Detection method for DC insulation fault of inverter

Abstract

The invention provides a method for detecting a direct current insulation fault of an inverter, and the method is suitable for a non-isolated energy storage converter in industrial and commercial energy storage, a micro-grid or an optical storage integrated system. A leakage current detection device is arranged on the alternating current output side of the energy storage converter; a dynamic controllable grounding path control device is introduced to an alternating current output side, and an operation mode control unit and a leakage current detection device are combined, so that self-adaptive switching of a system grounding strategy is realized, and during grid-connected operation, a stable reference ground is provided by a power grid; during off-grid operation, a local protection grounding path is actively constructed, and it is ensured that the leakage current detection device has the complete fault current sensing capacity under various operation conditions. According to the mechanism, an insulation degradation fault of a direct current side is converted into a detectable earth leakage current signal of an alternating current side, so that reliable identification and accurate positioning of the fault are realized.

Description

Technical Field

[0001] This invention relates to the field of power equipment technology, and more specifically, to a method for detecting DC insulation faults in inverters. Background Technology

[0002] With the transformation of the global energy structure and the advancement of "dual carbon" goals, the installed capacity of new energy power generation (such as photovoltaic and wind power) continues to grow. However, new energy power generation is characterized by intermittency and strong volatility, posing a severe challenge to the stability of the power grid. As a key component for realizing power time shifting, smoothing fluctuations, and improving power quality, energy storage systems have experienced rapid development and widespread application in recent years.

[0003] Energy storage systems typically consist of a battery system, a power conversion system (PCS), a step-up transformer, an auxiliary power supply system, and a monitoring and management system. Among these, the PCS, as the core device connecting the battery storage system to the power grid (or load), undertakes the tasks of bidirectional energy conversion and precise control, serving as a crucial hub for realizing functions such as charge / discharge management, grid-connected operation, and energy dispatch. Therefore, real-time and accurate monitoring and control of the PCS and its connected systems are essential prerequisites for ensuring the safe and stable operation of the entire energy storage system.

[0004] Among numerous monitoring projects, monitoring the insulation status of the DC side is particularly critical. The DC side of an energy storage system typically consists of battery packs, a DC bus, and connecting cables, operating under high voltage and high current conditions for extended periods. Factors such as environmental humidity, equipment aging, and mechanical damage can lead to a decline in the DC system's insulation performance to ground. It is worth noting that in a DC system, when a single-point ground fault occurs on one pole (positive or negative), the system can continue to operate without immediate shutdown. However, if this insulation fault is not detected and addressed in time, and the other pole also becomes grounded, a low-impedance short-circuit loop to ground will be formed between the positive and negative poles, triggering a serious DC-side short-circuit accident. This can cause the DC switch to trip, equipment damage, and even major safety accidents such as arcing, fire, and explosion.

[0005] Therefore, implementing effective DC-side insulation monitoring to promptly detect and warn of single-point grounding faults is of great significance for preventing secondary faults and ensuring personal and equipment safety, and has become a mandatory requirement in the safety design of energy storage systems.

[0006] Currently, the two main DC insulation monitoring methods commonly used in the industry are the bridge balancing method and the low-frequency signal injection method.

[0007] The bridge balancing method involves constructing a high-resistance measuring bridge between the positive and negative terminals of the DC bus and ground. The voltage divider principle is used to detect changes in the voltage between the positive and negative terminals and ground, thereby calculating the insulation resistance value of each phase to ground. This method is mature and cost-effective in single, independent DC systems. However, in systems with multiple energy storage converter PCS operating in parallel, each PCS is equipped with an independent insulation monitoring circuit. Their test resistors are electrically connected in parallel, leading to a significant reduction in the overall equivalent resistance to ground. This resistance may fall below the system's set alarm threshold, triggering false alarms and severely impacting the system's reliability and availability.

[0008] Low-frequency signal injection method: This method involves injecting a low-frequency AC signal of a specific frequency (e.g., 5Hz–20Hz) into a DC system, detecting the response voltage and current of this signal in the system, and using an impedance calculation model to assess the system's insulation level to ground. This method has high sensitivity and strong anti-interference capability, making it suitable for complex systems. However, in practical applications, especially in commercial and industrial energy storage scenarios equipped with step-up transformers, problems become apparent: transformers typically use a TN grounding system with a fixed grounding point; while most energy storage converters (PCS) have a non-isolated topology, with an electrical path between their AC and DC sides. In this case, the injected low-frequency test signal may form a closed loop through the path "DC bus → test ground → transformer neutral point → AC bus → PCS internal circuit → DC bus," causing an abnormally low measured impedance, which may trigger false alarms or mis-alarms, seriously affecting the accuracy of the monitoring system. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to provide a method for detecting DC insulation faults in inverters that addresses the shortcomings of the above-mentioned technical solutions and solves the problem of malfunctions of the traditional bridge balancing method and low-frequency signal injection method during DC parallel operation and grid connection.

[0010] This invention provides a method for detecting DC insulation faults in inverters, applicable to non-isolated energy storage converters in industrial and commercial energy storage, microgrids, or integrated photovoltaic-energy storage systems. The method includes the following steps:

[0011] S1; A leakage current detection device is installed on the AC output side of the energy storage converter to detect the current vector sum of the three-phase conductors A, B, and C and the neutral conductor N in real time, so as to identify the leakage current to ground.

[0012] S2; A grounding path control device is set up, the grounding path control device includes an on / off electrical switch, one end of the electrical switch is connected to the protective grounding PE path led out from the neutral line N, and the other end is connected to the system protective grounding terminal, forming a controllable grounding path;

[0013] S3; The operating mode control unit determines whether the energy storage converter is currently in grid-connected or off-grid operation, and controls the on / off state of the grounding path control device according to the determination result: it is controlled to be disconnected in grid-connected operation and closed in off-grid operation.

[0014] S4; When the energy storage converter is in grid-connected operation and an insulation fault occurs on the DC side, the fault current flows from the DC positive terminal through the power conversion circuit of the energy storage converter into the AC side, and then flows sequentially through the AC output circuit, the leakage current detection device, the protective grounding wire PE of the grid-side TN-S system, and the ground, finally returning to the DC negative terminal, forming the first fault current path; the leakage current detection device detects the leakage current signal and outputs the leakage signal to the fault judgment and response device;

[0015] S5; When the energy storage converter is in off-grid operation and an insulation fault occurs on the DC side, the grounding path control device is in a closed state. The fault current returns from the DC positive terminal through the power conversion circuit of the energy storage converter, the neutral conductor N, the PE lead, the grounding path control device, and the ground to the DC negative terminal, forming a second fault current path; the leakage current detection device detects the leakage current signal and outputs the leakage current signal to the fault judgment and response device;

[0016] S6; A first threshold and a second threshold are preset. The fault judgment and response device performs graded judgment based on the received leakage current signal. When the leakage current is greater than the first threshold but does not exceed the second threshold, an insulation fault alarm signal is generated and uploaded to the local monitoring unit or remote monitoring background system to prompt maintenance personnel to check and handle the issue. When the leakage current exceeds the second threshold, an alarm signal is generated and the AC side circuit breaker and / or DC side switch are tripped. The insulation fault alarm signal or tripping event is uploaded to the local monitoring unit or remote monitoring background system to cut off the fault current path and achieve electrical isolation and system protection.

[0017] In the method for detecting DC insulation faults in inverters described in this invention, the leakage current detection device in step S1 is a zero-sequence current transformer (RCT), which is sleeved outside the three-phase conductors A, B, and C and the neutral conductor N, and is used to detect the vector sum of the currents in the four conductors.

[0018] In the method for detecting DC insulation faults in inverters described in this invention, the grounding path control device in step S2 is one of an electromagnetic contactor, a solid-state relay, or a semiconductor-based electronic switch. The on / off state of the electromagnetic contactor is logically controlled by the main controller of the energy storage converter according to the grid-connected operation status signal or the off-grid operation status command. When in grid-connected operation state, the main controller of the energy storage converter outputs a disconnect signal to cut off the PE path; when in off-grid operation state, it outputs a closed signal to establish a fault current return path.

[0019] In the method for detecting DC insulation faults in inverters according to the present invention, in step S6, the first threshold is 30mA and the second threshold is 50mA.

[0020] In the method for detecting DC insulation faults in inverters according to the present invention, in step S6, when the leakage current is greater than 30mA but not more than 50mA, an insulation fault alarm signal is generated and uploaded to the local monitoring unit or the remote monitoring backend system to prompt maintenance personnel to check and handle the problem.

[0021] In the method for detecting DC insulation faults in inverters described in this invention, in step S6, when the leakage current exceeds 50mA, an alarm signal is generated and the AC side circuit breaker and / or DC side switch is tripped. The insulation fault alarm signal or tripping event is then uploaded to the local monitoring unit or remote monitoring backend system to cut off the fault current path and achieve electrical isolation and system protection.

[0022] In the method for detecting DC insulation faults in inverters described in this invention, the fault judgment and response device in step S6 is integrated into the protection and control system of the energy storage converter and is equipped with a communication interface. The generated insulation fault alarm signal or tripping event is uploaded to the local monitoring unit or the remote monitoring background system through the communication interface.

[0023] In the method for detecting DC insulation faults in inverters described in this invention, the energy storage converter in step S1 adopts a three-phase four-bridge-arm voltage source inverter topology, wherein the first three bridge arms are respectively connected to the three-phase output terminals A, B, and C, and the fourth bridge arm is connected to the neutral point and outputs the neutral line N.

[0024] The present invention provides a method for detecting DC insulation faults in inverters. This method introduces a dynamically controllable grounding path control device on the AC output side, combined with an operating mode control unit and a leakage current detection device, to achieve adaptive switching of the system grounding strategy: during grid-connected operation, it relies on a stable reference ground provided by the power grid; during off-grid operation, it actively constructs a local protective grounding path, ensuring that the leakage current detection device has complete fault current sensing capabilities under various operating conditions. This mechanism transforms DC-side insulation degradation faults into detectable AC-side ground leakage current signals, thereby achieving reliable fault identification and accurate fault location. Furthermore, by setting multi-level threshold criteria, it achieves intelligent judgment and differentiated response to fault severity—for minor leakage, only an alarm is triggered to prompt maintenance personnel to intervene promptly; for severe leakage, electrical isolation is immediately implemented, effectively balancing system safety and power supply continuity. Simultaneously, fault information can be uploaded to a local or remote monitoring platform via a communication interface, supporting remote alarms, event tracing, and intelligent operation and maintenance management, significantly improving the manageability and operational reliability of new energy power electronic systems. Attached Figure Description

[0025] Figure 1 This is a flowchart of the method for detecting DC insulation faults in inverters according to the present invention;

[0026] Figure 2 This is a circuit topology diagram of the method for detecting DC insulation faults in inverters according to the present invention. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0028] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0029] like Figure 1-2 As shown, Figure 1This is a flowchart illustrating an embodiment of a method for detecting DC insulation faults in inverters according to the present invention. A method for detecting DC insulation faults in inverters is provided, applicable to non-isolated energy storage converters in industrial and commercial energy storage, microgrids, or integrated photovoltaic-energy storage systems. The method includes the following steps:

[0030] In step S1, a leakage current detection device is installed on the AC output side of the energy storage converter to detect the current vector sum of the three-phase conductors A, B, and C and the neutral conductor N in real time, so as to identify the leakage current to ground.

[0031] In step S2; a grounding path control device is set up, the grounding path control device includes an on / off electrical switch, one end of the electrical switch is connected to the protective grounding PE path led out from the neutral line N, and the other end is connected to the system protective grounding terminal, forming a controllable grounding path;

[0032] In step S3, the operating mode control unit determines whether the energy storage converter is currently in grid-connected or off-grid operation, and controls the on / off state of the grounding path control device according to the determination result: it is controlled to be disconnected in grid-connected operation and closed in off-grid operation.

[0033] In step S4; when the energy storage converter is in grid-connected operation and an insulation fault occurs on the DC side, the fault current flows from the DC positive terminal through the power conversion circuit of the energy storage converter into the AC side, and then flows sequentially through the AC output circuit, the leakage current detection device, the protective grounding wire PE of the grid-side TN-S system, and the ground, finally returning to the DC negative terminal, forming the first fault current path; the leakage current detection device detects the leakage current signal and outputs the leakage signal to the fault judgment and response device;

[0034] In step S5; when the energy storage converter is in off-grid operation and an insulation fault occurs on the DC side, the grounding path control device is in a closed state, and the fault current returns from the DC positive terminal through the power conversion circuit of the energy storage converter, the neutral conductor N, the PE lead, the grounding path control device, and the ground to the DC negative terminal, forming a second fault current path; the leakage current detection device detects the leakage current signal and outputs the leakage current signal to the fault judgment and response device;

[0035] In step S6, a first threshold and a second threshold are preset. The fault judgment and response device performs graded judgment based on the received leakage current signal. When the leakage current is greater than the first threshold but does not exceed the second threshold, an insulation fault alarm signal is generated and uploaded to the local monitoring unit or remote monitoring backend system to prompt maintenance personnel to check and handle the issue. When the leakage current exceeds the second threshold, an alarm signal is generated and the AC side circuit breaker and / or DC side switch are tripped. The insulation fault alarm signal or tripping event is uploaded to the local monitoring unit or remote monitoring backend system to cut off the fault current path and achieve electrical isolation and system protection.

[0036] In one embodiment, the leakage current detection device in step S1 is a zero-sequence current transformer (RCT), which is sleeved outside the three-phase conductors A, B, and C and the neutral conductor N, and is used to detect the vector sum of the currents in the four conductors.

[0037] In one embodiment, the grounding path control device in step S2 is one of an electromagnetic contactor, a solid-state relay, or a semiconductor-based electronic switch. The on / off state of the electromagnetic contactor is logically controlled by the main controller of the energy storage converter according to the grid-connected operation status signal or the off-grid operation status command. When in grid-connected operation state, the main controller of the energy storage converter outputs a disconnect signal to cut off the PE path; when in off-grid operation state, it outputs a closed signal to establish a fault current return path.

[0038] In one embodiment, in step S6, the first threshold is 30mA and the second threshold is 50mA.

[0039] In one embodiment, in step S6, when the leakage current is greater than 30mA but not more than 50mA, an insulation fault alarm signal is generated and uploaded to the local monitoring unit or the remote monitoring backend system to prompt maintenance personnel to check and handle the problem.

[0040] In one embodiment, in step S6, when the leakage current exceeds 50mA, an alarm signal is generated and the AC side circuit breaker and / or DC side switch are triggered to trip. The insulation fault alarm signal or tripping event is then uploaded to the local monitoring unit or the remote monitoring backend system to cut off the fault current path and achieve electrical isolation and system protection.

[0041] In one embodiment, the fault judgment and response device in step S6 is integrated into the protection and control system of the energy storage converter and is equipped with a communication interface. The generated insulation fault alarm signal or tripping event is uploaded to the local monitoring unit or the remote monitoring background system through the communication interface.

[0042] In one embodiment, the energy storage converter in step S1 adopts a three-phase four-arm voltage source inverter topology, wherein the first three arms are connected to the three-phase output terminals A, B, and C respectively, and the fourth arm is connected to the neutral point and outputs the neutral line N.

[0043] Specifically, this application constructs fault current paths in grid-connected and off-grid states by dynamically controlling the grounding path control device, such as the on / off state of the electromagnetic contactor PE, thereby solving the technical problem that traditional non-isolated energy storage converters cannot effectively detect DC side insulation faults when operating off-grid, and significantly improving the safety and protection integrity of the system under all operating conditions.

[0044] When operating off-grid, the PE is led out from the neutral line N through the closed grounding path control device to form a controllable grounding loop, providing a return path for the leakage current generated by the DC side insulation fault. This allows the leakage current to be effectively sensed by the leakage current detection device (such as a zero-sequence current transformer), transforming the insulation fault into a detectable AC side leakage event, and achieving accurate fault identification.

[0045] A tiered protection mechanism is adopted (e.g., 30mA / 50mA dual threshold judgment):

[0046] When the leakage current is at a moderate level (>30mA and ≤50mA), only an alarm signal is issued to avoid false tripping and ensure power supply continuity.

[0047] When the leakage current exceeds the high threshold (>50mA), the trip protection is immediately activated to quickly disconnect the fault current path and prevent equipment damage or personal injury. This tiered strategy balances safety and operational reliability.

[0048] The fault diagnosis and response device integrates a communication interface, which can upload information such as insulation fault alarms and tripping events to the local monitoring unit or remote monitoring backend system. It supports remote alarms, fault tracing and preventive maintenance, and is suitable for unattended industrial and commercial energy storage, microgrids and photovoltaic-storage integrated systems, improving operation and maintenance efficiency and system manageability.

[0049] This application is designed specifically for non-isolated energy storage converters and is suitable for complex scenarios such as industrial and commercial energy storage, microgrids, and photovoltaic-storage-charging integration in TN-S grounded systems. It supports three-phase four-wire output and 100% unbalanced load operation, and has good engineering applicability and promotion value.

[0050] The required new components (such as leakage current detection devices and grounding path control devices) are all standard power electronic components, without the need for additional high-voltage isolation or dedicated insulation monitoring circuits (such as the low-frequency voltage signal injection method in IEC 61557-9), which reduces system complexity and cost and facilitates rapid integration and deployment in the existing PCS platform.

[0051] Combining a three-phase four-arm inverter topology, the fourth arm can actively adjust the neutral point potential, further improving voltage stability under unbalanced loads and providing a hardware foundation for the reliable extraction of the PE path, thereby enhancing the overall power quality and safety performance of the system.

[0052] It should be noted that, for the sake of simplicity, the foregoing method embodiments are all described as a series of actions. However, those skilled in the art should understand that the present invention is not limited to the described order of actions, because according to the present invention, some steps can be performed in other orders or simultaneously. Furthermore, those skilled in the art should also understand that the embodiments described in the specification are preferred embodiments, and the actions and modules involved are not necessarily essential to the present invention.

[0053] Through the above description of the embodiments, those skilled in the art can clearly understand that the methods according to the above embodiments can be implemented by means of software plus necessary general-purpose hardware platforms. Of course, they can also be implemented by hardware, but in many cases the former is a better implementation method. Based on this understanding, the technical solution of the present invention, in essence, or the part that contributes to the prior art, can be embodied in the form of a software product. This computer software product is stored in a storage medium (such as ROM / RAM, magnetic disk, optical disk) and includes several instructions to cause a terminal device (which may be a mobile phone, computer, server, or network device, etc.) to execute the methods described in the various embodiments of the present invention.

[0054] Therefore, the above description is only a preferred embodiment of the present invention, and the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. The scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for detecting DC insulation faults in inverters, applicable to non-isolated energy storage converters in industrial and commercial energy storage, microgrids, or integrated photovoltaic-energy storage systems, characterized in that... The method includes the following steps: S1; A leakage current detection device is installed on the AC output side of the energy storage converter to detect the current vector sum of the three-phase conductors A, B, and C and the neutral conductor N in real time, so as to identify the leakage current to ground. S2; A grounding path control device is set up, the grounding path control device includes an on / off electrical switch, one end of the electrical switch is connected to the protective grounding PE path led out from the neutral line N, and the other end is connected to the system protective grounding terminal, forming a controllable grounding path; S3; The operating mode control unit determines whether the energy storage converter is currently in grid-connected or off-grid operation, and controls the on / off state of the grounding path control device according to the determination result: it is controlled to be disconnected in grid-connected operation and closed in off-grid operation. S4; When the energy storage converter is in grid-connected operation and an insulation fault occurs on the DC side, the fault current flows from the DC positive terminal through the power conversion circuit of the energy storage converter into the AC side, and then flows sequentially through the AC output circuit, the leakage current detection device, the protective grounding wire PE of the grid-side TN-S system, and the ground, finally returning to the DC negative terminal, forming the first fault current path; the leakage current detection device detects the leakage current signal and outputs the leakage signal to the fault judgment and response device; S5; When the energy storage converter is in off-grid operation and an insulation fault occurs on the DC side, the grounding path control device is in a closed state. The fault current returns from the DC positive terminal through the power conversion circuit of the energy storage converter, the neutral conductor N, the PE lead, the grounding path control device, and the ground to the DC negative terminal, forming a second fault current path; the leakage current detection device detects the leakage current signal and outputs the leakage current signal to the fault judgment and response device; S6; A first threshold and a second threshold are preset. The fault judgment and response device performs graded judgment based on the received leakage current signal. When the leakage current is greater than the first threshold but does not exceed the second threshold, an insulation fault alarm signal is generated and uploaded to the local monitoring unit or remote monitoring background system to prompt maintenance personnel to check and handle the issue. When the leakage current exceeds the second threshold, an alarm signal is generated and the AC side circuit breaker and / or DC side switch are tripped. The insulation fault alarm signal or tripping event is uploaded to the local monitoring unit or remote monitoring background system to cut off the fault current path and achieve electrical isolation and system protection.

2. The method for detecting DC insulation faults in inverters according to claim 1, characterized in that, In step S1, the leakage current detection device is a zero-sequence current transformer (RCT), which is installed outside the three-phase conductors A, B, and C and the neutral conductor N, and is used to detect the vector sum of the currents in the four conductors.

3. The method for detecting DC insulation faults in inverters according to claim 2, characterized in that, In step S2, the grounding path control device is one of an electromagnetic contactor, a solid-state relay, or a semiconductor-based electronic switch. The on / off state of the electromagnetic contactor is controlled by the main controller of the energy storage converter according to the grid-connected operation status signal or the off-grid operation status command. When in grid-connected operation state, the main controller of the energy storage converter outputs a disconnect signal to cut off the PE path; when in off-grid operation state, it outputs a closed signal to establish a fault current return path.

4. The method for detecting DC insulation faults in inverters according to claim 2, characterized in that, In step S6, the first threshold is 30mA and the second threshold is 50mA.

5. The method for detecting DC insulation faults in inverters according to claim 4, characterized in that, In step S6, when the leakage current is greater than 30mA but not more than 50mA, an insulation fault alarm signal is generated and uploaded to the local monitoring unit or the remote monitoring backend system to prompt maintenance personnel to check and handle the problem.

6. The method for detecting DC insulation faults in an inverter according to claim 5, characterized in that, In step S6, when the leakage current exceeds 50mA, an alarm signal is generated and the AC side circuit breaker and / or DC side switch are triggered to trip. The insulation fault alarm signal or tripping event is uploaded to the local monitoring unit or remote monitoring background system to cut off the fault current path and achieve electrical isolation and system protection.

7. The method for detecting DC insulation faults in an inverter according to claim 6, characterized in that, In step S6, the fault judgment and response device is integrated into the protection and control system of the energy storage converter and is equipped with a communication interface. The generated insulation fault alarm signal or tripping event is uploaded to the local monitoring unit or the remote monitoring background system through the communication interface.

8. The method for detecting DC insulation faults in inverters according to claim 1, characterized in that, In step S1, the energy storage converter adopts a three-phase four-bridge-arm voltage source inverter topology, wherein the first three bridge arms are connected to the three-phase output terminals A, B, and C respectively, and the fourth bridge arm is connected to the neutral point and outputs the neutral line N.

Images