Photovoltaic inverter and control method thereof

Abstract

The invention discloses a photovoltaic inverter and a control method thereof. A bypass unit comprises a controllable switch; the alternating current side of the inversion unit is connected with a power grid through an alternating current connecting end, the direct current side of the inversion unit is connected with the direct current side of the DCDC conversion unit through a positive direct current bus and a negative direct current bus, and the other direct current side of the DCDC conversion unit is connected with a plurality of photovoltaic group strings through a direct current connecting end; the alternating current side of the bypass unit is connected with a power grid through an alternating current connecting end, and the direct current side of the bypass unit is connected with the positive direct current bus and the negative direct current bus; the control unit is used for controlling the controllable switch to be switched on, so that the bypass unit works; when the bypass unit works, branch current information of all the photovoltaic strings is obtained; and according to the change condition of the branch current, the photovoltaic string branch where the grounding fault is located is positioned. According to the invention, the photovoltaic string grounding fault can be positioned, the consumed resources are small, the cost is low, and the effect is good.

Description

Technical Field

[0001] The present application relates to the field of photovoltaic power generation technology, and in particular to a photovoltaic inverter and a control method thereof. Background Art

[0002] Based on safety considerations, industry standards require that the inverter must be shut down after a ground fault occurs on the DC side of the photovoltaic system, especially for the TN-C system. If the PV side grounding is not identified and the inverter is turned on directly, it will not only damage the inverter, but also cause the inverter to catch fire in serious cases. On the PV side, due to the large number of string lines and cables, grounding faults are more likely to occur due to construction, aging and other factors. If the grounding fault is not eliminated in time, it will cause a large loss in power generation. Especially for the Boost type boost unit on the DC side, because its negative pole is connected together and the positive pole is blocked by the boost diode, when the PV side is grounded, the PV terminals need to be disconnected one by one to locate the specific fault point, which makes it difficult to troubleshoot the problem, takes a long time to troubleshoot, wastes manpower, and affects power generation.

[0003] Currently, the industry can only detect faults, but cannot detect ground faults in the middle of PV strings. There is no low-cost solution for fault location. Therefore, accurate location of ground faults on the PV side has always been a pain point in the industry. Summary of the invention

[0004] In view of this, the purpose of the present application is to provide a photovoltaic inverter and a control method thereof to locate a photovoltaic string grounding fault.

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

[0006] On one hand, the present application provides a photovoltaic inverter, comprising an inverter unit, a DCDC conversion unit, a positive DC bus, a negative DC bus, a bypass unit, an AC connection terminal, a DC connection terminal and a control unit, wherein the bypass unit comprises a controllable switch;

[0007] The AC side of the inverter unit is connected to the grid through the AC connection terminal, the DC side of the inverter unit is connected to a DC side of the DCDC conversion unit through the positive DC bus and the negative DC bus, and the other DC side of the DCDC conversion unit is connected to a plurality of photovoltaic strings through the DC connection terminal;

[0008] The AC side of the bypass unit is connected to the power grid through the AC connection terminal, and the DC side of the bypass unit is connected to the positive DC bus and the negative DC bus;

[0009] The control unit is used to control the controllable switch to be turned on so that the bypass unit works; when the bypass unit works, the branch current information of all photovoltaic strings is obtained; according to the change of the branch current, the photovoltaic string branch where the ground fault is located is located.

[0010] On the other hand, the present application provides a control method for a photovoltaic inverter, wherein the photovoltaic inverter includes an inverter unit, a DCDC conversion unit, a positive DC bus, a negative DC bus, a bypass unit, an AC connection terminal, a DC connection terminal, and a control unit, wherein the bypass unit includes a controllable switch;

[0011] The AC side of the inverter unit is connected to the grid through the AC connection terminal, the DC side of the inverter unit is connected to a DC side of the DCDC conversion unit through the positive DC bus and the negative DC bus, and the other DC side of the DCDC conversion unit is connected to a plurality of photovoltaic strings through the DC connection terminal;

[0012] The AC side of the bypass unit is connected to the power grid through the AC connection terminal, and the DC side of the bypass unit is connected to the positive DC bus and the negative DC bus;

[0013] The control method comprises:

[0014] Controlling the controllable switch to be turned on so that the bypass unit works;

[0015] When the bypass unit is working, obtaining branch current information of all photovoltaic strings;

[0016] The photovoltaic string branch where the grounding fault is located is located according to the change of the branch current.

[0017] The photovoltaic inverter and control method provided in the present application obtain branch current information of all photovoltaic strings by controlling the conduction of controllable switches, thereby locating the photovoltaic string branch where the grounding fault is located according to the change of branch current; it consumes little resources, has low cost and good effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 A schematic diagram of a photovoltaic inverter provided in an embodiment of the present application;

[0019] Figure 2 A schematic diagram of a bypass unit provided in an embodiment of the present application;

[0020] Figure 3 A schematic diagram of another bypass unit provided in an embodiment of the present application;

[0021] Figure 4 A schematic diagram of the current flow of a PV-negative ground fault provided in an embodiment of the present application;

[0022] Figure 5 A schematic diagram of the current flow of a PV+ negative ground fault provided in an embodiment of the present application;

[0023] Figure 6 A schematic diagram of the flow of ground fault current in the intermediate component provided in an embodiment of the present application;

[0024] Figure 7 A schematic diagram of the flow of negative ground fault current in an IT system provided in an embodiment of the present application;

[0025] Figure 8 A schematic diagram of a control method for a photovoltaic inverter provided in an embodiment of the present application.

[0026] The realization of the purpose, functional features and advantages of this application will be further explained in conjunction with embodiments and with reference to the accompanying drawings. DETAILED DESCRIPTION

[0027] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present application clearer and more understandable, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0028] like Figure 1 As shown, a photovoltaic inverter provided in an embodiment of the present application includes an inverter unit, a DCDC conversion unit, a positive DC bus BUS+, a negative DC bus BUS-, a bypass unit, an AC connection terminal, a DC connection terminal and a control unit.

[0029] The AC side of the inverter unit is connected to the grid through the AC connection terminal, the DC side of the inverter unit is connected to a DC side of the DCDC conversion unit through the positive DC bus BUS+ and the negative DC bus BUS-, and the other DC side of the DCDC conversion unit is connected to a plurality of photovoltaic strings through the DC connection terminal; the AC side of the bypass unit is connected to the grid through the AC connection terminal, and the DC side of the bypass unit is connected to the positive DC bus BUS+ and the negative DC bus BUS-.

[0030] In one example, the inverter unit includes a single-phase inverter circuit or a three-phase inverter circuit. The single-phase inverter circuit includes one of a Heric circuit, an H5 circuit, an H6 circuit, and a single-phase H-bridge circuit; the three-phase inverter circuit includes one of an I-type three-level three-phase bridge circuit, an ANPC three-level three-phase bridge circuit, a T-type three-level three-phase bridge circuit, and a three-phase full-bridge circuit.

[0031] In one example, the DCDC conversion unit includes any one of a boost circuit, a buck circuit, and a buck-boost circuit. If it is a boost circuit, it can be a two-level boost circuit or a three-level boost circuit. The two-level boost circuit includes a single BOOST circuit or a BUCK-BOOST circuit; the three-level boost circuit includes a dual BOOST circuit or a flying capacitor BOOST circuit. For example, Figure 6 The single BOOST circuit is composed of diode D0, switch tube Q0 and inductor L0.

[0032] like Figure 2 As shown, in one example, the bypass unit includes a controllable switch Q11, a resistor R11, an inductor L11 and a diode D11 connected in series. In other alternative examples, Figure 2 The bypass unit in can remove the diode D11. In another alternative example, Figure 2 The diode D11 and the inductor L11 can be removed from the bypass unit. The controllable switch Q11 includes but is not limited to a MOS tube, an IGBT, and the like.

[0033] like Figure 3 As shown, in one example, the bypass unit includes a diode rectifier circuit, that is, a rectifier circuit composed of diodes D1 to D4, one AC end of the diode rectifier circuit is connected to one phase of the power grid through a first controllable switch Q2, the other AC end of the diode rectifier circuit is connected to another phase of the power grid through a second controllable switch Q3, and the DC end of the diode rectifier circuit is connected to the positive DC bus BUS+ and the negative DC bus BUS-.

[0034] In a further implementation, one AC end of the diode rectifier circuit is connected to one phase of the grid through the resistor R1 and the first controllable switch Q2, and the other AC end of the diode rectifier circuit is connected to another phase of the grid through the resistor R2 and the second controllable switch Q3.

[0035] In one example, the control unit is used to control the controllable switch to be turned on so that the bypass unit works; when the bypass unit works, the branch current information of all photovoltaic strings is obtained; according to the change of the branch current, the photovoltaic string branch where the ground fault is located is located.

[0036] In this example, the control unit can obtain the branch current information of the photovoltaic string through the Hall sensor, including but not limited to the direction of branch current change, the magnitude of branch current change, or the slope of branch current change. The control unit can accurately locate the photovoltaic string branch where the ground fault is located based on the direction of branch current change, the magnitude of branch current change, or the slope of branch current change.

[0037] In one example, the control unit is further used to detect the ground voltage and open circuit voltage of the photovoltaic string branch after locating the photovoltaic string branch where the ground fault is located, so as to locate the grounding position of the intermediate component.

[0038] In this example, the so-called middle component is a photovoltaic panel in the middle of a photovoltaic string. For example, the photovoltaic string consists of n photovoltaic panels. The control unit can locate the grounding position of the middle component k based on the detected ground voltage and open-circuit voltage of the photovoltaic string branch, 1≤k≤n.

[0039] In one example, the control unit is further configured to control the controllable switch to disconnect and report fault information after locating the photovoltaic string branch where the ground fault is located.

[0040] In one example, the control unit is further configured to control the photovoltaic inverter to start up normally when no ground fault is detected.

[0041] The following combination Figure 4-Figure 7 To explain:

[0042] like Figure 4 As shown, it is assumed that the system has two photovoltaic strings connected, and the two photovoltaic strings are connected in series to the same DCDC conversion unit. Assume that the negative pole PV1- of photovoltaic string 1 has a ground fault. Take phase A as an example. When the power grid is marked as positive on the left and negative on the right as shown in the figure, when the controllable switches Q2 and Q3 in the bypass unit are closed, there is a red current flow path as shown in the figure. Assuming that the direction of the Hall sensor Hall is positive on the left and negative on the right, the Hall current flowing through the Hall sensor H1 is negative at this time. Before the controllable switch of the bypass unit is closed, even if the Hall sensor H1 is located at PV1-, no current will flow at this time. Only when the controllable switch of the bypass unit is closed, the current flows through the Hall sensor H1. By collecting the voltage of the negative pole to PE at this time, the impedance value of the negative pole to the ground can be calculated, so as to reversely verify the detection accuracy of the insulation impedance detection. At the same time, the PV branch where the ground fault is located can be accurately located according to the direction of the Hall current change of the Hall sensor H1, the magnitude of the current change, or the slope of the current change, and then the fault is reported.

[0043] like Figure 5As shown, it is assumed that the system has two photovoltaic strings connected, and the two photovoltaic strings are connected in series to the same DCDC conversion unit. Assume that PV1+ of photovoltaic string 1 has a ground fault, and take phase A as an example. When the power grid is marked as negative on the left and positive on the right as shown in the figure, when the controllable switches Q2 and Q3 in the bypass unit are closed, there is a red current flow path as shown in the figure. When the current flows to the input end of the DCDC conversion unit, there will be two parallel paths, as shown in the yellow and green paths. Assuming that the direction of the Hall sensor Hall is positive on the left and negative on the right, the Hall current flowing through the Hall sensor H1 is positive at this time. Before the controllable switch of the bypass unit is closed, even if the PV1+ where the Hall sensor H1 is located is grounded, no current will flow at this time. When the controllable switch of the bypass unit is closed, the Hall sensor H1 will have current flowing. Collecting the voltage of the negative electrode to PE at this time, the impedance value of the negative electrode to the ground can be calculated, so that the detection accuracy of the insulation impedance detection can be reversely verified. At the same time, the PV branch where the ground fault is located can be accurately located according to the direction of the Hall current change of the Hall sensor H1, the magnitude of the current change, or the slope of the current change, and then the fault can be reported.

[0044] like Figure 6 As shown, it is assumed that the system has two photovoltaic strings connected, and the two photovoltaic strings are connected to the same DCDC conversion unit. Assume that the middle component of photovoltaic string 1 has a ground fault, and take phase A as an example. When the power grid is marked as left positive and right negative as shown in the figure, when the controllable Q2 and Q3 of the bypass unit are closed, there will be at least a red current flow path as shown in the figure. When the current flows to the input end of the DCDC conversion unit, there will be two parallel paths, as shown in the yellow and green paths. Assuming that the direction of the Hall sensor Hall is left positive and right negative, the Hall current flowing through H1 is positive at this time. Before the controllable switch of the bypass unit is closed, even if the photovoltaic string 1 where H1 is located is grounded, no current will flow at this time. When the controllable switch of the bypass unit is closed, H1 will have current flowing. According to the direction of the Hall current change of H1, the magnitude of the current change or the slope of the current change, the PV string branch where the grounding fault is located can be accurately located. Then, the voltage of the negative electrode to PE and the open-circuit voltage of PV string 1 are collected at this time. The two are divided to obtain the percentage position of the grounding position of the middle component from PV-. The percentage position is multiplied by the number of strings of PV string 1 to accurately locate the specific grounding position of the middle component.

[0045] like Figure 7 As shown, for an IT system, it is only necessary to add a controllable switch to any one side of the AC end of the photovoltaic inverter relative to PE, such as the controllable switch Q4 shown in the figure.

[0046] Assume that the system has two PV strings connected, and the two PV strings are connected in series to the same DCDC conversion unit. Assume that PV1- of PV string 1 has a ground fault. Take phases B and C as an example. When the controllable switches Q2 and Q3 of the bypass unit are closed, there is a red current flow path as shown in the figure. Assuming that the direction of the Hall sensor Hall is positive on the left and negative on the right, the Hall current flowing through H1 is negative at this time. Before the controllable switch of the bypass unit is closed, even if PV1- of the PV string where H1 is located is grounded, no current will flow at this time. When the controllable switch of the bypass unit is closed, current will flow through H1. By collecting the voltage of the negative electrode to PE at this time, the impedance value of the negative electrode to the ground can be calculated, so as to reversely verify the detection accuracy of the insulation impedance detection. At the same time, the PV branch where the ground fault is located can be accurately located according to the direction of the Hall current change of H1, the magnitude of the current change, or the slope of the current change, and then the fault can be reported.

[0047] With the above Figure 4-Figure 7 Similarly, PV+ string grounding and PV module middle grounding can also be identified by referring to other similar examples, which will not be described in detail here.

[0048] like Figure 8 As shown, another embodiment of the present application provides a control method for a photovoltaic inverter, and the photovoltaic inverter can refer to the content of the above part. The control method includes the following steps:

[0049] S11, controlling the controllable switch to be turned on so that the bypass unit works;

[0050] S12, when the bypass unit is working, obtaining branch current information of all photovoltaic strings;

[0051] S13. Locate the photovoltaic string branch where the ground fault is located according to the change of the branch current.

[0052] In one example, the control method further includes:

[0053] After locating the photovoltaic string branch where the ground fault is located, the ground voltage and open circuit voltage of the photovoltaic string branch are detected to locate the grounding position of the intermediate component.

[0054] In one example, the control method further includes:

[0055] After locating the photovoltaic string branch where the ground fault is located, the controllable switch is controlled to be disconnected and the fault information is reported.

[0056] In one example, the control method further includes:

[0057] When no ground fault is detected, the photovoltaic inverter is controlled to start normally.

[0058] Those skilled in the art can clearly understand that, for the convenience and brevity of description, the specific working processes of the systems, devices and units described above can refer to the corresponding processes in the aforementioned method embodiments and will not be repeated here.

[0059] In the several embodiments provided in the present application, it should be understood that the disclosed systems, devices and methods can be implemented in other ways. For example, the device embodiments described above are only schematic. For example, the division of the units is only a logical function division. There may be other division methods in actual implementation, such as multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed can be an indirect coupling or communication connection through some interfaces, devices or units, which can be electrical, mechanical or other forms.

[0060] The units described as separate components may or may not be physically separated, and the components shown as units may or may not be physical units, that is, they may be located in one place or distributed on multiple network units. Some or all of the units may be selected according to actual needs to achieve the purpose of the solution of this embodiment.

[0061] In addition, each functional unit in each embodiment of the present application may be integrated into one processing unit, or each unit may exist physically separately, or two or more units may be integrated into one unit. The above-mentioned integrated unit may be implemented in the form of hardware or in the form of software functional units.

[0062] If the integrated unit is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present application is essentially or the part that contributes to the prior art or all or part of the technical solution can be embodied in the form of a software product, and the computer software product is stored in a storage medium, including a number of instructions to enable a computer device (which can be a personal computer, a server, or a network device, etc.) to perform all or part of the steps of the method described in each embodiment of the present application. The aforementioned storage medium includes: various media that can store program codes, such as a USB flash drive, a mobile hard disk, a read-only memory (ROM), a random access memory (RAM), a disk or an optical disk.

[0063] As described above, the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A photovoltaic inverter, characterized in that: It includes an inverter unit, a DCDC conversion unit, a positive DC bus, a negative DC bus, a bypass unit, an AC connection terminal, a DC connection terminal and a control unit, wherein the bypass unit includes a controllable switch; The AC side of the inverter unit is connected to the grid through the AC connection terminal, the DC side of the inverter unit is connected to a DC side of the DCDC conversion unit through the positive DC bus and the negative DC bus, and the other DC side of the DCDC conversion unit is connected to a plurality of photovoltaic strings through the DC connection terminal; The AC side of the bypass unit is connected to the power grid through the AC connection terminal, and the DC side of the bypass unit is connected to the positive DC bus and the negative DC bus; The control unit is used to control the controllable switch to be turned on so that the bypass unit works; when the bypass unit works, the branch current information of all photovoltaic strings is obtained; according to the change of the branch current, the photovoltaic string branch where the ground fault is located is located.

2. The photovoltaic inverter according to claim 1, characterized in that: The DCDC conversion unit includes any one of a boost circuit, a buck circuit, and a buck-boost circuit.

3. The photovoltaic inverter according to claim 1, characterized in that: The bypass unit includes a controllable switch and a resistor connected in series.

4. The photovoltaic inverter according to claim 3, characterized in that: The bypass unit further includes an inductor connected in series.

5. The photovoltaic inverter according to claim 4, characterized in that: The bypass unit further includes a diode connected in series.

6. The photovoltaic inverter according to claim 1, characterized in that: The bypass unit includes a diode rectifier circuit, an AC end of the diode rectifier circuit is connected to one phase of the power grid through a first controllable switch, the other AC end of the diode rectifier circuit is connected to another phase of the power grid through a second controllable switch, and the DC end of the diode rectifier circuit is connected to the positive DC bus and the negative DC bus.

7. The photovoltaic inverter according to claim 1, characterized in that: The control unit is further used to detect the ground voltage and open circuit voltage of the photovoltaic string branch after locating the photovoltaic string branch where the ground fault is located, so as to locate the grounding position of the intermediate component.

8. The photovoltaic inverter according to claim 1, characterized in that: The control unit is further used to control the controllable switch to disconnect and report fault information after locating the photovoltaic string branch where the ground fault is located.

9. The photovoltaic inverter according to claim 1, characterized in that: The control unit is also used to control the photovoltaic inverter to start normally when no ground fault is detected.

10. A control method for a photovoltaic inverter, characterized in that: The photovoltaic inverter includes an inverter unit, a DCDC conversion unit, a positive DC bus, a negative DC bus, a bypass unit, an AC connection terminal, a DC connection terminal and a control unit, and the bypass unit includes a controllable switch; The AC side of the inverter unit is connected to the grid through the AC connection terminal, the DC side of the inverter unit is connected to a DC side of the DCDC conversion unit through the positive DC bus and the negative DC bus, and the other DC side of the DCDC conversion unit is connected to a plurality of photovoltaic strings through the DC connection terminal; The AC side of the bypass unit is connected to the power grid through the AC connection terminal, and the DC side of the bypass unit is connected to the positive DC bus and the negative DC bus; The control method comprises: Controlling the controllable switch to be turned on so that the bypass unit works; When the bypass unit is working, obtaining branch current information of all photovoltaic strings; According to the change of the branch current, the photovoltaic string branch where the grounding fault is located is located.

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