capacitor, the control circuit can control the first relay and / or the second relay to close before the grid-connected inverter is connected to the grid, obtain the capacitor voltage of the first capacitor and the phase voltage of the power grid, and determine the fault detection results of the first relay and the second relay according to the capacitor voltage and the phase voltage. This can not only detect whether the relay has a fault in time, but also prevent the situation where the single-phase relay fault cannot be identified, thereby improving the accuracy of relay failure detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a structural schematic diagram of a grid-connected inverter provided in an embodiment of the present application.

[0017] Figure 2 It is a structural schematic diagram of a photovoltaic inverter system provided in an embodiment of the present application. DETAILED DESCRIPTION

[0018] It should be noted that the terms "first" and "second" in the specification, claims and drawings of the present application are used to distinguish similar objects, rather than to describe a specific order or sequence. In the specification, claims and drawings of the present application, unless otherwise specified, " / " means or, for example, A / B can mean A or B. For example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone. In addition, the term "multiple" in the specification, claims and drawings of the present application refers to two or more than two.

[0019] It is understandable that the connection relationship described in the present application refers to direct or indirect connection. For example, A is connected to B or A is electrically connected to B, which can be either A and B are directly connected or A and B are indirectly connected through one or more other electrical components, for example, A and C are directly connected, and C and B are directly connected, so that A and B are connected through C.

[0020] It should also be noted that the method disclosed in the embodiments of the present application or the method shown in the flowchart includes one or more steps for implementing the method. Without departing from the scope of the claims, the execution order of multiple steps can be interchangeable with each other, and some of the steps can also be deleted.

[0021] Some embodiments will be described below in conjunction with the accompanying drawings. In the absence of conflict, the following embodiments and features in the embodiments may be combined with each other.

[0022] It should be noted that the terms "first" and "second" in the specification, claims and drawings of the present application are used to distinguish similar objects, rather than to describe a specific order or sequence. In the specification, claims and drawings of the present application, unless otherwise specified, " / " means or, for example, A / B can mean A or B. For example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone. In addition, the term "multiple" in the specification, claims and drawings of the present application refers to two or more than two.

[0023] It is understandable that the connection relationship described in the present application refers to direct or indirect connection. For example, A is connected to B or A is electrically connected to B, which can be either A and B are directly connected or A and B are indirectly connected through one or more other electrical components, for example, A and C are directly connected, and C and B are directly connected, so that A and B are connected through C.

[0024] It should also be noted that the method disclosed in the embodiments of the present application or the method shown in the flowchart includes one or more steps for implementing the method. Without departing from the scope of the claims, the execution order of multiple steps can be interchangeable with each other, and some of the steps can also be deleted.

[0025] Some embodiments will be described below in conjunction with the accompanying drawings. In the absence of conflict, the following embodiments and features in the embodiments may be combined with each other.

[0026] In a photovoltaic power generation system, a grid-connected inverter is generally required to convert the DC power generated by the solar panels into AC power that can be connected to the AC power grid. During the conversion process, the DC power output by the solar panels needs to be input into the grid-connected inverter to invert the DC power into AC power that can be used by the power grid.

[0027] Among them, in order to control the flow of electric energy from the grid-connected inverter to the mains power grid, a relay will be set between the AC side of the grid-connected inverter and the grid. In order to achieve the purpose of safety redundancy, a double relay will also be set between the inverter and the mains power grid. However, the failure of the relay may cause the power to be unable to be normally output from the grid-connected inverter to the mains power grid, affecting the performance and stability of the grid-connected inverter and the mains power grid. Therefore, timely detection of whether the relay is faulty can ensure the performance and stability of the grid-connected inverter and the mains power grid. Relay failure usually includes two situations: relay adhesion failure (that is, the relay cannot be disconnected normally) and relay open circuit failure (that is, the relay cannot be normally attracted). At present, the fault condition of the relay is generally determined by detecting the output voltage on the grid-connected inverter side and the grid voltage and comparing them. However, before the grid-connected inverter is connected to the grid, the inverter has not started working. If only one phase of the relays connected to the output phase line of the grid-connected inverter has a sticking fault, the grid-connected inverter and the mains power grid have no loop at this time, and the voltage on the AC side of the grid-connected inverter will not rise with the grid voltage. It is impossible to confirm that the relay has a sticking fault based on the small difference between the voltage on the AC side of the grid-connected inverter and the grid voltage. In other words, it is impossible to detect the situation where the grid-connected inverter has a single-phase relay sticking fault.

[0028] In view of this, the embodiments of the present application provide a grid-connected inverter and a photovoltaic inverter system, which can timely detect whether there is a fault in the grid-connected inverter and improve the accuracy of relay failure detection.

[0029] See also Figure 1 , an exemplary grid-connected inverter 100 provided in an embodiment of the present application is introduced. The grid-connected inverter 100 includes an inverter circuit 110, at least two phase line branches 120, at least two first capacitors C1, and a control circuit 130. The input end of the inverter circuit 110 is used to connect a DC power supply 200, and the output end of the inverter circuit 110 is connected to the power grid through the phase line branch 120. Each phase line branch 120 includes a first relay S1 and a second relay S2 connected in series. The first end of each first capacitor C1 is connected between the output end of the inverter circuit 110 and the corresponding phase line branch 120, and the second end of the first capacitor C1 is grounded.

[0030] The control circuit 130 can be used to control the first relay S1 and / or the second relay S2 to close before the grid-connected inverter 100 is connected to the grid, and obtain the capacitor voltage of the first capacitor C1 and the phase voltage of the grid. The control circuit 130 confirms the fault detection results of the first relay S1 and the second relay S2 based on the obtained capacitor voltage and phase voltage. Among them, the relay fault includes adhesion fault and open circuit fault.

[0031] In the embodiment of the present application, the capacitor voltage may include the voltage of the first capacitor connected to each phase line branch 120 , and the phase voltage of the power grid includes the voltage of the connection terminal connecting the power grid and each phase line branch 120 .

[0032] It can be understood that before the grid-connected inverter 100 is connected to the grid, if the first relay S1 and the second relay S2 of the phase line branch 120 are both closed, since the first capacitor C1 is grounded, the grid will charge the first capacitor C1 connected to it through the phase line branch 120, so that the voltage of the first capacitor C1 increases, and the first capacitor C1 will follow the grid voltage change. Therefore, by connecting the grounded first capacitor between the output end of the inverter circuit and each phase line branch 120, the control circuit 130 can control the first relay S1 and / or the second relay S2 to close when performing fault detection on the relay, and the fault detection result can be determined according to the obtained capacitor voltage and the phase voltage of the grid.

[0033] Specifically, in order to detect whether the first relay S1 and the second relay S2 have a sticking fault, the control circuit 130 can control one of the first relay S1 and the second relay S2 in each phase line branch 120 to be disconnected and the other to be closed, and obtain the capacitor voltage of each first capacitor C1 and the phase voltage of the power grid at this time. When the voltage difference between the capacitor voltage of the first capacitor C1 connected to the phase line branch 120 and the corresponding phase voltage is less than the preset voltage threshold, the control circuit 130 confirms that the disconnected relay of the phase line branch 120 has a sticking fault. When the voltage difference between the capacitor voltage of the first capacitor C1 connected to each phase line branch 120 and the corresponding phase voltage is greater than the preset voltage threshold, the control circuit 130 confirms that the disconnected relays of the phase line branch 120 in the grid-connected inverter 100 do not have a sticking fault.

[0034] Specifically, before the grid-connected inverter 100 is connected to the grid, and one of the first relay S1 and the second relay S2 is disconnected and the other is closed, since no path is formed between the inverter circuit 110 and the grid, the grid cannot charge the first capacitor C1, and the capacitor voltage of the first capacitor C1 will be different from the phase voltage of the grid. At this time, if the voltage difference between the capacitor voltage obtained by the control circuit 130 and the corresponding phase voltage is less than the preset voltage threshold, it means that the capacitor voltage of the first capacitor C1 is the same as or similar to the corresponding phase voltage, indicating that the first relay S1 and the second relay S2 of the corresponding phase line branch 120 are both turned on, indicating that the relay controlled to be disconnected by the control circuit 130 cannot be disconnected normally, that is, the relay has a sticking fault. Therefore, it can be determined whether the currently disconnected relay has a sticking fault based on the capacitor voltage of the first capacitor C1 and the phase voltage of the grid. When a relay disconnected from a single phase line branch 120 has a sticking fault, since the first capacitor C1 is grounded, the power grid and the first capacitor C1 connected to the phase line branch 120 can form a loop, that is, the power grid can charge the first capacitor C1 connected to the phase line branch 120, so that the capacitor voltage of the first capacitor C1 changes with the power grid, and therefore, it is also possible to confirm whether the relay has a sticking fault based on the capacitor voltage and the phase voltage of the power grid. Therefore, the embodiment of the present application can confirm whether the currently disconnected relay has a sticking fault by obtaining the capacitor voltage of the first capacitor C1 and the phase voltage of the power grid, and can also detect and identify when the relay of a single phase line branch 120 has a sticking fault, so that it can not only detect whether the relay has a fault in time, but also prevent the situation where the single-phase relay fault cannot be identified, thereby improving the accuracy of relay failure detection.

[0035] It can be understood that in the embodiment of the present application, after the grid-connected inverter 100 receives the power-on command to start, the first relay S1 and the second relay S2 can be automatically started to detect faults. If it is confirmed that the relay has a sticking fault, the control circuit 130 can send an indication signal, which can be sent to a display of an electronic device provided with the grid-connected inverter 100 to indicate that the relay has a sticking fault, or can be sent to a user end through a communication device to prompt the user that the grid-connected inverter 100 has a sticking fault.

[0036] In an embodiment of the present application, after the control circuit 130 confirms that the currently disconnected relay does not have a sticking fault, it can control the relay to close and the other relay to disconnect, and obtain the capacitor voltage of each first capacitor C1 and the phase voltage of the power grid again, and judge whether the disconnected relay has a sticking fault based on the capacitor voltage and the phase voltage.

[0037] Specifically, before the grid-connected inverter 100 is connected to the grid, the control circuit 130 first controls the first relay S1 to be disconnected and the second relay S2 to be closed, and obtains the capacitor voltage of each first capacitor C1 and the phase voltage of the grid. When the voltage difference between the capacitor voltage and the corresponding phase voltage is less than the preset voltage threshold, it is confirmed that the first relay S1 of at least one phase line branch 120 has a sticking fault. When the voltage difference between the capacitor voltage of each capacitor C1 and the corresponding phase voltage is less than the preset voltage threshold, it is confirmed that the first relay S1 of each phase line branch 120 does not have a sticking fault. In this way, the detection of whether the first relay S1 has a sticking fault is achieved.

[0038] Then, the control circuit 130 controls the first relay S1 to close and the second relay S2 to open, and reacquires the capacitor voltage of each first capacitor C1 and the phase voltage of the power grid. When the voltage difference between the capacitor voltage and the corresponding phase voltage is less than the preset voltage threshold, it is confirmed that there is a sticking fault in the second relay S2 of at least one phase line branch 120. When the voltage difference between the capacitor voltage of each capacitor C1 and the corresponding phase voltage is less than the preset voltage threshold, it is confirmed that there is no sticking fault in the second relay S2 of each phase line branch 120. In this way, the detection of whether the second relay S2 has a sticking fault is achieved.

[0039] In some embodiments, after the control circuit 130 confirms that the first relay S1 has a sticking fault, it may stop detecting whether the second relay S2 has a sticking fault, and the grid-connected inverter 100 stops the grid-connected operation.

[0040] It can be understood that the control circuit 130 can obtain the capacitance voltage of each first capacitor C1 and the phase voltage of the power grid through the voltage detection circuit.

[0041] In addition to adhesion faults, relays may also have open circuit faults. Therefore, in some embodiments, the control circuit 130 can also be used to control the first relay S1 and the second relay S2 to close, and obtain the capacitor voltage of the first capacitor C1 and the phase voltage of the power grid. When the voltage difference between the capacitor voltage of each first capacitor C1 and the corresponding phase voltage is less than the preset voltage threshold, the control circuit 130 confirms that there is no open circuit fault in the first relay S1 and the second relay S2. When there is a voltage difference between the capacitor voltage and the corresponding phase voltage greater than the preset voltage threshold, the control circuit 130 confirms that at least one of the first relay S1 and the second relay S2 has an open circuit fault.

[0042] Among them, when the first relay S1 and the second relay S2 are closed, a path is formed between the inverter circuit 110 and the power grid, and the power grid can charge the first capacitor C1, so that the voltage of the first capacitor C1 and the power grid voltage will be the same or similar. At this time, if the voltage difference between the capacitor voltage of each first capacitor C1 and the corresponding phase voltage is less than the preset voltage threshold, it means that a path is formed between the inverter circuit 110 and the power grid, and there is no open circuit fault in the first relay S1 and the second relay S2 of each phase line branch 120. If there is a voltage difference between the capacitor voltage of the first capacitor C1 and the corresponding phase voltage greater than the preset voltage threshold, it means that there is no path between the inverter circuit 110 and the power grid, that is, at least one of the first relay S1 and the second relay S2 of at least one phase line branch 120 has an open circuit fault.

[0043] When the first relay S1 or the second relay S2 in the phase line branch 120 is disconnected based on the first capacitor C1, the leakage current generated by the DC power supply 200 through the inverter circuit 110 will flow into the ground through the first capacitor C1, so that it can be confirmed whether the first relay S1 and the second relay S2 in the phase line branch 120 are disconnected based on the current on the phase line branch 120, and then confirm whether the first relay S1 and the second relay S2 can be normally switched on and off.

[0044] In the embodiment of the present application, the control circuit 30 may be a central processing unit (CPU), or other general-purpose processors, digital signal processors (DSP), application-specific integrated circuits (ASIC), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. The general-purpose processor may be a microprocessor or the processor may be any conventional processor, etc.

[0045] It can be understood that when the grid-connected inverter 100 is connected to the grid, the inverter circuit 110 starts to work and converts the DC power of the DC power supply 200 into AC power and provides it to the grid. At this time, if the DC power supply 200 generates leakage current, the leakage current of the DC power supply 200 will flow into the ground through the first capacitor C1. In the embodiment of the present application, by connecting the first end of each first capacitor C1 between the output end of the inverter circuit 110 and the corresponding phase line branch 120, and the second end of the first capacitor C1 is grounded, the leakage current generated by the DC power supply 200 through the inverter circuit 110 flows into the ground through the grounded first capacitor C1, thereby reducing the leakage current flowing into the grid through the phase line branch 120, thereby reducing the impact of the leakage current on the grid.

[0046] In the embodiment of the present application, the DC power supply 200 may be a photovoltaic component, an energy storage battery, etc., wherein the photovoltaic component may be a solar cell package, a photovoltaic panel, etc.

[0047] In addition, in the grid-connected inverter 100 of the present application, by providing a grounded first capacitor C1 between the output end of the inverter circuit 110 and the phase line branch 120, the electromagnetic interference (EMI) of the grid-connected inverter 100 can be improved. In some embodiments, the first capacitor C1 can be configured as a Y capacitor (Safety Fixed capacitor).

[0048] In some embodiments, the grid-connected inverter 100 further includes a DC bus (not labeled) and two bus capacitors Co, and the input end of the inverter circuit 110 is connected to the DC power source 200 via the DC bus. The two bus capacitors Co are connected in series to the DC bus.

[0049] The DC power source 200 charges two bus capacitors Co through the DC bus, and then provides the power of the DC power source 200 to the inverter circuit 110 to convert it into AC power.

[0050] Furthermore, the grid-connected inverter 100 also includes at least two second capacitors C2, a first end of each second capacitor C2 is correspondingly connected between the output end of the inverter circuit 110 and the phase line branch 120, and a second end of each second capacitor C2 is connected to the connection midpoint of the two bus capacitors Co.

[0051] The second capacitor C2 can perform voltage stabilization and filtering on the voltage at the output end of the inverter circuit 110 .

[0052] In some embodiments, the phase line branch 120 may also include a first inductor L1, a first end of the first inductor L1 is connected to the output end of the inverter circuit 110, a second end of the first inductor L1 is correspondingly connected to the first end of the first relay S1, a second end of the first relay S1 is connected to the first end of the second relay S2, and a second end of the second relay S2 is connected to the power grid.

[0053] Furthermore, the phase line branch 120 further includes a second inductor L2, and the second end of the second relay S2 is connected to the grid via the corresponding second inductor L2.

[0054] The second capacitor C2 , the first inductor L1 and the second inductor L2 together form an LCL filter capacitor to filter out clutter of each phase line branch 120 .

[0055] In some embodiments, a current detection circuit 140 can be connected between each phase line branch 120 and the power grid. The current detection circuit 140 detects the current of each phase line branch 120 and provides the detected current to the control circuit 130. The control circuit 130 can control the operation of the inverter circuit 110 according to the obtained current.

[0056] Specifically, the current detection circuit 140 may include a current detection meter, which is connected in series between the phase branch 120 and the power grid to detect the current of the phase branch 120 . The control circuit 130 obtains the current of each phase branch 120 through the current detection meter.

[0057] In some specific examples, the phase line branch 120 may include three (eg Figure 1 The inverter circuit 110 includes three output terminals, and the three output terminals of the inverter circuit 110 are respectively connected to the neutral terminal and two live terminals of the power grid through three phase line branches 120. In this way, the grid-connected inverter 100 is a three-phase grid-connected inverter.

[0058] When the control circuit 130 detects whether the first relay S1 and the second relay S2 are faulty, the obtained capacitance voltage of the first capacitor C1 may include: the capacitance voltage effective value of each first capacitor C1 and the phase voltage effective value of the power grid within a preset time period, wherein the preset time period includes at least one power grid cycle. The control circuit 130 determines the voltage difference according to the capacitance voltage effective value of the first capacitor C1 and the corresponding phase voltage effective value.

[0059] Specifically, the obtained capacitor voltage effective value includes: the capacitor voltage effective value UA_AVE of the first capacitor C1 connected to the phase line branch A, the capacitor voltage effective value UB_AVE of the first capacitor C1 connected to the phase line branch B, and the capacitor voltage effective value UC_AVE of the first capacitor C1 connected to the phase line branch C. The obtained phase voltage effective value includes: the phase voltage effective value EA_AVE of the live terminal connected to the phase line branch A of the power grid, the phase voltage effective value EB_AVE of the neutral terminal connected to the phase line branch B, and the phase voltage effective value EC_AVE of the live terminal connected to the phase line branch C. Correspondingly, the voltage difference includes: the difference between the capacitor voltage effective value UA_AVE and the phase voltage effective value EA_AVE |UA_AVE-EA_AVE|, the difference between the capacitor voltage effective value UB_AVE and the phase voltage effective value EB_AVE |UB_AVE-EB_AVE|, and the difference between the capacitor voltage effective value UC_AVE and the phase voltage effective value EC_AVE |UC_AVE-EC_AVE|.

[0060] Among them, since the first capacitor C1 is grounded, when the relay of a single phase line branch 120 of the grid-connected inverter 100 has a sticking fault, the control circuit 130 controls one of the first relay S1 and the second relay S2 to be disconnected and the other to be closed when performing sticking fault detection. Then, the phase line branch 120 with the sticking fault is turned on, and the power grid charges the corresponding first capacitor C1 through the phase line branch 120, so that the voltage of the first capacitor C1 is raised, and then, based on whether the voltage difference between the effective value of the capacitor voltage of the first capacitor C1 and the corresponding phase voltage effective value is greater than the preset voltage threshold, it can be judged whether the relay in the phase line branch 120 has a sticking fault.

[0061] When an open-circuit fault occurs in the relay of a single phase line branch 120 of the grid-connected inverter 100, when the control circuit 130 controls the first relay S1 and the second relay S2 to be closed during open-circuit fault detection, the phase line branch 120 with the open-circuit fault is disconnected, and the power grid cannot charge the corresponding first capacitor C1 through the phase line branch 120, so that the voltage of the first capacitor C1 is not raised. Furthermore, based on whether the voltage difference between the effective value of the capacitor voltage of the first capacitor C1 and the corresponding phase voltage effective value is less than a preset voltage threshold, it can be determined whether the relay in the phase line branch 120 has an open-circuit fault.

[0062] In the embodiment of the present application, the preset voltage threshold is set to 10V, 20V, etc.

[0063] It can be understood that the grid-connected inverter 100 may also include more functional modules or fewer functional modules, and this application does not make any specific limitation on this.

[0064] See also Figure 2 The present application further provides a photovoltaic inverter system 10, comprising a photovoltaic module 201 and the aforementioned grid-connected inverter 100. The photovoltaic module 201 is connected to an input end of an inverter circuit 110.

[0065] The photovoltaic module 201 is used to convert solar energy into electrical energy, and then provide the electrical energy to the inverter circuit 110 for conversion into alternating current, and then provide it to the power grid.

[0066] In some embodiments, the photovoltaic inverter system 10 may further include a maximum power point tracking (MPPT) circuit (not shown). The input end of the MPPT circuit is connected to the output end of the photovoltaic module 201, and the output end of the MPPT circuit is connected to the DC input end of the inverter circuit 110. The MPPT circuit can be used to perform maximum power tracking on the photovoltaic module 201, so that the photovoltaic module 201 operates at the maximum power point, and the inverter circuit 110 is connected to the maximum power PMAX of the photovoltaic module 201, and then the connected power is inverted.

[0067] The MPPT circuit may be, for example, a boost circuit, and the present application does not limit the specific topology of the MPPT circuit.

[0068] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application and are not intended to limit them. In the actual application process, the entire content of the technical solutions described in any embodiment of the present application can be implemented, or some content can be added, or some content can be deleted, or some content can be changed / replaced. Although the present application is described in detail with reference to the preferred embodiments, it should be understood by those of ordinary skill in the art that the technical solutions of the present application can be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present application.

Claims

1. A grid-connected inverter, characterized in that: The invention comprises an inverter circuit, at least two phase line branches, at least two first capacitors and a control circuit, wherein the input end of the inverter circuit is used to connect to a DC power supply, and the output end of the inverter circuit is connected to a power grid via the phase line branches; each of the phase line branches comprises a first relay and a second relay connected in series; the first end of each of the first capacitors is connected between the output end of the inverter circuit and the corresponding phase line branch, and the second end of the first capacitor is grounded; the control circuit connects the first relay and the second relay, and the control circuit is used to control the first relay and / or the second relay to close before the grid-connected inverter is connected to the grid, obtain the capacitor voltage of the first capacitor and the phase voltage of the power grid, and confirm the fault detection results of the first relay and the second relay according to the capacitor voltage and the phase voltage.

2. The grid-connected inverter according to claim 1, characterized in that: The control circuit is used to: before the grid-connected inverter is connected to the grid, control one of the first relay and the second relay to be disconnected and the other to be closed; obtain the capacitor voltage of the first capacitor and the phase voltage of the grid; when the voltage difference between the capacitor voltage and the corresponding phase voltage is less than a preset voltage threshold, confirm that the currently disconnected relay has a sticking fault; when the voltage difference is greater than the preset voltage threshold, confirm that the currently disconnected relay does not have a sticking fault.

3. The grid-connected inverter according to claim 1 or 2, characterized in that: The control circuit is used to: before the grid-connected inverter is connected to the grid, control the first relay and the second relay to be closed; obtain the capacitor voltage of the first capacitor and the phase voltage of the grid; when the voltage difference between the capacitor voltage and the corresponding phase voltage is less than a preset voltage threshold, confirm that the currently disconnected relay does not have an open circuit fault; when the voltage difference is greater than the preset voltage threshold, confirm that at least one of the first relay and the second relay has an open circuit fault.

4. The grid-connected inverter according to claim 1, characterized in that: It also includes a DC bus and two bus capacitors. The input end of the inverter circuit is connected to the DC power supply through the DC bus; and the two bus capacitors are connected in series to the DC bus.

5. The grid-connected inverter according to claim 4, characterized in that: It also includes at least two second capacitors, wherein the first end of each second capacitor is correspondingly connected between the output end of the inverter circuit and the phase line branch, and the second end of each second capacitor is connected to the connection midpoint of the two bus capacitors.

6. The grid-connected inverter according to claim 4, characterized in that: The phase line branch also includes a first inductor, a first end of the first inductor is connected to the output end of the inverter circuit, a second end of the first inductor is correspondingly connected to the first end of the first relay, a second end of the first relay is connected to the first end of the second relay, and a second end of the second relay is connected to the power grid.

7. The grid-connected inverter according to claim 6, characterized in that: The phase line branch further includes a second inductor, and the second end of the second relay is connected to the power grid via the corresponding second inductor.

8. The grid-connected inverter according to claim 1, characterized in that: It also includes a current detection circuit connected between each of the phase line branches and the power grid, and the current detection circuit is used to detect the current of the phase line branch.

9. The grid-connected inverter according to claim 1, characterized in that: The phase line branches include three, the inverter circuit includes three output terminals, and the three output terminals of the inverter circuit are respectively connected to the neutral line terminal and two live line terminals of the power grid through the three phase line branches.

10. A photovoltaic inverter system, characterized in that: It comprises a photovoltaic component and a grid-connected inverter as claimed in any one of claims 1 to 9; the photovoltaic component is connected to the input end of the inverter circuit.

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