A method for detecting the insulation impedance of a photovoltaic system at night

Abstract

The application discloses a photovoltaic system night insulation impedance detection method, comprising the following steps: before the photovoltaic system works at night, the bus negative electrode of the system is powered by taking power from the power grid through a power taking architecture, so that the positive and negative electrodes of all photovoltaic groups form a grounding loop with the power taking architecture, and then the insulation impedance of the DC side of the system to the ground is calculated by measuring the voltage of the power taking architecture; when the photovoltaic system works at night, the grounding insulation fault of the switch tube of the power taking architecture and each DC / DC unit is positioned. The application has the beneficial effects that the application can realize online detection of the insulation impedance of the photovoltaic system at night, the structure of the whole detection architecture is simple, and even the existing hardware circuit does not need to be modified, so that the cost can be effectively reduced.

Description

Technical Field

[0001] This application relates to the field of photovoltaic power generation technology, and in particular to a method for detecting the insulation impedance of a photovoltaic system at night. Background Technology

[0002] Before the inverter can be started, the DC-side insulation resistance to ground must meet or exceed a predetermined safety standard. Currently, the industry widely uses the bridge method to measure the DC-side insulation resistance to ground, which can effectively assess the insulation status of the photovoltaic system during the day, thereby ensuring the overall safety of the entire system.

[0003] Photovoltaic inverters are often configured to perform SVG compensation functions at night, therefore it is necessary to ensure that the insulation resistance to ground of the photovoltaic inverter meets safety standards at night. During this period, the photovoltaic system cannot generate electricity due to the lack of sunlight, meaning the photovoltaic modules have no output voltage. Current mainstream insulation resistance testing technologies rely on the output voltage of the PV string or the DC bus voltage, making it impossible to test the insulation resistance to ground of the photovoltaic system at night. Summary of the Invention

[0004] One objective of this application is to provide a method for detecting the nighttime insulation impedance of a photovoltaic system that can overcome at least one of the defects in the aforementioned background art.

[0005] To achieve at least one of the above objectives, the technical solution adopted in this application is as follows: A method for detecting the insulation impedance of a photovoltaic system at night, comprising the following steps: before the photovoltaic system operates at night, power is drawn from the grid through a power-taking structure to supply power to the negative terminal of the system bus, so that the insulation impedance to ground of the positive and negative terminals of all photovoltaic strings forms a grounding loop with the power-taking structure, and then the DC side insulation impedance to ground of the system is calculated by measuring the voltage of the power-taking structure; if the calculated insulation impedance to ground does not exceed a set threshold, a fault is determined to have occurred; when the photovoltaic system operates at night, the ground insulation fault is located by controlling the switching transistors of the power-taking structure and each DC / DC unit.

[0006] Preferably, the power supply architecture includes a switching module, a current limiting module, and a power conversion module; the switching module, the current limiting module, and the power conversion module are connected in series between the negative terminal of the bus and ground; the switching module is adapted to control the power supply architecture to be connected or disconnected; the power conversion module is adapted to draw power from the grid and supply power to the negative terminal of the bus; the DC side insulation impedance of the system to ground is calculated by measuring the voltage of the current limiting module.

[0007] Preferably, let the number of photovoltaic strings in the system be n, and the positive terminal-to-ground insulation resistance of each photovoltaic string be R. P1 R P2... R Pn The negative terminal insulation resistance to ground of each photovoltaic string is R. N1 R N2 ... R Nn The total insulation impedances of the positive and negative sides of the system on the DC side are respectively: R P =R P1 / / R P2 / / …… / / R Pn R N =R N1 / / R N2 / / …… / / R Nn The DC-side insulation resistance to ground of the system is R. P / / R N The calculation formula is:

[0008] V RT / R T =V T / (R T +R N / / R P );

[0009] Among them, V RT R represents the voltage value of the current limiting module. T V represents the resistance value of the current limiting module. T This indicates the output voltage of the power conversion module.

[0010] Preferably, when the photovoltaic system is operating at night, the three-phase-to-ground voltage of the AC side of the photovoltaic system is detected; if the voltage of one phase to ground drops to zero, and the voltages of the other two phases to ground rise from phase voltage to line voltage, it is determined that there is a fault in the AC side insulation to ground of the photovoltaic system.

[0011] Preferably, the negative bus voltage to ground is detected when the AC side insulation performance of the photovoltaic system is normal; if the negative bus voltage to ground fluctuation exceeds the set threshold, it is determined that the photovoltaic string has a ground insulation fault, and then the positive and negative ground insulation faults are determined by controlling the power extraction architecture and the switching transistors of each DC / DC unit.

[0012] Preferably, the photovoltaic string negative electrode to ground insulation fault judgment includes the following process: isolating the power extraction structure from the bus negative electrode so that the negative electrode to ground insulation impedance of all photovoltaic strings are connected in parallel; detecting the bus negative electrode to ground voltage, if the bus negative electrode to ground voltage exceeds the set threshold, it is determined that the photovoltaic string negative electrode to ground insulation performance meets the requirements, otherwise the photovoltaic string has a negative electrode to ground insulation fault.

[0013] Preferably, when the negative electrode insulation performance of the photovoltaic string meets the requirements, the positive electrode insulation fault judgment of the photovoltaic string includes the following process: isolating the power extraction structure from the negative electrode of the bus, so that the negative electrode insulation impedance of all photovoltaic strings is connected in parallel; controlling the switching transistors corresponding to each DC / DC unit to conduct sequentially, so that the positive electrode insulation impedance of each photovoltaic string is connected in parallel with the negative electrode insulation impedance of all photovoltaic strings; detecting the bus negative electrode voltage to ground when each switching transistor is conducted; if the bus negative electrode voltage to ground exceeds the set threshold, it is determined that the positive electrode insulation performance of the photovoltaic string meets the requirements; otherwise, the photovoltaic string has a positive electrode insulation fault.

[0014] Preferably, the power conversion module includes an isolated DC / DC unit, an AC / DC unit, and a control switch connected in sequence; the isolated DC / DC unit is adapted to be connected to the negative terminal of the bus as an output terminal, and the control switch is connected to the power grid to control the connection or disconnection of the power conversion module with the power grid.

[0015] Preferably, the switching module is one of a trigger, a thyristor, and a MOSFET.

[0016] Preferably, the current limiting module uses a current limiting resistor.

[0017] Compared with the prior art, the beneficial effects of this application are as follows:

[0018] This application enables online detection of the insulation impedance of photovoltaic systems at night, and the entire detection architecture is simple in structure, requiring no modification to existing hardware circuits, thereby effectively reducing costs. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of an existing photovoltaic system insulation impedance detection architecture.

[0020] Figure 2 for Figure 1 A schematic diagram of the equivalent circuit structure corresponding to the photovoltaic string PV#1 in the middle.

[0021] Figure 3 This is a schematic diagram of the nighttime insulation impedance architecture of this application.

[0022] Figure 4 This is a schematic diagram of the equivalent structure of the switching module, current limiting module and power conversion module in this application.

[0023] Figure 5 This is a schematic diagram of the power conversion module in this application.

[0024] Figure 6 This is a schematic diagram of the DC-side insulation resistance detection process in this application.

[0025] Figure 7 This is a schematic diagram of the ground loop of the photovoltaic system in this application. Figure 1 .

[0026] Figure 8 This is a schematic diagram of the ground loop of the photovoltaic system in this application. Figure 2 .

[0027] Figure 9 This is a schematic diagram of the equivalent circuit structure of the photovoltaic system's ground loop in this application.

[0028] Figure 10 This is a schematic diagram of the architecture for testing the string negative electrode insulation performance to ground in this application.

[0029] Figure 11 For this application Figure 10 The corresponding equivalent circuit structure diagram.

[0030] Figure 12 This is a schematic diagram of the architecture for testing the string positive electrode insulation performance to ground in this application.

[0031] Figure 13 For this application Figure 12 The corresponding equivalent circuit structure diagram.

[0032] Figure 14 This is a schematic diagram of the workflow for testing the insulation performance of the positive and negative terminals of the string to ground in this application.

[0033] In the diagram: switch module 100, current limiting module 200, power conversion module 300. Detailed Implementation

[0034] The present application will now be further described in conjunction with specific embodiments. It should be noted that, in the description of this specification, the use of terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicates that the specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms should not be construed as necessarily referring to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. In addition, those skilled in the art can combine and integrate the different embodiments or examples described in this specification.

[0035] In the description of this application, it should be noted that the terms "center", "lateral", "longitudinal", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", etc., which indicate the orientation and positional relationship based on the orientation or positional relationship shown in the accompanying drawings, are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and should not be construed as limiting the specific protection scope of this application.

[0036] It should be noted that the terms "first," "second," etc., in the specification and claims of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence.

[0037] In this application, unless otherwise expressly specified and limited, the terms "installation," "connection," "joining," and "fixing," etc., should be interpreted broadly. For example, they can refer to a connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0038] In this application, unless otherwise expressly specified and limited, "above" or "below" the second feature can include direct contact between the first and second features, or contact between the first and second features through another feature between them. Furthermore, "above," "over," and "on top" of the second feature includes the first feature being directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature includes the first feature being directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature.

[0039] The terms “comprising” and “having”, and any variations thereof, in the specification and claims of this application are intended to cover non-exclusive inclusion, for example, a process, method, system, product, or device that includes a series of steps or units is not necessarily limited to those steps or units that are explicitly listed, but may include other steps or units that are not explicitly listed or that are inherent to such process, method, product, or device.

[0040] To facilitate understanding of the technical solution of this application, a brief description of the specific process of ground insulation impedance detection in existing photovoltaic systems will be provided below.

[0041] like Figure 1 The diagram shows a schematic of an existing photovoltaic system insulation impedance detection architecture, with an insulation detection circuit added to the DC bus side of the system. R1 and R2 are detection resistors connected between the positive terminal of the bus and ground, and between the negative terminal of the bus and ground, respectively. S d This is a controllable switch. During insulation impedance testing, the DC-side switch of one string can be turned off, while the DC-side switches of the remaining strings are closed, thus allowing the insulation performance of that string to be tested. The photovoltaic system has n photovoltaic strings, each labeled as PV#1, PV#2, ..., PV#n. For ease of understanding, the insulation impedance testing process for PV#1 will be described in detail below using PV#1 as an example.

[0042] Specifically, such as Figure 2 The diagram shows the equivalent circuit when the DC-side switch of photovoltaic string PV#1 is off, while the DC-side switches of the other photovoltaic strings are closed. When the controllable switch S... d When closed, the sensing resistor R2 is short-circuited, at which point only the positive and negative insulation resistance R to ground corresponding to the photovoltaic string PV#1 is considered. P1 and R N1 And the output voltage V of the photovoltaic string PV#1 detected by resistor R1. PV1 Perform voltage division. Measure the voltage V1 between the negative terminal of photovoltaic string PV#1 and ground, and based on the voltage division principle, we can obtain:

[0043] V1=V PV1 ×R N1 / (R P1 / / R1+R N1 ).

[0044] Then, disconnect the controllable switch S. d By retesting the negative terminal voltage V2 of photovoltaic string PV#1, another value containing R can be identified. P1 and R N1 Relationship:

[0045] V2 = V PV1 ×(R N1 / / R2) / (R P1 / / R1+R N1 / / R2).

[0046] By combining the above two equations, the positive and negative ground insulation resistance R of the photovoltaic string PV#1 can be obtained. P1 and R N1 The insulation resistance to ground of the remaining photovoltaic strings can be obtained through the above process, and will not be described again here.

[0047] It is important to know that the above testing process is applicable to daytime operation, because at this time the photovoltaic system can generate electricity normally. By adjusting the connection status of the detection resistors R1 and R2, the voltage division of the insulation impedance to ground can be effectively changed, thereby enabling the evaluation of the insulation performance of the DC side of the system.

[0048] However, in practical applications, the above solution faces a significant challenge: at night or under conditions of severe light shortage, the photovoltaic string cannot generate an output voltage, i.e., V. PV1 The value is 0. At this time, since the inverter is typically disconnected from the grid before startup, there is no bus voltage. And since neither the bus voltage nor the photovoltaic string output voltage exists, there is actually no voltage difference across the sensing resistors R1 and R2. Therefore, regardless of the controllable switch S... d Whether it's in a closed or open state, the negative terminal voltage to ground cannot be detected, directly resulting in the inability to list the voltage containing R based on the voltage divider principle. P1 and R N1 The relationship equation leads to the failure of traditional insulation performance testing methods; that is, the above testing scheme cannot be performed at night.

[0049] To address the drawbacks of traditional insulation impedance testing methods, this application provides a method for detecting the insulation impedance of photovoltaic systems at night, such as... Figure 3 and Figure 6 As shown, one preferred embodiment includes the following steps: drawing power from the grid through the power-taking architecture to supply power to the negative terminal of the system bus, so that the positive and negative terminals of all photovoltaic strings form a grounding loop with the power-taking architecture, and then calculating the DC side insulation impedance of the system to ground by measuring the voltage of the power-taking architecture.

[0050] Understandably, an external power supply is required when testing the DC-side insulation resistance to ground of a photovoltaic (PV) system. Since PV strings cannot output voltage at night or in cases of severe sunlight deficiency, power must be drawn from the grid through the power supply structure. Because each PV string shares a common negative terminal, the negative terminal insulation resistance to ground of each PV string forms a grounding loop with the power supply structure. Simultaneously, since the PV strings have no output voltage and no bus voltage, the body diodes of the DC-side switches in the corresponding DC / DC units of each PV string are all in a forward-conducting state. Therefore, the positive terminal insulation resistance to ground of each PV string also forms a grounding loop with the power supply structure. The grounding loop formed by the positive and negative terminals of each PV string's insulation resistance to ground enables the testing of the insulation resistance to ground.

[0051] In this embodiment, there are various specific structures for the power extraction architecture that can achieve the above-mentioned functions. Since photovoltaic strings generate PID effects, photovoltaic systems are generally equipped with PID repair devices. Because the PID device suppresses the PID effect by drawing power from the grid, in order to simplify the hardware structure of the power extraction architecture and reduce costs, this application can reuse the PID device as the power extraction architecture to complete the detection of ground insulation impedance. For ease of understanding, the specific structure of the power extraction architecture that reuses the PID device will be described in detail below.

[0052] Specifically, such as Figure 3 As shown, the power supply architecture includes a switch module 100, a current limiting module 200, and a power conversion module 300; the switch module 100, the current limiting module 200, and the power conversion module 300 are connected in series between the negative terminal of the bus and ground. The switch module 100 can control the connection or disconnection between the power supply architecture and the negative terminal of the bus; the power conversion module 300 can draw power from the grid and supply power to the negative terminal of the bus; the DC side insulation impedance to ground of the system is calculated by measuring the voltage of the current limiting module 200.

[0053] It should be noted that the series connection order of the switch module 100, the current limiting module 200, and the power conversion module 300 is not restricted and can be adjusted according to actual needs.

[0054] It is understandable that, such as Figure 3 and Figure 4 As shown, the switch module 100 mainly consists of a controllable switch S T Composition, when the controllable switch S T When closed, the power conversion module 300 is connected between the negative terminal of the bus and ground. Since the photovoltaic strings have no output voltage at night, the body diodes of the corresponding switching transistors in each DC / DC unit will conduct in the forward direction. At this time, the positive-to-ground insulation resistance of each photovoltaic string will be connected in parallel across the negative-to-ground insulation resistance, forming a circuit with the switch module 100, the current limiting module 200, and the power conversion module 300. The controllable switch S, which enables the above functions... T There are various specific types, such as triggers, thyristors, and MOSFETs, which can be selected according to the actual needs of those skilled in the art.

[0055] The current limiting module 200 is mainly composed of current limiting resistors. Its function is to limit the current in the grounding circuit and prevent current surges from damaging the circuit.

[0056] The power conversion module 300 provides necessary power support in the ground loop, enabling the system to still perform effective ground insulation impedance detection. At night, when DC power is unavailable, the power conversion module 300 needs to draw power from the grid. Figure 5As shown, the power conversion module 300 mainly includes an isolated DC / DC unit, an AC / DC unit, and a control switch S connected in sequence. VT The isolated DC / DC unit can be connected to the negative terminal of the bus as an output terminal, controlling switch S. VT It connects to the power grid to control the connection or disconnection of the power conversion module 300 from the power grid. When the control switch S... VT After closing, the AC / DC unit rectifies the AC power supplied by the grid, and then the required power supply voltage V is obtained through the isolated DC / DC unit. T That is, the output voltage of the power conversion module 300.

[0057] To facilitate understanding, the following detailed description of the process for detecting the DC-side insulation impedance to ground of a photovoltaic system, based on the specific structure of the power extraction architecture, will be provided. The switching transistors of the corresponding DC / DC units for each photovoltaic string are labeled as switching transistors Q1 to Q... n .

[0058] like Figure 6 As shown, due to the absence of DC voltage or very low bus voltage on the inverter side before startup at night, it is necessary to control the control switch S in the power conversion module 300. VT The circuit is closed, and power is drawn from the grid side to output voltage V. T Provides power for subsequent testing processes.

[0059] When the voltage V T Once stable, the controllable switch S can be closed. T The ground insulation resistance test begins. Since all photovoltaic strings share a common negative terminal, the ground insulation resistance of the negative terminal of each photovoltaic string is related to that of the controllable switch S. T The current-limiting resistor and power conversion module 300 form a grounding loop. Meanwhile, since the photovoltaic string has no output voltage and no bus voltage, the DC-side switching transistors Q1 to Q2... n If all the body diodes are in the forward conducting state, then the positive terminal insulation resistance to ground of each photovoltaic string is also related to that of the controllable switch S. T The current-limiting resistor and the power conversion module 300 form a grounding loop.

[0060] After the grounding loop is constructed, the voltage across the current-limiting resistor can be measured, and then the measured voltage value V across the current-limiting resistor can be used as a basis for further analysis. RTCalculate the DC-side insulation impedance to ground of the system. After calculating the insulation impedance to ground, compare the calculated insulation impedance to ground with a set threshold. If the insulation impedance to ground exceeds the set threshold, it indicates that the DC-side insulation performance of the photovoltaic system meets the requirements, and the inverter can start normally for subsequent operation. If the insulation impedance to ground does not exceed the set threshold, it indicates that the DC-side insulation performance of the photovoltaic system does not meet the requirements, and the inverter will be prohibited from starting.

[0061] In this embodiment, each photovoltaic string is connected to a controllable switch S. T Two grounding loops can be formed between the current-limiting resistor and the power conversion module 300. One of the grounding loops is as follows: Figure 7 As shown, the current path of the grounding loop originates from the power supply voltage V. T The positive terminal of the power supply first flows through the current-limiting resistor, then through the negative terminal of the DC / DC unit, and then to the negative terminal of each photovoltaic string, where it is insulated from the ground. Finally, it flows back to the power supply voltage V through the ground. T The negative terminal, thus forming the negative terminal-to-ground insulation resistance and the controllable switch S T The current loop between the current-limiting resistor and the power conversion module 300. Figure 7 The thick dashed line represents a current loop, and the arrow indicates the direction of the current.

[0062] Another grounding loop, such as Figure 8 As shown, the current path of the grounding loop also originates from the power supply voltage V. T Starting from the positive terminal, the current first flows through the current-limiting resistor, and then through the switching transistor Q1 of the DC / DC unit to Q. n The body diode and inductor, and then the current flows to the positive terminal of each photovoltaic string to ground insulation impedance, and finally flows back to the power supply voltage V through the ground. T The negative terminal, thus forming the positive terminal's insulation resistance to ground and the controllable switch S T The current loop between the current-limiting resistor and the power conversion module 300. Figure 8 The thick dashed line represents a current loop, and the arrow indicates the direction of the current.

[0063] according to Figure 7 and Figure 8 The grounding loop shown is constructed as follows: Figure 9 The equivalent circuit diagram is shown. The positive terminal-to-ground insulation resistance of each photovoltaic string is R. P1 R P2 ... R Pn The negative terminal insulation resistance to ground of each photovoltaic string is R. N1 R N2 ... R Nn The resistance of the current limiting module 200 is R. T Then according to Figure 9As shown in the equivalent circuit diagram, the total insulation impedances to ground on the DC side of the system are: R P =R P1 / / R P2 / / …… / / R Pn R N =R N1 / / R N2 / / …… / / R Nn The DC-side insulation resistance to ground of the system is R. P / / R N .

[0064] When the controllable switch S T When closed, the insulation resistance of the positive and negative terminals of each photovoltaic string to ground is connected in parallel with the current-limiting resistor and the power supply voltage V. T At this point, the voltage V across the current-limiting resistor is measured. RT Then, based on the voltage divider principle, the DC-side insulation resistance to ground, R, can be obtained. P / / R N The calculation formula is:

[0065] V RT / R T =V T / (R T +R N / / R P ).

[0066] Understandably, once the DC-side insulation impedance to ground of the system meets the predetermined requirements, the inverter can start operating. At night, the DC bus voltage V of the inverter can be established using the power grid. BUS This is to enable the inverter to perform SVG compensation and other tasks. To ensure system safety, online detection of the ground insulation impedance is also required. The online detection process for the ground insulation impedance during inverter operation is the same as the DC-side ground insulation impedance detection process described above, and therefore will not be repeated here. The difference is that when the inverter is in operation, the calculated ground insulation impedance is the overall impedance of the photovoltaic system, as described above; once the ground insulation impedance is detected to be lower than the set threshold, the fault point needs to be determined.

[0067] It should be noted that insulation faults in photovoltaic systems can be located on either the AC or DC side. The following section will provide a detailed description of how to determine the location of insulation faults.

[0068] I. The insulation fault point occurs on the AC side of the photovoltaic system.

[0069] In this embodiment, the AC-side voltage to ground of the photovoltaic system can be used to measure whether an insulation fault has occurred in the system. When the photovoltaic system is performing nighttime SVG compensation, the three-phase voltage to ground on the AC side of the photovoltaic system is detected; if the voltage to ground of one phase drops to zero, and the voltages to ground of the other two phases rise from phase voltage to line voltage, it indicates that the AC-side insulation to ground of the photovoltaic system has failed.

[0070] Second, the insulation fault point occurs on the DC side of the photovoltaic system.

[0071] In this embodiment, the stability of the bus negative voltage to ground is one of the important indicators for measuring the insulation status of the system. The bus negative voltage to ground is detected when the AC side insulation performance of the photovoltaic system is normal. If the bus negative voltage to ground fluctuation exceeds a set threshold, it indicates that the photovoltaic string has experienced a ground insulation fault. The positive and negative ground insulation faults are then determined by controlling the power extraction architecture and the switching transistors of each DC / DC unit.

[0072] It is understandable that the ground insulation impedance of a photovoltaic system needs to exceed a set threshold to ensure the inverter can start and operate normally. Therefore, a ground insulation fault indicates that the ground insulation impedance of the photovoltaic system is below the set threshold. According to the voltage divider principle, the bus negative terminal-to-ground voltage when the ground insulation performance meets the requirements must be greater than the bus negative terminal-to-ground voltage when a ground insulation fault occurs. Therefore, the ground insulation fault status of the photovoltaic system can be determined by observing the fluctuations in the bus negative terminal-to-ground voltage.

[0073] Specifically, the process for judging the negative electrode insulation fault of a photovoltaic string includes the following steps: isolating the power take-off structure from the negative electrode of the busbar so that the negative electrode insulation impedance of all photovoltaic strings is connected in parallel; detecting the negative electrode voltage to ground of the busbar, if the negative electrode voltage to ground exceeds the set threshold, it indicates that the negative electrode insulation performance of the photovoltaic string meets the requirements; otherwise, the photovoltaic string has a negative electrode insulation fault to ground. When the negative-to-ground insulation performance of the photovoltaic string meets the requirements, the fault judgment of the positive-to-ground insulation of the photovoltaic string includes the following process: Isolate the power extraction structure from the negative terminal of the busbar so that the negative-to-ground insulation impedances of all photovoltaic strings are connected in parallel; sequentially control the switching transistors corresponding to each DC / DC unit to conduct, so that the positive-to-ground insulation impedances of each photovoltaic string are sequentially connected in parallel with the negative-to-ground insulation impedances of all photovoltaic strings; when each switching transistor is conducted, detect the negative-to-ground voltage of the busbar; if the negative-to-ground voltage of the busbar exceeds the set threshold, it indicates that the positive-to-ground insulation performance of the photovoltaic string meets the requirements; otherwise, the photovoltaic string has a negative-to-ground insulation fault.

[0074] It should be understood that the prerequisite for judging the positive-to-ground insulation fault of a photovoltaic (PV) string is that the negative-to-ground insulation performance of the PV string meets the requirements. That is, when judging the positive and negative-to-ground insulation faults of a PV string, the negative-to-ground insulation fault is judged first. If a negative-to-ground insulation fault occurs, it indicates that the ground insulation fault of the PV system occurs at the PV string location, and there is no need to judge the positive-to-ground insulation fault. If no ground insulation fault occurs at the negative terminal of the PV string, then the positive-to-ground insulation fault is judged. If the positive-to-ground insulation performance of the PV string meets the requirements, it indicates that no insulation fault has occurred in the PV string. If the positive-to-ground insulation performance does not meet the requirements, it indicates that the ground insulation fault of the PV system occurs at the PV string location.

[0075] To facilitate understanding, the following section will describe in detail the process of judging the insulation faults between the positive and negative terminals of a photovoltaic string and the ground, taking into account the specific architecture of the photovoltaic system.

[0076] Specifically, such as Figure 10 and Figure 14 As shown, firstly, the controllable switch S of the switch module 100 T By disconnecting the circuit, the power conversion module 300 can be blocked from the ground circuit formed by the positive and negative terminals of each photovoltaic string. At this time, the positive terminal of each photovoltaic string will have no direct electrical connection to the negative terminal of the busbar. Since all photovoltaic strings share a common negative terminal, the negative terminal insulation resistance of all photovoltaic strings can be connected in parallel to form a common circuit. Figure 11 The equivalent circuit diagram is shown. At this point, the voltage between the negative terminal of the busbar and ground can be detected, and whether it exceeds a set threshold can be used to determine if there is an insulation performance fault in the negative terminals of all photovoltaic strings. If the detected voltage between the negative terminal of the busbar and ground exceeds the set threshold, it indicates that the photovoltaic string has a negative terminal insulation fault; otherwise, it indicates that the negative terminal insulation performance of the photovoltaic string meets the requirements.

[0077] If the ground insulation performance of the negative terminals of all photovoltaic strings meets the standard, the ground insulation performance of the positive terminals of all photovoltaic strings can be further evaluated.

[0078] like Figure 12 and Figure 14 As shown, first close the switch Q1 corresponding to DC / DC#1 on the DC side, i.e., m=1; at this time, the positive terminal to ground insulation resistance of the photovoltaic string PV#1 will be connected to the negative terminal of the bus, and in parallel to the negative terminal to ground insulation resistance of all photovoltaic strings. Figure 13As shown, the positive-to-ground insulation impedance of photovoltaic string PV#1 and the negative-to-ground insulation impedances of all photovoltaic strings are connected in parallel to form the ground insulation impedance of the busbar negative terminal. In this state, the presence of an insulation fault at the positive terminal of photovoltaic string PV#1 can be determined by detecting the busbar negative terminal-to-ground voltage and checking if this voltage exceeds a threshold. If the detected busbar negative terminal-to-ground voltage exceeds the set threshold, it indicates that photovoltaic string PV#1 has a positive-to-ground insulation fault; otherwise, it indicates that the positive-to-ground insulation performance of photovoltaic string PV#1 meets the requirements.

[0079] After completing the positive-to-ground insulation fault judgment for photovoltaic string PV#1, the switch Q1 corresponding to DC / DC#1 on the DC side is disconnected. Then, after executing m=m+1, the above positive-to-ground insulation fault judgment process is performed on the next photovoltaic string PV#m until m>n+1, that is, after all photovoltaic strings have completed the positive-to-ground insulation fault judgment, the photovoltaic string ground insulation fault judgment process ends.

[0080] The basic principles, main features, and advantages of this application have been described above. Those skilled in the art should understand that this application is not limited to the above embodiments. The embodiments and descriptions in the specification are merely the principles of this application. Various changes and modifications can be made to this application without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claims. The scope of protection claimed by this application is defined by the appended claims and their equivalents.

Claims

1. A method for detecting the insulation impedance of a photovoltaic system at night, characterized in that, Includes the following steps: Before the photovoltaic system operates at night, power is drawn from the grid through the power-taking structure to supply power to the negative terminal of the system bus, so that the ground insulation impedance of the positive and negative terminals of all photovoltaic strings forms a grounding loop with the power-taking structure. Then, the DC side ground insulation impedance of the system is calculated by measuring the voltage of the power-taking structure. If the ground insulation impedance calculation result does not exceed the set threshold, a fault is judged to have occurred. When the photovoltaic system operates at night, the ground insulation fault is located by controlling the power extraction architecture and the switching transistors of each DC / DC unit; When the photovoltaic system is operating at night, the three-phase-to-ground voltage on the AC side of the photovoltaic system is detected; if the voltage of one phase to ground drops to zero, and the voltages of the other two phases to ground rise from the phase voltage to the line voltage, it is determined that the photovoltaic system has an AC side insulation fault to ground. Under the condition that the AC side of the photovoltaic system has normal ground insulation performance, the voltage of the negative terminal of the busbar to ground is detected; if the voltage fluctuation of the negative terminal of the busbar to ground exceeds the set threshold, it is determined that the photovoltaic string has a ground insulation fault, and then the positive and negative ground insulation faults are determined by controlling the power extraction architecture and the switching transistors of each DC / DC unit. The power supply architecture includes a switching module, a current limiting module, and a power conversion module; the switching module, the current limiting module, and the power conversion module are connected in series between the negative terminal of the bus and ground; The switching module is adapted to control the power-taking architecture to be connected or disconnected; the power conversion module is adapted to draw power from the grid and supply power to the negative terminal of the bus; the DC side insulation impedance of the system to ground is calculated by measuring the voltage of the current limiting module; The power conversion module includes an isolated DC / DC unit, an AC / DC unit, and a control switch connected in sequence. The isolated DC / DC unit is adapted to be connected to the negative terminal of the bus as an output terminal, and the control switch is connected to the power grid to control the connection or disconnection of the power conversion module with the power grid.

2. The method for detecting the insulation impedance of a photovoltaic system at night as described in claim 1, characterized in that, Let the number of photovoltaic strings in the system be n, and the positive terminal insulation resistance to ground of each photovoltaic string be R. P1 R P2 ... R Pn The negative terminal insulation resistance to ground of each photovoltaic string is R. N1 R N2 ... R Nn The total insulation impedances of the positive and negative sides of the system on the DC side are respectively: R P =R P1 / / R P2 / / …… / / R Pn R N =R N1 / / R N2 / / …… / / R Nn The DC-side insulation resistance to ground of the system is R. P / / R N The calculation formula is: V RT / R T =V T / (R T +R N / / R P ); Among them, V RT R represents the voltage value of the current limiting module. T V represents the resistance value of the current limiting module. T This indicates the output voltage of the power conversion module.

3. The method for detecting the nighttime insulation impedance of a photovoltaic system as described in claim 1, characterized in that, The process for diagnosing insulation faults to ground at the negative terminal of a photovoltaic string includes the following steps: The power extraction architecture is isolated from the negative terminal of the bus, so that the negative terminal insulation resistance to ground of all photovoltaic strings is connected in parallel. The voltage of the negative terminal of the busbar to ground is detected. If the voltage of the negative terminal of the busbar to ground exceeds the set threshold, it is determined that the insulation performance of the negative terminal of the photovoltaic string to ground meets the requirements; otherwise, the photovoltaic string has a negative terminal to ground insulation fault.

4. The method for detecting the insulation impedance of a photovoltaic system at night as described in claim 3, characterized in that, When the insulation performance of the negative terminal of the photovoltaic string to ground meets the requirements, the fault diagnosis of the insulation performance of the positive terminal of the photovoltaic string to ground includes the following process: The power extraction architecture is isolated from the negative terminal of the bus, so that the negative terminal insulation resistance to ground of all photovoltaic strings is connected in parallel. The switching transistors corresponding to each DC / DC unit are turned on sequentially so that the positive-to-ground insulation impedance of each photovoltaic string is connected in parallel with the negative-to-ground insulation impedance of all photovoltaic strings. The voltage between the negative terminal of the bus and ground is detected when each switch is turned on; If the voltage between the negative pole of the busbar and the ground exceeds the set threshold, it is determined that the insulation performance between the positive pole and the ground of the photovoltaic string meets the requirements; otherwise, the photovoltaic string will experience a positive pole-to-ground insulation fault.

5. The method for detecting the nighttime insulation impedance of a photovoltaic system as described in claim 1, characterized in that, The switching module uses one of the following: a trigger, a thyristor, or a MOSFET.

6. The method for detecting the insulation impedance of a photovoltaic system at night as described in claim 1, characterized in that, The current limiting module uses a current limiting resistor.

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