A photovoltaic string ground fault locating method and system and a photovoltaic inverter

Abstract

This application discloses a method, system, and photovoltaic inverter for locating photovoltaic string grounding faults. The method includes the following steps: installing protection circuits to cut off faulty circuits in each MPPT; detecting the ground insulation impedance of the common-side DC bus of the inverter system; if the ground insulation impedance of the common-side DC bus is less than a preset insulation impedance threshold, a photovoltaic string grounding fault is determined to have occurred; when a photovoltaic string grounding fault is determined to have occurred in the inverter system, the inverter system is first shut down, and then short-circuit tests are sequentially performed on each MPPT to locate the faulty photovoltaic string. The system is used to implement the above method. The photovoltaic inverter includes the above system. Beneficial effects: When a short-circuit test is performed on one MPPT, the photovoltaic strings of other MPPTs are isolated by their respective protection circuits, preventing shunting interference to the MPPT branch currently performing the short-circuit test, thus ensuring accurate location.

Description

Technical Field

[0001] This application relates to the field of new energy power generation technology, and in particular to a method, system and photovoltaic inverter for locating ground faults in photovoltaic strings. Background Technology

[0002] Non-isolated photovoltaic inverters are widely used due to their high efficiency and low cost. For example... Figure 1 The diagram shows the architecture of a traditional multi-MPPT inverter system. Each MPPT includes a photovoltaic (PV) string and a boost converter. The positive terminals of multiple PV strings are connected to independent boost converter circuits, while the negative terminals of all PV strings converge to the same DC bus reference ground within the inverter. Because there is no electrical isolation between the DC and AC sides, if the cable of one PV string is accidentally grounded due to damage, water ingress, or other reasons, a dangerous fault loop will be formed. The fault current path of the fault loop is as follows... Figure 1 As shown by the red arrow, the fault circuit sequentially passes through the power grid, inverter power diodes, negative terminal of the bus capacitor, internal reference ground, negative cable of the fault string, grounding point, earth, and power grid neutral point. Due to the extremely low impedance of this fault circuit, the fault current can reach hundreds of amperes, enough to instantly burn out the Boost switch and PCB copper foil.

[0003] Traditional protection schemes use fuses or circuit breakers, which have slow response times (milliseconds) and cannot protect semiconductor switching transistors. Some schemes use a PV-series relay at the negative terminal of the PV string to disconnect in case of a fault, but the mechanical contacts cannot handle sudden ground faults during operation, and multi-MPPT PV inverter systems require multiple relays, significantly increasing cost and size. Furthermore, when a ground fault occurs in a multi-MPPT inverter system, existing technologies for locating the ground fault mainly include: Sensor-by-channel sampling scheme: Hall current sensors are connected in series with the positive and negative terminals of each MPPT to monitor the leakage current to ground. This method is costly (tens of yuan per channel), bulky, and requires multiple high-precision ADCs for sampling.

[0004] Relay switching scheme: This involves sequentially switching the connection status of each circuit using an external relay array, and then calculating the insulation impedance of each branch using simultaneous equations. This method is complex, has limited relay lifespan, and requires a long measurement time.

[0005] Manual troubleshooting: Repair personnel carry a megohmmeter and disconnect the wires one by one on-site for measurement. This method is inefficient, poses safety risks, and is difficult to reproduce for intermittent faults. Summary of the Invention

[0006] One objective of this application is to provide a photovoltaic string grounding fault location method that can solve at least one of the defects in the above-mentioned background art.

[0007] Another object of this application is to provide a system for implementing a photovoltaic string grounding fault location method to resolve at least one of the defects in the above-mentioned background art.

[0008] Another object of this application is to provide a photovoltaic inverter that can solve at least one of the defects in the above-mentioned background art.

[0009] To achieve at least one of the above objectives, one aspect of this application provides a method for locating photovoltaic string grounding faults, applied to an inverter system including multiple MPPTs; comprising the following steps: installing protection circuits in each MPPT to cut off the fault circuit when a grounding fault occurs in the photovoltaic string; detecting the ground insulation impedance of the common-side DC bus of the inverter system; if the ground insulation impedance of the common-side DC bus is less than a preset insulation impedance threshold, determining that a photovoltaic string grounding fault has occurred; when a photovoltaic string grounding fault is determined to have occurred in the inverter system, first shutting down the inverter system, and then sequentially performing short-circuit tests on each MPPT to locate the faulty photovoltaic string.

[0010] Preferably, the grounding faults of the photovoltaic string include positive grounding faults and negative grounding faults; when detecting the ground insulation impedance of the common side DC positive and negative buses of the inverter system, if the ground insulation impedance of the common side DC positive bus of the inverter system is less than a preset insulation impedance threshold, it is determined that the photovoltaic string has a positive grounding fault; if the ground insulation impedance of the common side DC negative bus of the inverter system is less than a preset insulation impedance threshold, it is determined that the photovoltaic string has a negative grounding fault.

[0011] Preferably, the specific process for performing a short-circuit test on any MPPT is as follows: the switching transistor of the Boost circuit corresponding to the MPPT is continuously turned on, so that the positive and negative terminals of the photovoltaic string corresponding to the MPPT are short-circuited through the turned-on switching transistor and the protection circuit; the ground insulation impedance of the common side DC bus of the inverter system is detected again. If the ground insulation impedance of the common side DC bus drops below a preset threshold, it is determined that the MPPT has a photovoltaic string grounding fault; otherwise, the MPPT is determined to be normal.

[0012] Preferably, when the fault type is determined to be a photovoltaic string positive electrode grounding fault, during the short-circuit test, only the ground insulation impedance of the common side DC negative bus is tested; when the fault type is determined to be a photovoltaic string negative electrode grounding fault, during the short-circuit test, only the ground insulation impedance of the common side DC positive bus is tested.

[0013] Preferably, the photovoltaic string includes multiple photovoltaic panels connected in series; after locating the faulty photovoltaic string, the specific location of the fault point of the photovoltaic string relative to each photovoltaic panel is determined based on the relationship between the port voltage of the photovoltaic string that changes due to the ground fault and the open-circuit voltage of a single photovoltaic panel.

[0014] Preferably, the specific fault point is located by calculating the ratio n of the port voltage of the faulty photovoltaic string to the open-circuit voltage of a single photovoltaic panel; if the calculated ratio n is an integer, the fault point of the photovoltaic string is determined to be located between the nth and (n+1)th photovoltaic panels starting from the non-faulty end of the photovoltaic string; if the calculated ratio n is not an integer, the closest integer is taken as the comparison value n, and the nearest integer is used as the confidence interval, and the fault point of the photovoltaic string is determined to be located between the photovoltaic panels corresponding to the confidence interval starting from the non-faulty end of the photovoltaic string.

[0015] Preferably, during the operation of the inverter system, the positive and negative voltages of each photovoltaic string to ground are continuously detected and stored; after locating the faulty photovoltaic string, the positive and negative voltages of the faulty photovoltaic string to ground are read; based on the read positive and negative voltages to ground, the difference is calculated to obtain the port voltage corresponding to the faulty photovoltaic string.

[0016] Preferably, the open-circuit voltage of a single photovoltaic panel can be directly read from the configuration parameters of the inverter system; or, a photovoltaic string that has not experienced a fault can be selected, and the voltage can be calculated by reading the ratio of the port voltage of the photovoltaic string to the number of corresponding photovoltaic panels; or, the port voltage detected during the most recent normal operation of the photovoltaic string before the fault occurred can be read, and the voltage can be calculated based on the ratio of the port voltage to the number of photovoltaic panels.

[0017] Another aspect of this application provides a photovoltaic string grounding fault location system for implementing the aforementioned photovoltaic string grounding fault location method; it includes multiple reverse cutoff diodes, a grounding impedance detection circuit, a controller, and multiple differential voltage sampling circuits; each of the reverse cutoff diodes is installed at the negative terminal of the photovoltaic string output of each MPPT to form a protection circuit to cut off the fault circuit; the grounding impedance detection circuit is used to detect the ground insulation impedance of the common side DC bus of the inverter system; each differential voltage sampling circuit is used to sample the ground voltage of the positive and negative terminals of the photovoltaic string output corresponding to each MPPT; the controller is signal-connected to the grounding impedance detection circuit and the differential voltage sampling circuit respectively, and the controller judges the grounding fault and locates the faulty photovoltaic string based on the ground insulation impedance detected by the grounding impedance detection circuit; the controller locates the specific fault point of the faulty photovoltaic string based on the sampling results of the differential voltage sampling circuit.

[0018] Another aspect of this application provides a photovoltaic inverter including the aforementioned photovoltaic string ground fault location system.

[0019] Compared with the prior art, the beneficial effects of this application are as follows: (1) The characteristics of the short-circuit current of the photovoltaic panel and the inductive reactance naturally limit the short-circuit current to avoid generating dangerous DC short-circuit current. The inspection is carried out after the system is completely shut down and the busbar is discharged, which is safe and reliable.

[0020] (2) Since each path is equipped with a protection circuit, when a short circuit test is performed on a certain MPPT, the photovoltaic strings of other MPPTs are isolated by their respective protection circuits, which will not cause shunting interference to the MPPT branch currently performing the short circuit test, thus ensuring the accuracy of the location. Attached Figure Description

[0021] Figure 1 A schematic diagram of the architecture of a traditional multi-MPPT inverter system; Figure 2 This is a schematic diagram illustrating the specific steps of the photovoltaic string grounding fault location method of this application; Figure 3 This is a schematic diagram illustrating the workflow for locating faulty photovoltaic strings in this application; Figure 4 This is a schematic diagram of the specific structure of a single photovoltaic string in this application; Figure 5 This is a schematic diagram of the specific structure of the photovoltaic string when a ground fault occurs in this application; Figure 6 This is a schematic diagram illustrating the workflow of locating the fault point of an identified faulty photovoltaic string in this application. Figure 7 This is a schematic diagram of the architecture of the photovoltaic string grounding fault location system in this application; Figure 8 This is a schematic diagram of the differential voltage sampling circuit in this application. Detailed Implementation

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] To facilitate understanding of the technical solution of this application, the specific structure of the multi-MPPT inverter system will be briefly described below. For example... Figure 1 As shown, the multi-MPPT inverter system includes N MPPTs and a DC / AC unit; each MPPT includes a series-connected photovoltaic string (PV) and a boost converter circuit. The N MPPTs are connected in parallel to the DC side of the DC / AC unit through the output of the boost converter circuit, and the AC side of the DC / AC unit is used to connect to the power grid. Each boost converter circuit includes an input capacitor, a boost inductor, a switching transistor (Q), and an output diode; the input capacitor is connected across the positive and negative terminals of the photovoltaic string's output, the boost inductor is connected in series to the positive terminal of the photovoltaic string's output, the output diode is also connected in series to the positive terminal of the photovoltaic string's output, and the switching transistor (Q) is connected across the positive and negative terminals of the photovoltaic string's output, with its connection point to the positive terminal of the photovoltaic string's output located between the boost inductor and the output diode.

[0029] One aspect of this application provides a method for locating photovoltaic string grounding faults, applicable to the aforementioned inverter system including multiple MPPTs; such as Figure 2 and Figure 3 As shown, one preferred embodiment includes the following steps: S1: Install protection circuits on each MPPT to cut off the fault circuit when a ground fault occurs in the photovoltaic string, thereby ensuring the normal operation of the remaining MPPTs.

[0030] It should be understood that in traditional non-isolated multi-MPPT inverter systems, when a ground fault occurs at the output of a photovoltaic string, an extremely low impedance fault loop is formed through the inverter system's common-side DC target reference ground, generating a fault current of hundreds of amperes, which can instantly burn out the power devices of that MPPT. This application, however, uses protection circuits to cut off the fault loop, physically interrupting the fault current path. Furthermore, because the protection circuits for each MPPT are independent, the cut-off action of the faulty MPPT does not affect the conduction state of other non-faulty MPPTs, allowing other MPPTs to continue operating normally.

[0031] S2: Detect the ground insulation impedance of the common side DC bus of the inverter system. If the ground insulation impedance of the common side DC bus is less than the preset insulation impedance threshold, it is determined that a photovoltaic string grounding fault has occurred.

[0032] It should be understood that in a non-isolated multi-MPPT inverter system, there are equivalent ground insulation impedances between the common-side DC positive bus (BUS+) and ground (PE), and between the common-side DC negative bus (BUS-) and ground (PE). These ground insulation impedances reflect the insulation level of the DC-side live conductors to ground. During normal operation of the inverter system, the ground insulation impedance of the common-side DC bus is in the megaohm range, at which point the leakage current to ground is extremely small and negligible. However, when a ground fault occurs in one of the photovoltaic strings, the ground insulation impedance of the common-side DC bus corresponding to the fault polarity drops sharply to tens of ohms. Therefore, the presence of a photovoltaic string ground fault can be monitored in real time by detecting the ground insulation impedance of the common-side DC bus.

[0033] S3: When a photovoltaic string grounding fault is detected in the inverter system, the inverter system is first shut down, and then short-circuit tests are performed on each MPPT in sequence to locate the faulty photovoltaic string.

[0034] It should be understood that after a ground fault is detected in step S2, although the occurrence of the fault is known, the negative terminals of the photovoltaic strings output by the N MPPTs converge at the internal common reference ground. This means that the insulation impedance of the common-side DC bus to ground can only provide an overall fault assessment, and cannot directly pinpoint which MPPT's corresponding photovoltaic string is faulty. In this embodiment, by safely shutting down the inverter system and sequentially performing short-circuit tests on each MPPT to identify the faulty photovoltaic string, precise location of the faulty photovoltaic string can be achieved.

[0035] Understandably, since each MPPT is equipped with a protection circuit, when a short-circuit test is performed on one MPPT, the photovoltaic strings of other MPPTs are isolated by their respective protection circuits, preventing shunting interference to the MPPT branch currently undergoing the short-circuit test and ensuring accurate location. For ease of understanding, the specific process of the short-circuit test will be described in detail below.

[0036] In a specific embodiment, such as Figure 3 As shown, the N MPPTs of the inverter system can be sequentially ordered. When performing a short-circuit test on any i-th MPPT, the switching transistor Q of the Boost circuit corresponding to the i-th MPPT can be switched first. i The circuit remains on, allowing the positive and negative terminals of the photovoltaic string corresponding to the i-th MPPT to pass through the on-state switch Q. i The protection circuit is short-circuited. Then, the insulation resistance to ground of the common side DC bus of the inverter system is detected again. If the insulation resistance to ground of the common side DC bus drops below the preset threshold, it is determined that the i-th MPPT has a photovoltaic string grounding fault; otherwise, it is determined that the i-th MPPT is normal.

[0037] It is important to note that before performing a short-circuit test, in order to ensure the accuracy and safety of the test results, the inverter system needs to be initialized when it is shut down; that is, disconnect the AC relays on the AC side of the inverter system used to connect to the load and / or the power grid; at the same time, discharge the bus capacitors and block all PWM modulation signals.

[0038] It should be understood that the output terminals of a photovoltaic string include a positive terminal (PV+) and a negative terminal (PV-). When a ground fault occurs in the photovoltaic string, it could be a PV+ ground fault or a PV- ground fault. If the fault polarity is not distinguished, the insulation resistance to ground of both the positive and negative DC buses on the common side needs to be tested during short-circuit testing. This would increase the duration of the short-circuit test, while power devices can only withstand short-term continuous conduction. Furthermore, failing to distinguish the fault polarity would also affect the subsequent location of the specific fault point. Therefore, in this embodiment, when a ground fault is detected in the photovoltaic string, it is possible to further analyze whether it is a PV+ or PV- ground fault. For ease of understanding, the analysis process will be described in detail below.

[0039] In one specific embodiment, such as Figure 3 As shown, when testing the ground insulation resistance of the common-side DC positive and negative buses of the inverter system, if the ground insulation resistance R of the common-side DC positive bus of the inverter system is... iso+ Less than the preset insulation resistance threshold R isoiThe photovoltaic string is determined to have a PV+ ground fault; if the ground insulation resistance R of the common side DC negative bus of the inverter system is... iso- Less than the preset insulation resistance threshold R isoi The photovoltaic string was determined to have a PV-ground fault.

[0040] As can be understood from the foregoing, during normal operation of the inverter system, the ground insulation impedance of the common-side DC positive and negative buses is in the megaohm range; after a ground fault occurs, the ground insulation impedance is significantly reduced to tens of ohms; therefore, the insulation impedance threshold R... isoi The specific value can be several hundred ohms or thousands of ohms, and can be selected according to the actual needs of those skilled in the art. At least it is necessary to ensure the insulation resistance threshold R. isoi The value of is an order of magnitude different from the ground insulation impedance of the inverter system during normal operation.

[0041] Based on the determination of the fault polarity of the photovoltaic string, the short-circuit test process under different fault polarities will be described in detail below.

[0042] In a specific example, for short-circuit testing of a photovoltaic string experiencing a PV+ ground fault, the troubleshooting logic is as follows: Short-circuit the PV+ and PV- of a certain MPPT. If the photovoltaic string corresponding to that MPPT happens to be the faulty photovoltaic string, then the short-circuited PV- will be pulled to the PE voltage. At this time, the insulation impedance to ground of the inverter system's common-side DC negative bus will decrease significantly. The specific short-circuit test process will be described in detail below.

[0043] Specifically, such as Figure 3 As shown, we can first set i=1; then continuously turn on the switch Q of the Boost circuit corresponding to the i-th MPPT. i (100% duty cycle), so that the photovoltaic string PV+ of the i-th MPPT passes through the conducting switch Q. i And the protection circuit is shorted to PV. Then, the insulation resistance R to ground of the DC negative bus on the common side of the inverter system is checked. iso- If the ground insulation resistance R is detected, the test will be performed. iso- If the value drops significantly, i.e., falls below a preset threshold, it can be determined that the photovoltaic string corresponding to the i-th MPPT has a PV+ ground fault. At this point, the switch Q can be turned off. i Record the fault information, then report the fault and end the troubleshooting; if the ground insulation resistance R is detected... iso- If no significant decrease occurs, i.e., the value is higher than the preset threshold, it can be determined that the photovoltaic string corresponding to the i-th MPPT is normal, and the switching transistor Q can be turned off. iThen determine whether the fault investigation of all photovoltaic strings has been completed. If the fault investigation has not been completed, execute i=i+1 and repeat the above process to investigate the fault of the next MPPT. If no faulty photovoltaic string is found after all MPPTs have been investigated, it may be due to the abnormal ground insulation impedance detection caused by the reduction of the bus insulation. In this case, it can be reported and the investigation can be ended.

[0044] It is important to note that when the PV+ and PV- terminals of a photovoltaic string are short-circuited, the short-circuit current will be limited to the maximum short-circuit current I. sc In the vicinity, the short-circuit current is not very large at this time, so the power devices in the Boost converter and protection circuit can withstand short-term continuous conduction, such as a few seconds. Maximum short-circuit current I sc Under standard test conditions, the maximum current flowing through the photovoltaic string when its positive and negative terminals are directly short-circuited is generally considered to be I. sc The value is around 10A, which is much lower than the rated current for normal operation of the inverter system.

[0045] In a specific example, for short-circuit testing of a photovoltaic string experiencing a PV-to-ground fault, the troubleshooting logic is as follows: Short-circuit the PV+ and PV- of a certain MPPT. If the photovoltaic string corresponding to that MPPT happens to be the faulty photovoltaic string, then the short-circuited PV+ will be pulled to the PE voltage. At this time, the insulation impedance to ground of the inverter system's common-side DC positive bus will decrease significantly. The specific short-circuit test process will be described in detail below.

[0046] Specifically, such as Figure 3 As shown, we can first set i=1; then continuously turn on the switch Q of the Boost circuit corresponding to the i-th MPPT. i (100% duty cycle), so that the photovoltaic string PV+ of the i-th MPPT passes through the conducting switch Q. i And the protection circuit is shorted to PV. Then, the insulation resistance R to ground of the DC positive bus on the common side of the inverter system is checked. iso+ If the ground insulation resistance R is detected, the test will be performed. iso+ If the value drops significantly, i.e., falls below a preset threshold, it can be determined that a PV-to-ground fault has occurred in the photovoltaic string corresponding to the i-th MPPT. At this point, the switch Q can be turned off. i Record the fault information, then report the fault and end the troubleshooting; if the ground insulation resistance R is detected... iso+ If no significant decrease occurs, i.e., the value is higher than the preset threshold, it can be determined that the photovoltaic string corresponding to the i-th MPPT is normal, and the switching transistor Q can be turned off. iThen determine whether the fault investigation of all photovoltaic strings has been completed. If the fault investigation has not been completed, execute i=i+1 and repeat the above process to investigate the fault of the next MPPT. If no faulty photovoltaic string is found after all MPPTs have been investigated, it may be due to the abnormal ground insulation impedance detection caused by the reduction of the bus insulation. In this case, it can be reported and the investigation can be ended.

[0047] It is important to note that the successful execution of the short-circuit test in step S3 depends on the protection circuit set in step S1. If no protection circuit is set in each MPPT, the PV- of the photovoltaic strings corresponding to each MPPT are directly connected within the inverter system. After the inverter system is shut down, when a short-circuit test is performed on the i-th MPPT, since the negative terminals of all photovoltaic strings are connected together, the negative terminal potential of the photovoltaic string output corresponding to the i-th MPPT will affect the photovoltaic strings corresponding to other MPPTs through the common reference ground. More seriously, when the short-circuit test pulls the PV+ of the i-th photovoltaic string down to a potential close to that of the PV- of the photovoltaic string, this low potential will propagate through the common DC bus to the PV- of the photovoltaic strings corresponding to other MPPTs, causing the measured ground insulation resistance R on the common DC bus to increase. iso+ or R iso- The change cannot uniquely correspond to the i-th path (because the PV- of the photovoltaic string corresponding to the MPPT of other paths is also simultaneously pulled down).

[0048] It should be known that, as Figure 4 As shown, a single photovoltaic (PV) string typically includes multiple PV panels connected in series. When a ground fault occurs in a PV string, it usually occurs on the positive or negative terminal of a specific PV panel. In this case, only the faulty PV panel needs to be repaired. After locating the faulty PV string, performing fault detection on each PV panel sequentially may result in a long detection time. Therefore, in this embodiment, to further improve fault detection efficiency, the fault point of the PV string relative to the specific location of each PV panel can be located after locating the faulty PV string. Compared to traditional methods, this application can significantly improve the ground fault location efficiency of the inverter system through three-level precise positioning. The three levels of positioning are: the first level is polarity prediction, the second level is fault string location, and the third level is PV panel location within the string. For ease of understanding, a detailed description will be provided below.

[0049] In one specific embodiment, after locating the faulty photovoltaic string, the specific location of the fault point of the photovoltaic string relative to each photovoltaic panel is determined based on the relationship between the port voltage of the photovoltaic string that changes due to the ground fault and the open-circuit voltage of the individual photovoltaic panels.

[0050] Understandably, because the fault point is connected to ground, the potential at the fault point is forcibly clamped at 0V; for example... Figure 5 As shown, when a ground fault occurs at a point in a photovoltaic (PV) string, the fault point can "divide" the PV string into two parts: the positive grounding portion from PV+ to the fault point, and the negative grounding portion from PV- to the fault point. In the positive grounding portion, the positive terminal of the PV panel is connected to PV+, and the negative terminal is connected to the fault point; in the negative grounding portion, the positive terminal of the PV panel is connected to the fault point, and the negative terminal is connected to PV-.

[0051] If a PV+ ground fault occurs in a photovoltaic (PV) string, the PV+ potential of the PV string is pulled low by the fault point (through the fault path), forcibly clamping the voltage across the PV panel directly opposite to ground to 0V, effectively short-circuiting it and preventing it from outputting voltage. Since the fault point potential is 0V, the PV- potential of the PV string is negative relative to the fault point, causing the PV panel opposite to ground to experience a reverse voltage. However, since the PV panel itself is a unidirectional conductor, it still normally outputs its open-circuit voltage, only with the voltage polarity reversed.

[0052] If a PV- ground fault occurs in the photovoltaic (PV) string, since the fault point potential is 0V and the PV+ potential of the PV string is positive, the PV panels facing ground are subjected to a positive voltage and can operate normally, with each PV panel outputting its open-circuit voltage. Because the PV- of the PV string is connected to ground through the fault point, its potential is also pulled close to 0V, forcibly clamping the voltage across the negative-to-ground PV panels to 0V, i.e., short-circuiting them. Figure 5 (represented by the dashed box in the middle) does not output voltage externally.

[0053] Based on the above analysis, the following will describe in detail the process of locating the fault point of a photovoltaic string with a known grounding fault polarity.

[0054] In a specific example, such as Figure 6 As shown, the open-circuit voltage V of a single photovoltaic panel is obtained. oc Simultaneously, obtain the port voltage V of the faulty photovoltaic string. PV ; By calculating the port voltage V of the faulty photovoltaic string PV With respect to the open-circuit voltage V of a single photovoltaic panel oc The ratio n is used to locate the specific fault point, i.e., n = V. PV / V oc If the calculated ratio n is an integer, the fault point of the photovoltaic string is determined to be located between the nth and (n+1)th photovoltaic panels, starting from the non-faulty end of the photovoltaic string. If the calculated ratio n is not an integer, the nearest integer is taken as the ratio n, and the nearest integer is used as the confidence interval. The fault point of the photovoltaic string is determined to be located between the photovoltaic panels corresponding to the confidence interval, starting from the non-faulty end of the photovoltaic string.

[0055] Understandably, when the ratio n is an integer, if a PV- ground fault occurs in the photovoltaic string, the fault point is located between the nth and (n+1th)th photovoltaic panels starting from the PV+ section, or near the negative side of the nth photovoltaic panel. If a PV+ ground fault occurs, the fault point is located between the nth and (n+1th)th photovoltaic panels starting from the PV- section, or near the positive side of the nth photovoltaic panel. For example, if a PV- ground fault is determined in the photovoltaic string, which includes 10 photovoltaic panels, each with an open-circuit voltage of 50V, and the port voltage of the photovoltaic string drops from 500V to 300V, we can calculate n = 300 / 50 = 6; indicating that the fault point is located between the 6th and 7th photovoltaic panels starting from the PV+ section.

[0056] When the ratio n is a non-integer, such as 6.8, the nearest integer 7 can be taken, and the adjacent integers can be used as the confidence interval, i.e., the confidence interval is [6, 8]. If a PV- ground fault occurs in the photovoltaic string, the fault point is located between the 6th and 8th photovoltaic panels starting from the PV+ panel of the photovoltaic string; if a PV+ ground fault occurs in the photovoltaic string, the fault point is located between the 6th and 8th photovoltaic panels starting from the PV- panel of the photovoltaic string.

[0057] In this embodiment, there are several ways to obtain the open-circuit voltage of a single photovoltaic panel. Three methods are described below. The first is to read it directly from the inverter system's configuration parameters. The second is to select a photovoltaic string that has not experienced a fault and calculate the voltage by reading the ratio of the string's port voltage to the number of photovoltaic panels. The third is to read the port voltage detected during the most recent normal operation of the photovoltaic string before a fault occurred and calculate the voltage based on the ratio of that port voltage to the number of photovoltaic panels.

[0058] It should be noted that for the first method, the inverter system's string configuration information (module model, number of series connections) has already been entered by the maintenance personnel during installation and commissioning, so the open-circuit voltage data can be directly read. For the second method, generally speaking, the photovoltaic panel model of the entire inverter system should be consistent, so the parameters can be calculated using other normally operating photovoltaic strings. The third method is generally used when all MPPTs are faulty, and is suitable for extreme operating conditions.

[0059] It should also be understood that when acquiring the port voltage of a photovoltaic string, conventional sampling circuits cannot be used to directly calculate the difference between the PV+ and PV- voltages. Specifically, regardless of whether a ground fault occurs in the photovoltaic string, the voltage difference between PV+ and PV- remains a fixed value. This makes the acquired differential voltage unable to reflect the characteristic of the photovoltaic string port voltage changing with the specific location of the fault point. Therefore, in this embodiment, the port voltage of the photovoltaic string is acquired through hierarchical calculation, which will be described in detail below for ease of understanding.

[0060] In a specific example, during the operation of the inverter system, the positive and negative voltages of each photovoltaic string to ground are continuously detected and stored. After locating the faulty photovoltaic string, the positive and negative voltages of the faulty photovoltaic string to ground are read and used as the front-end; the back-end calculates the difference based on the read positive and negative voltages to ground to obtain the port voltage corresponding to the faulty photovoltaic string.

[0061] It is understandable that the voltage between the positive and negative terminals of a photovoltaic (PV) string and ground is affected by whether or not a ground fault occurs in the PV string. That is, when no ground fault occurs in the PV string, the difference between the voltage between the positive and negative terminals and ground is the sum of the open-circuit voltages of all the PV panels corresponding to the PV string. When a ground fault occurs in the PV string, the voltage between the positive terminal and ground or the voltage between the negative terminal and ground will decrease, so that the difference between the voltage between the positive and negative terminals and ground is the sum of the open-circuit voltages of the PV panels in the PV string that are not short-circuited.

[0062] Another aspect of this application provides a photovoltaic string grounding fault location system for implementing the above-described photovoltaic string grounding fault location method; such as Figure 7 As shown, one preferred embodiment includes multiple reverse cutoff diodes, a ground impedance detection circuit, a controller, and multiple differential voltage sampling circuits. Each reverse cutoff diode is installed at the negative terminal of the photovoltaic string output of each MPPT, forming a protection circuit to cut off the fault circuit. The ground impedance detection circuit is used to detect the ground insulation impedance of the common-side DC bus of the inverter system. Each differential voltage sampling circuit is used to sample the positive and negative voltages to ground at the output terminals of the photovoltaic strings corresponding to each MPPT. The controller is connected to both the ground impedance detection circuit and the differential voltage sampling circuit. The controller determines the ground fault and locates the faulty photovoltaic string based on the ground insulation impedance detected by the ground impedance detection circuit; the controller locates the specific fault point of the faulty photovoltaic string based on the sampling results of the differential voltage sampling circuit.

[0063] It is understandable that the number of reverse-blocking diodes is the same as the number of MPPTs in the inverter system, which is N; the N reverse-blocking diodes can be labeled D#1 to D#N respectively. For any reverse-blocking diode D#i, its anode is connected to the switch Q of the Boost converter circuit of the i-th MPPT. i The source of the diode is connected to the cathode of the photovoltaic string PV- of the i-th MPPT. The specific structure and working principle of the reverse cutoff diode are well known to those skilled in the art, and therefore will not be described in detail here. Common types of reverse cutoff diodes include Schottky diodes, fast recovery diodes, and silicon carbide Schottky diodes. For ease of understanding, the fault protection process of the reverse cutoff diode will be described in detail below.

[0064] Taking the i-th MPPT as an example, when the i-th MPPT is normally performing boost operation, the switching transistor Q... i When the circuit is turned on, the current path is: PV+ of the photovoltaic string, boost inductor, and switching transistor Q. i The drain and switching transistor Q i The source of the photovoltaic array, the anode of the reverse cutoff diode D#i, the cathode of the reverse cutoff diode D#i, and the photovoltaic string PV-. At this time, the reverse cutoff diode D#i is normally conducting and does not affect the energy storage of the boost inductor. When the switching transistor Q... i When turned off, the current path is: PV+ of the photovoltaic string, boost inductor, output diode, common-side reference ground, and switching transistor Q. i The source of the photovoltaic (MPPT), the anode of the reverse cutoff diode D#i, the cathode of the reverse cutoff diode D#i, and the photovoltaic string PV-. At this time, the reverse cutoff diode D#i remains normally conducting, and the freewheeling circuit is unobstructed. Therefore, when each MPPT is working normally, the reverse cutoff diode is always forward conducting and does not affect the boost function of the MPPT's boost circuit.

[0065] When a photovoltaic string grounding fault occurs in the i-th MPPT, taking a PV- grounding fault as an example, the ground potential is introduced into PV-. Since the inverter system is operating in grid-connected mode, the grid voltage establishes a potential relative to the ground on the common reference ground through the internal path of the inverter system. At this time, the cathode of the reverse cutoff diode D#i installed on the i-th MPPT is forcibly pulled to the ground potential, while the anode potential of the reverse cutoff diode D#i is the common reference ground potential (relatively higher than the ground potential). Therefore, the reverse cutoff diode D#i withstands the reverse voltage and quickly cuts off. The reverse cutoff diode D#i is a semiconductor device with a reverse recovery time in the nanosecond to microsecond range. It can interrupt current instantaneously when a fault occurs, with a response speed far exceeding that of traditional fuses and mechanical switches, truly achieving "non-destructive protection." Furthermore, in a multi-MPPT inverter system, the protection of the faulty branch by the reverse cutoff diode does not affect other branches; each branch only requires one additional reverse cutoff diode, with no mechanical contacts or complex analog circuits, significantly improving system reliability.

[0066] It should be understood that the grounding impedance detection circuit is connected between the positive terminal of the common-side DC bus and the ground, and between the negative terminal of the common-side DC bus and the ground. This circuit is a standard functional module already existing in the inverter system, and its specific structure and working principle are well known to those skilled in the art, so it will not be described in detail here.

[0067] The differential voltage sampling circuit is used to collect the port voltage of the photovoltaic string, which is also a standard module required for MPPT control in the inverter system. However, traditional sampling circuits often directly sample the positive and negative potentials of the photovoltaic string and calculate the difference as the port voltage of the photovoltaic string. This makes the port voltage obtained by the traditional sampling circuit unable to meet the requirements for fault location. Therefore, this embodiment needs to improve the traditional sampling circuit; for ease of understanding, a detailed description will be provided below.

[0068] In a specific embodiment, such as Figure 8 As shown, a traditional sampling circuit only includes a differential sampling circuit and a follower circuit. The two inputs of the differential circuit are connected to the positive and negative terminals of the photovoltaic string, respectively. The difference between the collected positive and negative potentials of the photovoltaic string is calculated, and the follower circuit is used to denoise the calculation result. This embodiment of the differential voltage sampling circuit adds a positive-to-ground differential sampling circuit and a negative-to-ground differential sampling circuit to the traditional sampling circuit. The positive-to-ground differential sampling circuit is used to measure the voltage V+ of the photovoltaic string PV+ to ground. PV+_1 Data acquisition is performed; that is, the differential sampling circuit directly faces the ground and samples the potential V of the photovoltaic string PV+. PV+ and ground potential V PE Perform the difference calculation to obtain V. PV+_1Similarly, the negative-to-ground differential sampling circuit obtains the ground potential V... PE The potential V of the photovoltaic string PV- PV- The difference is calculated to obtain the ground voltage V of the photovoltaic string PV-. PV-_1 The obtained V PV+_1 and V PV-_1 The difference is calculated again in the differential sampling circuit to obtain the port voltage V of the photovoltaic string based on the ground fault change. PV Finally, the output is denoised by a follower circuit.

[0069] It should be understood that the specific structures of the positive-to-ground differential sampling circuit, the negative-to-ground differential sampling circuit, and the differential sampling circuit are the same, only the sampling positions are different; the specific circuit structures and working principles of the positive-to-ground differential sampling circuit, the negative-to-ground differential sampling circuit, the differential sampling circuit, and the follower circuit are well known to those skilled in the art, and therefore will not be described in detail here.

[0070] Another aspect of this application provides a photovoltaic inverter, one preferred embodiment of which includes the photovoltaic string ground fault location system described above.

[0071] 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 locating photovoltaic string grounding faults, applied to an inverter system including multiple MPPTs, characterized in that, Includes the following steps: Install protection circuits on each MPPT to cut off the fault circuit when a ground fault occurs in the photovoltaic string; The ground insulation impedance of the common side DC bus of the inverter system is detected. If the ground insulation impedance of the common side DC bus is less than the preset insulation impedance threshold, it is determined that a photovoltaic string grounding fault has occurred. When a photovoltaic string grounding fault is detected in the inverter system, the inverter system is first shut down, and then short-circuit tests are performed on each MPPT in sequence to locate the faulty photovoltaic string.

2. The photovoltaic string grounding fault location method as described in claim 1, characterized in that, Grounding faults in photovoltaic strings include positive grounding faults and negative grounding faults; When testing the ground insulation impedance of the common side DC positive and negative busbars of the inverter system, if the ground insulation impedance of the common side DC positive busbar of the inverter system is less than the preset insulation impedance threshold, it is determined that the photovoltaic string has a positive grounding fault. If the ground insulation impedance of the common side DC negative bus of the inverter system is less than the preset insulation impedance threshold, it is determined that the photovoltaic string has a negative grounding fault.

3. The photovoltaic string grounding fault location method as described in claim 2, characterized in that, The specific process of performing a short-circuit test on any MPPT is as follows: The switch of the Boost circuit corresponding to the MPPT is kept on, so that the positive and negative terminals of the photovoltaic string corresponding to the MPPT are short-circuited through the on-state switch and the protection circuit. The ground insulation impedance of the common-side DC bus of the inverter system was tested again. If the ground insulation impedance of the common side DC bus is detected to drop below a preset threshold, it is determined that the MPPT has a photovoltaic string grounding fault; otherwise, the MPPT is determined to be normal.

4. The photovoltaic string grounding fault location method as described in claim 3, characterized in that, When the fault type is determined to be a positive grounding fault of the photovoltaic string, during the short circuit test, only the ground insulation impedance of the common side DC negative bus is tested. When the fault type is determined to be a photovoltaic string negative grounding fault, during the short-circuit test, only the ground insulation impedance of the common side DC positive bus is tested.

5. The photovoltaic string grounding fault location method according to any one of claims 2-4, characterized in that, A photovoltaic string consists of multiple photovoltaic panels connected in series; After locating the faulty photovoltaic string, the specific location of the fault point of the photovoltaic string relative to each photovoltaic panel is determined based on the relationship between the port voltage of the photovoltaic string that changes due to the ground fault and the open-circuit voltage of the individual photovoltaic panels.

6. The photovoltaic string grounding fault location method as described in claim 5, characterized in that, The specific fault point is located by calculating the ratio n of the port voltage of the faulty photovoltaic string to the open-circuit voltage of a single photovoltaic panel. If the calculated ratio n is an integer, the fault point of the photovoltaic string is determined to be located between the nth and (n+1)th photovoltaic panels, starting from the non-faulty end of the photovoltaic string. If the calculated ratio n is not an integer, the ratio n is taken as the nearest integer, and the nearest integer is used as the confidence interval. The fault point of the photovoltaic string is determined to be between the photovoltaic panels corresponding to the confidence interval starting from the non-faulty end of the photovoltaic string.

7. The photovoltaic string grounding fault location method as described in claim 5, characterized in that, During the operation of the inverter system, the positive and negative voltages of each photovoltaic string to ground are continuously detected and stored; After locating the faulty photovoltaic string, the positive and negative voltages to ground of the faulty photovoltaic string are read; based on the difference between the read positive and negative voltages to ground, the port voltage corresponding to the faulty photovoltaic string is obtained.

8. The photovoltaic string grounding fault location method as described in claim 5, characterized in that, The open-circuit voltage of a single photovoltaic panel is suitable for direct reading from the configuration parameters of the inverter system; Alternatively, select a photovoltaic string that has not experienced a fault, and calculate the result by reading the ratio of the port voltage of the photovoltaic string to the number of corresponding photovoltaic panels; Alternatively, the port voltage detected during the most recent normal operation of the photovoltaic string before a fault occurred can be read, and the result can be calculated based on the ratio of that port voltage to the number of photovoltaic panels.

9. A photovoltaic string grounding fault location system, used to implement the photovoltaic string grounding fault location method as described in any one of claims 1-8, characterized in that, include: Multiple reverse cutoff diodes; Each of the aforementioned reverse cutoff diodes is installed at the negative terminal of the photovoltaic string output of each MPPT to form a protection circuit that cuts off the fault circuit. Grounding impedance detection circuit; the grounding impedance detection circuit is used to detect the ground insulation impedance of the common side DC bus of the inverter system; Multiple differential voltage sampling circuits; each differential voltage sampling circuit is used to sample the positive and negative voltages of the photovoltaic string output terminals to ground corresponding to each MPPT. as well as The controller is connected to both the grounding impedance detection circuit and the differential voltage sampling circuit. The controller determines grounding faults and locates faulty photovoltaic strings based on the grounding impedance detection circuit's detection of grounding insulation impedance. The controller also locates the specific fault point of the faulty photovoltaic string based on the sampling results of the differential voltage sampling circuit.

10. A photovoltaic inverter, characterized in that, Including the photovoltaic string grounding fault location system as described in claim 9.

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