Method for detecting night insulation resistance of photovoltaic system

Abstract

The invention discloses a photovoltaic system night insulation impedance detection method, which comprises the following steps: before a photovoltaic system works at night, power is taken from a power grid through a power taking framework to supply power to a bus cathode of the system, so that ground insulation impedances of positive and negative electrodes of all photovoltaic group strings and the power taking framework form a grounding loop; the ground insulation resistance of the direct current side of the system is calculated by measuring the voltage of the power taking framework; when the photovoltaic system works at night, ground insulation fault positioning is carried out by controlling the power taking framework and the switching tubes of the DC / DC units. The beneficial effects of the invention are that the system can achieve the online detection of the night insulation resistance of a photovoltaic system, and the whole detection architecture is simple in structure, and even does not need to modify an existing hardware circuit, thereby effectively reducing the cost.

Description

Technical Field

[0001] The present application relates to the technical field of photovoltaic power generation, and particularly to a method for detecting the night insulation impedance of a photovoltaic system. Background Art

[0002] Before the inverter starts, it is required that the insulation impedance value of the DC side of the system to the ground reaches or exceeds the established safety standard, and then the inverter can start to work. At present, the bridge method is widely used in the industry to measure the insulation impedance of the DC side to the ground, which can effectively evaluate the insulation state of the photovoltaic system during the day, and thus ensure the overall safety of the entire system.

[0003] Photovoltaic inverters are often configured to perform SVG compensation functions at night, so it is necessary to ensure that the insulation impedance of the photovoltaic inverter to the ground at night meets the safety standard. Since the photovoltaic device cannot generate electric energy due to lack of sunlight during this period, that is, the photovoltaic module has no output voltage; and the current mainstream insulation impedance detection technology relies on the output voltage of the PV string or the DC bus voltage, resulting in the inability to detect the insulation impedance of the photovoltaic system to the ground at night. Summary of the Invention

[0004] One of the purposes of the present application is to provide a method for detecting the night insulation impedance of a photovoltaic system that can solve at least one of the defects in the above background art.

[0005] To achieve at least one of the above purposes, the technical solution adopted by the present application is: A method for detecting the night insulation impedance of a photovoltaic system, including the following steps: Before the photovoltaic system works at night, power is taken from the power grid through a power-taking architecture to supply power to the negative pole of the bus of the system, so that the positive and negative poles of all photovoltaic strings are grounded to the ground through 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 calculation result of the insulation impedance to the ground does not exceed the set threshold, it is determined that a fault has occurred; when the photovoltaic system works at night, the power-taking architecture and the switching tubes of each DC / DC unit are controlled to locate the insulation fault to the ground.

[0006] Preferably, the power-taking 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 pole of the bus and the ground; the switching module is suitable for controlling the power-taking architecture to be turned on or off; the power conversion module is suitable for taking power from the power grid and supplying power to the negative pole of the bus; the insulation impedance of the DC side of the system to the 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 insulation impedance of the positive pole of each photovoltaic string to the ground be R P1 、R P2, ……, R Pn , the negative pole to ground insulation impedance of each photovoltaic string is R N1 , R N2 , ……, R Nn ; the total insulation impedance of the positive and negative poles to ground on the DC side of the system are respectively: R P = R P1 / / R P2 / / …… / / R Pn , R N = R N1 / / R N2 / / …… / / R Nn ; the insulation impedance R P / / R N of the DC side of the system to ground has the following calculation formula: V RT / R T = V T / (R T + R N / / R P ); Among them, V RT represents the voltage value of the current limiting module, R T represents the resistance value of the current limiting module, and V T represents the output voltage of the power conversion module.

[0008] Preferably, when the photovoltaic system operates at night, the three-phase voltages of the AC side of the photovoltaic system to ground are detected; if the voltage of a certain 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 there is a fault in the insulation of the AC side of the photovoltaic system to ground.

[0009] Preferably, when the insulation performance of the AC side of the photovoltaic system to ground is normal, the voltage of the negative pole of the bus to ground is detected; if the voltage fluctuation of the negative pole of the bus to ground exceeds the set threshold, it is determined that there is a ground insulation fault in the photovoltaic string, and then the positive and negative ground insulation faults are judged by controlling the switching tubes of the power extraction structure and each DC / DC unit.

[0010] Preferably, the judgment of the ground insulation fault of the negative pole of the photovoltaic string includes the following process: isolating the power extraction structure from the negative pole of the bus so that the negative pole to ground insulation impedances of all photovoltaic strings are connected in parallel with each other; detecting the voltage of the negative pole of the bus to ground. If the voltage of the negative pole of the bus to ground exceeds the set threshold, it is determined that the insulation performance of the negative pole of the photovoltaic string meets the requirements, otherwise there is a ground insulation fault in the negative pole of the photovoltaic string.

[0011] Preferably, when the insulation performance of the negative electrode of the photovoltaic string to the ground meets the requirements, the insulation fault judgment of the positive electrode of the photovoltaic string to the ground includes the following process: isolating the power-taking architecture from the negative electrode of the bus, so that the insulation impedances of the negative electrodes of all photovoltaic strings are connected in parallel with each other; successively controlling the switching tubes corresponding to each DC / DC unit to conduct, so that the insulation impedances of the positive electrodes of the corresponding photovoltaic strings are successively connected in parallel to the insulation impedances of the negative electrodes of all photovoltaic strings; when each switching tube conducts, detecting the voltage of the negative electrode of the bus to the ground; if the voltage of the negative electrode of the bus to the ground exceeds the set threshold, it is determined that the insulation performance of the positive electrode of the photovoltaic string to the ground meets the requirements, otherwise the positive electrode of the photovoltaic string has an insulation fault to the ground.

[0012] 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 electrode of the bus as an output end, and the control switch is connected to the power grid to control the connection or disconnection of the power conversion module to the power grid.

[0013] Preferably, the switching module adopts one of a trigger, a thyristor, and a MOS transistor.

[0014] Preferably, the current limiting module adopts a current limiting resistor.

[0015] Compared with the prior art, the beneficial effects of the present application are as follows: The present application can realize the on-line detection of the insulation impedance of the photovoltaic system at night, and the structure of the entire 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 of the Drawings

[0016] Figure 1 It is a schematic diagram of the insulation impedance detection architecture of the existing photovoltaic system.

[0017] Figure 2 is Figure 1 The equivalent circuit structure diagram corresponding to the photovoltaic string PV#1 in

[0018] Figure 3 It is a schematic diagram of the night insulation impedance architecture of the present application.

[0019] Figure 4 It is an equivalent structure diagram of the switching module, the current limiting module, and the power conversion module in the present application.

[0020] Figure 5 It is a schematic diagram of the structure of the power conversion module in the present application.

[0021] Figure 6 It is a schematic diagram of the working process of the DC-side insulation impedance detection in the present application.

[0022] Figure 7 Schematic diagram of the ground loop of the photovoltaic system in this application Figure 1 .

[0023] Figure 8 Schematic diagram of the ground loop of the photovoltaic system in this application Figure 2 .

[0024] Figure 9 Schematic structural diagram of the equivalent circuit of the ground loop of the photovoltaic system in this application.

[0025] Figure 10 Schematic structural diagram of the architecture for detecting the insulation performance of the negative pole of the string to the ground in this application

[0026] Figure 11 For this application Figure 10 Corresponding schematic structural diagram of the equivalent circuit

[0027] Figure 12 Schematic structural diagram of the architecture for detecting the insulation performance of the positive pole of the string to the ground in this application

[0028] Figure 13 For this application Figure 12 Corresponding schematic structural diagram of the equivalent circuit

[0029] Figure 14 Schematic diagram of the working process for detecting the insulation performance of the positive and negative poles of the string to the ground in this application

[0030] In the figure: Switch module 100, current limiting module 200, power conversion module 300. Specific implementation manners

[0031] Next, in combination with specific implementation manners, the present application will be further described. It should be noted that in the description of this specification, the descriptions with reference to terms such as "one embodiment", "some embodiments", "example", "specific example", or "some examples" mean that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic descriptions of the above terms should not be understood as necessarily referring to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples. In addition, those skilled in the art can combine and combine the different embodiments or examples described in this specification.

[0032] In the description of the present application, it should be noted that for orientation terms, such as the terms "center", "horizontal", "vertical", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", etc., indicating the orientation and positional relationship are based on the orientation or positional relationship shown in the drawings. This is only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and should not be construed as limiting the specific protection scope of the present application.

[0033] It should be noted that the terms "first", "second", etc. in the description and claims of the present application are used to distinguish similar objects and do not necessarily have to be used to describe a specific order or sequence.

[0034] In the present application, unless otherwise clearly defined and limited, the terms "installed", "connected", "connected to", "fixed", etc. should be understood in a broad sense. For example, it can be a connection, a detachable connection, or integrated; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the communication inside two elements or the interaction relationship between two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0035] In the present application, unless otherwise clearly defined and limited, the first feature being "above" or "below" the second feature may include the direct contact between the first and second features, or may include the situation where the first and second features are not in direct contact but in contact through other features between them. Moreover, the first feature being "above", "over", and "on" the second feature includes the first feature being directly above and obliquely above the second feature, or merely indicating that the horizontal height of the first feature is higher than that of the second feature. The first feature being "under", "beneath", and "under" the second feature includes the first feature being directly below and obliquely below the second feature, or merely indicating that the horizontal height of the first feature is lower than that of the second feature.

[0036] The terms "comprising" and "having" in the description and claims of the present application, and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or device that includes a series of steps or units does not necessarily have to be limited to those steps or units clearly listed, but may include other steps or units not clearly listed or inherent to these processes, methods, products, or devices.

[0037] For the convenience of understanding the technical solution of the present application, the following can first briefly describe the specific process of detecting the ground insulation impedance of the existing photovoltaic system.

[0038] like Figure 1 The figure shows the schematic diagram of the existing photovoltaic system insulation impedance detection architecture, where an insulation detection circuit is added on the DC bus side of the system. R1 and R2 are detection resistors connected between the positive pole of the bus and the ground and between the negative pole of the bus and the ground, respectively. d It is a controllable switch. When performing insulation impedance detection, the DC side switch tube of one of the strings can be controlled to be disconnected, and the DC side switches of the remaining strings can be closed, so as to detect the insulation performance of the string. The number of photovoltaic strings in the photovoltaic system is n, and each photovoltaic string is marked as photovoltaic string PV#1, photovoltaic string PV#2, ..., photovoltaic string PV#n; for the sake of ease of understanding, the following will take photovoltaic string PV#1 as an example to describe the insulation impedance detection process of photovoltaic string PV#1 in detail.

[0039] Specifically, Figure 2 As shown in FIG. 1 , it is a schematic diagram of an equivalent circuit when the DC side switch tube of photovoltaic string PV#1 is disconnected and the DC side switch tubes of other photovoltaic strings are closed. d When closed, the detection resistor R2 is short-circuited, and only the positive and negative insulation impedances R P1 and R N1 And the output voltage V of the photovoltaic string PV#1 is detected by the detection resistor R1 PV1 Detect the voltage V1 of the negative pole of photovoltaic string PV#1 to the ground, and according to the voltage division principle, we can get: V1=V PV1 ×R N1 / (R P1 / / R1+R N1 ).

[0040] Then, turn off the controllable switch S d , and then detect the negative pole-to-ground voltage V2 of the photovoltaic string PV#1 again, and another P1 and R N1 The relationship is: V2 = V PV1 ×(R N1 / / R2) / (R P1 / / R1+R N1 / / R2).

[0041] By combining the above two equations, the positive and negative insulation resistance R of the photovoltaic string PV#1 can be obtained: P1 and R N1 The insulation impedance detection of the remaining photovoltaic strings to ground can be obtained through the above process, which will not be repeated here.

[0042] It should be noted that the above detection process is applicable to be carried out during the day, because at this time the photovoltaic system can generate electricity normally. By adjusting the access states of the detection resistors R1 and R2, the voltage division situation of the insulation impedance to the ground can be effectively changed, so that the insulation performance of the DC side of the system can be evaluated.

[0043] However, in practical applications, the above scheme faces a significant challenge: at night or under conditions of severe lack of light, the photovoltaic string cannot generate an output voltage, that is, V PV1 is 0. At this time, since the inverter is usually disconnected from the power grid before startup, that is, there is no bus voltage either. And in the case where both the bus voltage and the output voltage of the photovoltaic string do not exist, there is actually no voltage difference across the two ends of the detection resistors R1 and R2. Therefore, regardless of whether the controllable switch S d is in the closed or open state, the negative voltage to the ground cannot be detected, which directly leads to the inability to list the relationship equation containing R P1 and R N1 according to the voltage division principle, and further makes the traditional insulation performance detection method ineffective; that is, the above detection scheme cannot be carried out at night.

[0044] Based on the drawbacks of the traditional insulation impedance detection scheme, the present application provides a method for detecting the insulation impedance of a photovoltaic system at night, as shown in Figure 3 and Figure 6 . One preferred embodiment includes the following steps: power is taken from the power grid through a power-taking architecture to supply power to the negative pole of the bus of the system, so that the insulation impedances of the positive and negative poles of all photovoltaic strings to the ground 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.

[0045] It can be understood that when detecting the insulation impedance of the DC side of the photovoltaic system to the ground, an additional power supply is required for power supply. Since the photovoltaic string cannot output voltage at night or under conditions of severe lack of light, power needs to be taken from the power grid through a power-taking architecture at this time. Since all photovoltaic strings share the negative pole, the insulation impedance of the negative pole of each photovoltaic string to the ground can form a grounding loop with the power-taking architecture; at the same time, since there is no output voltage of the photovoltaic string and no bus voltage, among the DC side switching tubes of the DC / DC units corresponding to each photovoltaic string, the body diodes of the switching tubes are all in the forward conduction state, so the insulation impedance of the positive pole of each photovoltaic string to the ground also forms a grounding loop with the power-taking architecture. The detection of the insulation impedance to the ground can be realized through the grounding loops formed by the insulation impedances of the positive and negative poles of each photovoltaic string to the ground.

[0046] In this embodiment, there are various specific structures of the power-taking architecture that can achieve the above functions. Since the PV string will generate the PID effect, the PV system generally cooperates with a PID repair device. Since the PID device suppresses the PID effect by taking power from the power grid, in order to simplify the hardware structure of the power-taking architecture and reduce costs, this application can reuse the PID device as the power-taking architecture to complete the detection of the ground insulation impedance. For the convenience of understanding, the specific structure of the power-taking architecture that reuses the PID device will be described in detail below.

[0047] Specifically, as Figure 3 shown, the power-taking 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 pole of the bus and the ground. The switch module 100 can control the connection or disconnection of the power-taking architecture to the negative pole of the bus; the power conversion module 300 can take power from the power grid and supply power to the negative pole of the bus; the system DC-side ground insulation impedance is calculated by measuring the voltage of the current-limiting module 200.

[0048] 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 limited and can be adjusted according to actual needs.

[0049] It can be understood that, as Figure 3 and Figure 4 shown, the switch module 100 is mainly composed of a controllable switch S T . When the controllable switch S T is closed, the power conversion module 300 will be connected between the negative pole of the bus and the ground. Since there is no output voltage from the PV string at night, the body diodes of the corresponding switching tubes of each DC / DC unit will conduct forward. At this time, the positive-pole-to-ground insulation impedance of each PV string will be connected in parallel across the negative-pole-to-ground insulation impedance and form a loop with the switch module 100, the current-limiting module 200, and the power conversion module 300. There are various specific types of the controllable switch S T that can achieve the above functions, such as flip-flops, thyristors, and MOS transistors, etc., and can be specifically selected according to the actual needs of those skilled in the art.

[0050] The current-limiting module 200 is mainly composed of a current-limiting resistor, and its function is to limit the current magnitude in the grounding loop to prevent damage to the circuit caused by current surges.

[0051] The power conversion module 300 is used to provide necessary power support in the ground loop so that the system can still effectively detect the ground insulation impedance. Under night conditions, the DC side cannot supply power, so the power conversion module 300 needs to take power from the grid side. As 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 pole of the bus as the output terminal, and the control switch S VT is connected 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 is closed, the AC / DC unit will rectify the alternating current provided by the power grid, and then the required power supply voltage V T can be obtained through the isolated DC / DC unit, that is, the output voltage of the power conversion module 300.

[0052] For easy understanding, the detection process of the DC-side to-ground insulation impedance of the photovoltaic system will be described in detail below for the specific structure of the power-taking architecture. Among them, the switching tubes of each photovoltaic string corresponding to the DC / DC unit are respectively marked as switching tubes Q1 to Q n .

[0053] As Figure 6 shown, under night conditions, before the inverter starts, there is no voltage or the bus voltage is very low on the DC side. At this time, it is necessary to control the control switch S VT in the power conversion module 300 to be closed, so as to draw power from the grid side and output the voltage V T to provide power for the subsequent detection process.

[0054] When the voltage V T is stable, the controllable switch S T can be closed to start the detection of the to-ground insulation impedance. Since the negative poles of each photovoltaic string are common, the negative pole to-ground insulation impedance of each photovoltaic string forms a grounding loop with the controllable switch S T , the current-limiting resistor, and the power conversion module 300. At the same time, since there is no output voltage and no bus voltage in the photovoltaic string, the body diodes of the switching tubes Q1 to Q n on the DC side are all in the forward conduction state, then the positive pole to-ground insulation impedance of each photovoltaic string also forms a grounding loop with the controllable switch S T , the current-limiting resistor, and the power conversion module 300.

[0055] After the grounding loop is constructed, the voltage of the current-limiting resistor can be measured, and then according to the measured voltage value V RTCalculate the insulation impedance of the DC side of the system to the ground. After completing the calculation of the insulation impedance to the ground, the calculated insulation impedance to the ground can be compared with the set threshold; if the insulation impedance value exceeds the set threshold, it indicates that the insulation performance of the DC side of the PV system meets the requirements, and the inverter can be normally started for subsequent work; if the insulation impedance value does not exceed the set threshold, it indicates that the insulation performance of the DC side of the PV system does not meet the requirements, and the inverter will be prohibited from starting.

[0056] In this embodiment, each PV string and the controllable switch S T , the current-limiting resistor, and the power conversion module 300 can form two grounding loops. One of the grounding loops is as Figure 7 shown. The current path of the grounding loop starts from the positive pole of the power supply voltage V T , first flows through the current-limiting resistor, then through the negative terminal of the DC / DC unit, and then flows to the negative pole-to-ground insulation impedance of each PV string, and finally returns to the negative pole of the power supply voltage V T through the ground, thus forming a current loop between the negative pole-to-ground insulation impedance and the controllable switch S T , the current-limiting resistor, and the power conversion module 300. Figure 7 The thick dashed line in it represents the current loop, and the arrow represents the current direction.

[0057] The other grounding loop is as Figure 8 shown. The current path of the grounding loop also starts from the positive pole of the power supply voltage V T , first flows through the current-limiting resistor, then through the body diode and inductor of the switching tubes Q1 to Q n of the DC / DC unit, and then flows to the positive pole-to-ground insulation impedance of each PV string, and finally returns to the negative pole of the power supply voltage V T through the ground, thus forming a current loop between the positive pole-to-ground insulation impedance and the controllable switch S T , the current-limiting resistor, and the power conversion module 300. Figure 8 The thick dashed line in it represents the current loop, and the arrow represents the current direction.

[0058] According to Figure 7 and Figure 8 the shown grounding loops, construct the equivalent circuit schematic diagram as Figure 9 shown. Among them, the positive pole-to-ground insulation impedance of each PV string is R P1 , R P2 , ……, R Pn , the negative pole-to-ground insulation impedance of each PV string is R N1 , R N2 , ……, R Nn , and the resistance value of the current-limiting module 200 is R T . Then according to Figure 9As can be seen from the equivalent circuit diagram shown, the total insulation impedance of the positive and negative poles of the system DC side to ground is: R P =R P1 / / R P2 / / …… / / R Pn , R N =R N1 / / R N2 / / …… / / R Nn , and the insulation impedance of the system DC side to ground is R P / / R N .

[0059] When the controllable switch S T is closed, the positive and negative insulation impedances of each photovoltaic string to ground are all connected in parallel across the current-limiting resistor and the power supply voltage V T . At this time, by measuring the voltage V RT of the current-limiting resistor, and then according to the voltage division principle, the calculation formula for the insulation impedance R P / / R N of the system DC side to ground is: V RT / R T =V T / (R T +R N / / R P ).

[0060] It can be understood that when the insulation impedance of the system DC side to ground meets the established requirements, the inverter can start to work. At night, the power grid can be used to establish the DC bus voltage V BUS of the inverter, so that the inverter can perform work such as SVG compensation. To ensure the safety of the system, online detection of the insulation impedance to ground is also required at this time. The online detection process of the insulation impedance to ground during the operation of the inverter is the same as the detection process of the insulation impedance of the system DC side to ground described above, so it will not be repeated here. The difference is that when the inverter is in the working state, the overall insulation impedance of the photovoltaic system to ground is calculated, and the specific calculation process is as described above; once the insulation impedance to ground is detected to be lower than the set threshold, the fault point needs to be judged.

[0061] It should be known that for the photovoltaic system, the insulation fault point may be located on the AC side or the DC side. The following will describe in detail the judgment of the occurrence position of the insulation fault point.

[0062] I. The insulation fault point appears on the AC side of the photovoltaic system.

[0063] In this embodiment, the AC-side to-ground voltage of the photovoltaic system can be used to measure whether an insulation fault occurs in the system. When the photovoltaic system performs night SVG compensation work, the three-phase to-ground voltages on the AC side of the photovoltaic system are detected; if the to-ground voltage of a certain phase drops to zero and the to-ground voltages of the other two phases rise from phase voltage to line voltage, it indicates that there is an insulation fault in the AC-side to-ground insulation of the photovoltaic system.

[0064] II. The insulation fault point appears on the DC side of the photovoltaic system.

[0065] In this embodiment, the stability of the bus negative pole to-ground voltage is one of the important indicators for measuring the insulation state of the system. The bus negative pole to-ground voltage is detected when the AC-side to-ground insulation performance of the photovoltaic system is normal; if the fluctuation of the bus negative pole to-ground voltage exceeds the set threshold, it indicates that there is an insulation fault in the photovoltaic string to the ground, and then the positive and negative to-ground insulation faults are judged by controlling the power-taking structure and the switching tubes of each DC / DC unit.

[0066] It can be understood that the to-ground insulation impedance of the photovoltaic system needs to exceed the set threshold to ensure the normal startup and operation of the inverter. Then when an insulation fault to the ground occurs, it means that the to-ground insulation impedance of the photovoltaic system is lower than the set threshold. Then according to the voltage division principle, the bus negative pole to-ground voltage corresponding to when the to-ground insulation performance meets the requirements is greater than the bus negative pole to-ground voltage corresponding to when there is an insulation fault to the ground. Then the to-ground insulation fault situation of the photovoltaic system can be judged according to the fluctuation of the bus negative pole to-ground voltage.

[0067] Specifically, the judgment of the negative pole to-ground insulation fault of the photovoltaic string includes the following process: isolate the power-taking structure from the bus negative pole so that the negative pole to-ground insulation impedances of all photovoltaic strings are connected in parallel with each other; detect the bus negative pole to-ground voltage. If the bus negative pole to-ground voltage exceeds the set threshold, it indicates that the negative pole to-ground insulation performance of the photovoltaic string meets the requirements, otherwise the photovoltaic string has a negative pole to-ground insulation fault. When the negative pole to-ground insulation performance of the photovoltaic string meets the requirements, the judgment of the positive pole to-ground insulation fault of the photovoltaic string includes the following process: isolate the power-taking structure from the bus negative pole so that the negative pole to-ground insulation impedances of all photovoltaic strings are connected in parallel with each other; control the switching tubes corresponding to each DC / DC unit to conduct in sequence so that the positive pole to-ground insulation impedances corresponding to each photovoltaic string are successively connected in parallel with the negative pole to-ground insulation impedances of all photovoltaic strings; when each switching tube conducts, detect the bus negative pole to-ground voltage; if the bus negative pole to-ground voltage exceeds the set threshold, it indicates that the positive pole to-ground insulation performance of the photovoltaic string meets the requirements, otherwise the photovoltaic string has a negative pole to-ground insulation fault.

[0068] It should be noted that the prerequisite for judging the insulation fault of the positive pole of the photovoltaic string to the ground is that the insulation performance of the negative pole of the photovoltaic string to the ground meets the requirements. That is, when judging the insulation fault of the positive and negative poles of the photovoltaic string to the ground, first judge the insulation fault of the negative pole of the photovoltaic string to the ground. If the negative pole of the photovoltaic string has an insulation fault to the ground, it means that the insulation fault of the photovoltaic system to the ground occurs at the position of the photovoltaic string. At this time, there is no need to judge the insulation fault of the positive pole of the photovoltaic string to the ground. If the negative pole of the photovoltaic string does not have an insulation fault to the ground, then continue to judge the insulation fault of the positive pole of the photovoltaic string to the ground. If the insulation performance of the positive pole of the photovoltaic string meets the requirements, it means that the photovoltaic string has no insulation fault. If the insulation performance of the positive pole of the photovoltaic string does not meet the requirements, it means that the insulation fault of the photovoltaic system to the ground occurs at the position of the photovoltaic string.

[0069] For the convenience of understanding, the following will describe in detail the process of judging the insulation faults of the positive and negative poles of the photovoltaic string to the ground in combination with the specific architecture of the photovoltaic system.

[0070] Specifically, as Figure 10 and Figure 14 shown, first, the controllable switch S of the switch module 100 T is disconnected, so as to block the ground loop formed by the power conversion module 300 and the positive and negative poles of each photovoltaic string. At this time, the insulation impedance of the positive pole of each photovoltaic string to the ground will have no direct electrical connection with the negative pole of the bus. Since the negative poles of all photovoltaic strings are common, the insulation impedances of the negative poles of all photovoltaic strings can be combined in parallel to form an equivalent circuit diagram as Figure 11 shown. At this time, the voltage of the negative pole of the bus to the ground can be detected, and whether it exceeds the set threshold can be used to judge whether there are insulation performance problems with the negative poles of all photovoltaic strings. If the detected voltage of the negative pole of the bus to the ground exceeds the set threshold, it means that the photovoltaic string has an insulation fault of the negative pole to the ground, otherwise it means that the insulation performance of the negative pole of the photovoltaic string to the ground meets the requirements.

[0071] If the insulation performance of the negative poles of all photovoltaic strings meets the standard, the insulation performance of the positive poles of all photovoltaic strings can be further evaluated.

[0072] As Figure 12 and Figure 14 shown, first, the switch tube Q1 corresponding to the DC / DC #1 on the DC side is closed, that is, m = 1; at this time, the insulation impedance of the positive pole of the photovoltaic string PV #1 to the ground will be connected to the negative terminal of the bus and in parallel with the insulation impedances of the negative poles of all photovoltaic strings. As Figure 13As shown, the positive pole to ground insulation impedance of the photovoltaic string PV#1 and the negative pole to ground insulation impedance of all photovoltaic strings together constitute the negative pole to ground insulation impedance of the bus in a parallel manner. In this state, the positive pole of the photovoltaic string PV#1 can be determined whether there is an insulation fault by detecting the negative pole to ground voltage of the bus and judging whether the voltage exceeds the threshold. If the detected negative pole to ground voltage of the bus exceeds the set threshold, it indicates that there is a positive pole to ground insulation fault in the photovoltaic string PV#1, otherwise it indicates that the positive pole to ground insulation performance of the photovoltaic string PV#1 meets the requirements.

[0073] After completing the judgment of the positive pole to ground insulation fault of the photovoltaic string PV#1, disconnect the switching tube Q1 corresponding to the DC / DC#1 on the DC side, then execute m = m + 1, and perform the above positive pole to ground insulation fault judgment process on the next photovoltaic string PV#m until m > n + 1, that is, after all photovoltaic strings have completed the positive pole to ground insulation fault judgment, end the process of judging the insulation fault of the photovoltaic string to ground.

[0074] The above describes the basic principle, main features and advantages of the present application. Those skilled in the art should understand that the present application is not limited by the above embodiments, and what is described in the above embodiments and the specification is only the principle of the present application. Without departing from the spirit and scope of the present application, the present application will have various changes and improvements, and these changes and improvements all fall within the scope of the present application claimed. The scope of protection required by the present application is defined by the appended claims and their equivalents.

Claims

1. A method for detecting the night insulation impedance of a photovoltaic system, characterized in that, The method includes the following steps: Before the photovoltaic system operates at night, power is taken from the power grid through a power-taking structure to supply power to the negative pole of the bus of the system, so that the positive and negative pole-to-ground insulation impedances of all photovoltaic strings form a grounding loop with the power-taking structure. Then, the insulation impedance of the DC side of the system to the ground is calculated by measuring the voltage of the power-taking structure. If the calculated result of the insulation impedance to the ground does not exceed the set threshold, it is determined that a fault has occurred; When the photovoltaic system operates at night, the power-taking structure and the switching tubes of each DC / DC unit are controlled to locate the insulation fault to the ground.

2. The method for detecting the night insulation impedance of a photovoltaic system according to claim 1, wherein The power-taking structure 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 pole of the bus and the ground; The switching module is adapted to control the power-taking structure to be turned on or off; the power conversion module is adapted to take power from the power grid and supply power to the negative pole of the bus; the insulation impedance of the DC side of the system to the ground is calculated by measuring the voltage of the current-limiting module.

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

4. The method for detecting the night insulation impedance of a photovoltaic system according to claim 1, wherein When the photovoltaic system operates at night, the three-phase voltages of the AC side of the photovoltaic system to the ground are detected; If the voltage of a certain phase to the ground drops to zero, and the voltages of the other two phases to the ground rise from the phase voltage to the line voltage, it is determined that an insulation fault of the AC side of the photovoltaic system to the ground has occurred.

5. The method for detecting the night insulation impedance of a photovoltaic system according to claim 4, wherein When the insulation performance of the AC side of the photovoltaic system to the ground is normal, the voltage of the negative pole of the bus to the ground is detected; If the voltage fluctuation of the negative pole of the bus to the ground exceeds the set threshold, it is determined that an insulation fault of the photovoltaic string to the ground has occurred. Then, the power-taking structure and the switching tubes of each DC / DC unit are controlled to judge the positive and negative insulation faults to the ground.

6. The method for detecting the night insulation impedance of a photovoltaic system according to claim 5, wherein The judgment of the insulation fault of the negative pole of the photovoltaic string to the ground includes the following process: The power-taking structure is isolated from the negative pole of the bus, so that the negative pole-to-ground insulation impedances of all photovoltaic strings are connected in parallel with each other; The voltage of the negative pole of the bus to the ground is detected. If the voltage of the negative pole of the bus to the ground exceeds the set threshold, it is determined that the insulation performance of the negative pole of the photovoltaic string to the ground meets the requirements; otherwise, an insulation fault of the negative pole of the photovoltaic string to the ground has occurred.

7. The method for detecting the night insulation impedance of a photovoltaic system according to claim 6, wherein, When the insulation performance of the negative pole of the photovoltaic string to the ground meets the requirements, the judgment of the insulation fault of the positive pole of the photovoltaic string to the ground includes the following process: The power-taking structure is isolated from the negative pole of the bus, so that the negative pole-to-ground insulation impedances of all photovoltaic strings are connected in parallel with each other; The switching tubes corresponding to each DC / DC unit are controlled to conduct in sequence, so that the positive pole-to-ground insulation impedances corresponding to each photovoltaic string are connected in parallel with the negative pole-to-ground insulation impedances of all photovoltaic strings in sequence; When each switching tube conducts, the voltage of the negative pole of the bus to the ground is detected; If the voltage of the negative pole of the bus to the ground exceeds the set threshold, it is determined that the insulation performance of the positive pole of the photovoltaic string to the ground meets the requirements; otherwise, an insulation fault of the positive pole of the photovoltaic string to the ground has occurred.

8. The method for detecting the night insulation impedance of a photovoltaic system according to claim 2, wherein, 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 pole of the bus as an output end, and the control switch is connected to the power grid to control the connection or disconnection of the power conversion module to the power grid.

9. The method for detecting the night insulation impedance of the photovoltaic system according to claim 2, characterized in that, The switch module adopts one of a trigger, a thyristor, and a MOS transistor.

10. The method for detecting the night insulation impedance of a photovoltaic system according to claim 2, characterized in that, The current limiting module adopts a current limiting resistor.

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