Temporary grounding fault processing device for medium-voltage power distribution network

Abstract

The invention discloses a processing device for a temporary grounding fault in a medium-voltage power distribution network, and belongs to the technical field of temporary grounding fault processing in a power system, the medium-voltage power distribution network adopts a neutral point to be grounded through a small resistor, and the processing device is located in a transformer substation and is connected to a bus of the medium-voltage power distribution network in parallel. The processing device comprises single-phase circuit breakers Ka, Kb and Kc, first ends of the single-phase circuit breakers Ka, Kb and Kc are respectively connected with three phases of the power distribution network, and second ends of the single-phase circuit breakers Ka, Kb and Kc are And the controller is electrically connected with the single-phase circuit breakers Ka, Kb and Kc to control the opening and closing actions of the single-phase circuit breakers Ka, Kb and Kc, and the controller is used for collecting the three-phase voltage, the zero-sequence voltage and the neutral point current in real time, judging the occurrence of a single-phase grounding fault according to the three-phase voltage, the zero-sequence voltage and the neutral point current and instructing the single-phase circuit breaker corresponding to the fault phase to be According to the processing device, the transfer of the grounding current of the fault point is completed in the delay action of the line protection device, so that the temporary grounding fault of the fault phase does not trigger the action of the protection device, and the influence of the temporary grounding fault on the power supply continuity is avoided.

Description

Technical Field

[0001] This application relates to the technical field of temporary grounding fault handling in power systems, and more specifically, to a handling device for temporary grounding faults in a low-resistance grounding system. Background Art

[0002] In the initial stage of the development of the distribution system, medium-voltage distribution networks of 35 kV and below operated with the neutral point grounded through an arc suppression coil or not grounded. When a single-phase grounding fault occurs in a non-grounded neutral system, the fault current at the fault point is small, and it is generally allowed to continue operating for 2 hours. However, with the increase in cable lines, the system capacitive current continues to increase, and the problem of intermittent arc grounding overvoltage becomes increasingly serious, which may damage the equipment insulation and even cause phase-to-phase short circuits. Although the method of grounding the neutral point through an arc suppression coil can compensate for the capacitive current, reduce the fault current at the fault point, and reduce the possibility of arc reignition, there are also some problems, such as increasing the investment in the power grid, reducing the selectivity of protection devices, generating resonance overvoltage, and having a low accuracy rate for grounding line selection. With the acceleration of the urbanization process, the scale of the urban distribution network continues to expand, and the proportion of cable lines is increasing day by day, and the system capacitive current has risen significantly. For urban distribution networks mainly composed of cable lines and with a large system capacitive current, higher requirements are put forward for rapid fault removal and protection selectivity, and the methods of grounding through an arc suppression coil or not grounding are difficult to meet these requirements.

[0003] The method of grounding the neutral point through a small resistance means that the neutral point is grounded through a low-resistance resistor (usually a few ohms to dozens of ohms). This method can effectively suppress overvoltage, quickly cut off the fault line, and reduce the insulation requirements of equipment. Therefore, in occasions mainly composed of cables or with a large system capacitive current, the method of grounding the neutral point through a small resistance is increasingly selected.

[0004] However, operating experience also shows that most of the single-phase grounding faults in medium-voltage distribution networks are recoverable grounding faults, and the grounding faults can self-heal under certain conditions, that is, they are temporary grounding faults. In a low-resistance grounding system with the neutral point grounded, after a single-phase grounding fault occurs, when the grounding impedance at the fault point is small, a large current will flow through the fault line. At this time, the protection device of the fault line will start to trip, thereby cutting off the fault line, without distinguishing whether it is a temporary grounding fault or a permanent grounding fault. Handling single-phase grounding faults in this way will increase the risk of unplanned power outages and reduce the continuity of power supply. Even if power supply is restored again through reclosing later, it will still cause a short-term power supply interruption. For occasions with high requirements for power supply continuity, such as medical institutions, semiconductor manufacturing, chemical industry, steelmaking, stock exchanges, bank data centers, key laboratories, military communication networks, etc., this disposal method is obviously not very suitable. Summary of the Invention

[0005] In the existing medium-voltage distribution network with a neutral point grounded through a small resistor, when a protection device for a faulty line trips regardless of whether it is a temporary or permanent single-phase grounding fault when the grounding impedance at the fault point is small during a single-phase grounding fault, resulting in the problem of affecting the power supply continuity. The present application provides a processing device for single-phase grounding faults in a medium-voltage distribution network. This processing device transfers the grounding current at the fault point within the time delay action of the protection device, so that a temporary grounding fault does not trigger the action of the protection device, thereby avoiding the impact of temporary grounding faults on power supply continuity.

[0006] In an embodiment of the present application, a processing device for temporary grounding faults in a medium-voltage distribution network is provided. The medium-voltage distribution network has a neutral point grounded through a small resistor. The processing device is located in a substation and is connected in parallel to the bus of the medium-voltage distribution network. The processing device includes: single-phase circuit breakers, including single-phase circuit breakers Ka, Kb, and Kc with the first ends respectively connected to the A-phase, B-phase, and C-phase of the medium-voltage distribution network and the second ends grounded through current-limiting devices; the controller is electrically connected to the single-phase circuit breakers Ka, Kb, and Kc. Based on the electrical quantity data of the medium-voltage distribution network collected in real time, when it is determined that a single-phase grounding fault has occurred, the fault phase is identified, and the single-phase circuit breaker corresponding to the fault phase is instructed to close to transfer the grounding current at the fault point into the substation.

[0007] In a further embodiment of the present application, the electrical quantity data of the medium-voltage distribution network includes the three-phase voltage, zero-sequence voltage, and neutral point current flowing through the small resistor of the medium-voltage distribution network.

[0008] In the above embodiment, further, the sampling frequency of the controller is at least 9.6 kHz.

[0009] In a further embodiment of the present application, the processing device further includes current-limiting devices, and the single-phase circuit breakers Ka, single-phase circuit breaker Kb, and single-phase circuit breaker Kc are grounded through the current-limiting devices.

[0010] In the above embodiment, further, the current-limiting device includes a single-phase reactor L, and the power frequency inductive reactance Z of the single-phase reactor L is in the range of 10 Ω to 100 Ω.

[0011] In a further embodiment of the present application, the current-limiting device further includes a single-phase circuit breaker Kl connected in parallel with the single-phase reactor L. The single-phase circuit breaker Kl is electrically connected to the controller; the controller instructs the single-phase circuit breaker Kl to close within the time t after closing the single-phase circuit breaker corresponding to the fault phase.

[0012] In the above embodiment, further, the controller instructs the single-phase circuit breaker Kl to close within 20 ms to 30 ms after closing the single-phase circuit breaker corresponding to the fault phase.

[0013] In an embodiment of the present application, the circuit breaker is a vacuum circuit breaker.

[0014] In various embodiments of the present application, the controller includes a signal sampling module, an operation module, and an input / output module. The input / output module is connected to the single-phase circuit breaker. Among them, the signal sampling module is used to collect electrical quantity data of the medium-voltage distribution network in real time and transmit the electrical quantity data to the operation module; the operation module is used to identify the faulty phase when it determines that a single-phase grounding fault has occurred based on the electrical quantity data of the medium-voltage distribution network collected in real time, and output a control instruction to the input / output module; the input / output module is used to receive the control instruction and the circuit breaker status, and output a driving signal to the single-phase circuit breaker to control the opening and closing operations of the single-phase circuit breaker.

[0015] In a further embodiment of the present application, the controller further includes a human-machine interaction module and a communication module. Among them, the communication module is used to realize information interaction between the processing device and the outside; the human-machine interaction module is used to provide device status display and a human-machine interaction interface.

[0016] The beneficial effects of the present application are as follows:

[0017] The processing device of the present application completes the transfer of the grounding current at the fault point within the delay action of the protection device, so that the temporary grounding fault of the faulty phase does not trigger the action of the protection device, thereby avoiding the impact of the temporary grounding fault on the power supply continuity;

[0018] The processing device of the present application can effectively suppress the instantaneous high-frequency current by closing the circuit breaker of the faulty phase with a reactor, and can limit the circulating current between the non-faulty phase and the faulty phase without causing an interphase short circuit when closing the wrong phase. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The accompanying drawings forming a part of the present application are used to provide a further understanding of the present application, making other features, objectives, and advantages of the present application more obvious. The schematic embodiments and descriptions thereof of the present application are used to explain the present application and do not constitute an improper limitation to the present application.

[0020] In the drawings:

[0021] Figure 1 is a schematic diagram of an embodiment of a processing device for temporary grounding faults in a medium-voltage distribution network according to the present application;

[0022] Figure 2 is a schematic diagram of the current flow direction when a temporary grounding fault occurs in a medium-voltage distribution network;

[0023] Figure 3It is a schematic diagram of current flow after the processing device of the present application is enabled;

[0024] Figure 4 It is a functional block diagram of the controller in the processing device of the present application. Detailed implementation manners

[0025] When the neutral point of the medium-voltage distribution network is grounded through a small resistor, in an embodiment of the present application, the grounding impedance in the substation meets the requirements of the GB / T 50065-2011 standard, and the zero-sequence overcurrent action of the line protection device in the distribution network is set to be a delayed action (for example, a delay of 100 ms). As Figure 1 shown, the processing device for temporary grounding faults in the medium-voltage distribution network according to the present application is located in the substation and is connected in parallel to the bus of the medium-voltage distribution network. The processing device includes: single-phase circuit breakers, including single-phase circuit breakers Ka, Kb, and Kc whose first ends are respectively connected to the A-phase, B-phase, and C-phase of the medium-voltage distribution network and whose second ends are grounded through current-limiting devices; the controller is electrically connected to the single-phase circuit breakers Ka, Kb, and Kc. Based on the electrical quantity data of the medium-voltage distribution network collected in real time, when it is determined that a single-phase grounding fault occurs, the controller identifies the fault phase and instructs the single-phase circuit breaker corresponding to the fault phase to close, so as to transfer the grounding current of the fault phase to the substation. Through the processing device of the present application, when an arc grounding occurs, the controller can instruct the single-phase circuit breaker corresponding to the fault phase to close. Since the arc resistance during arc grounding is generally not less than 20 Ω (except for pure metallic grounding), and the grounding impedance of the processing device located in the substation is smaller, after the single-phase circuit breaker corresponding to the fault phase closes, the processing device will transfer the grounding current at the fault point to the substation, so that almost no current flows through the fault point, thereby enabling the fault point to extinguish the arc naturally without triggering the action of the protection device. After a delay of a certain period of time (for example, 2 s), the controller then opens the circuit breaker of the fault phase. If the grounding fault is a temporary fault, the system will resume normal operation, and thus the fault line will not be powered off during the entire grounding fault process. If the grounding fault is a permanent fault, after the circuit breaker corresponding to the fault phase is tripped, a grounding current provided by the small resistor will flow through the fault point, causing the protection device of the fault line to continue to start the zero-sequence overcurrent protection trip, thereby cutting off the fault line.

[0026] In various embodiments of the present application, the controller collects the three-phase voltage, zero-sequence voltage of the medium-voltage distribution network, and the neutral-point current flowing through the small resistor in real time to determine whether a single-phase grounding fault has occurred, and identifies the faulty phase when it is determined that a single-phase grounding fault has occurred. Specifically, the controller is electrically connected to a device for measuring the three-phase voltage, zero-sequence voltage, and the neutral-point current flowing through the small resistor. For example, a voltage transformer (PT) with a Yyn0 wiring method can be used to measure the phase voltage. In the Yyn0 wiring, the primary winding is connected in star, the secondary winding is also connected in star, and the neutral points of both the primary and secondary sides are grounded. This wiring method can measure the line voltage and phase voltage. By detecting the changes in the three-phase voltage, it is possible to preliminarily determine whether a single-phase grounding fault has occurred. When a single-phase grounding fault occurs, the voltage of the faulty phase will decrease (close to zero), and the voltage of the non-faulty phases will increase to close to the line voltage. The phase with the lowest voltage measured by the PT is the faulty phase.

[0027] Alternatively or additionally, a three-phase five-column voltage transformer can be used to measure the phase voltage and zero-sequence voltage. The three-phase five-column voltage transformer has three primary windings and one auxiliary secondary winding. During normal operation, the three-phase voltages are symmetric, and the output voltage of the auxiliary secondary winding is zero. When an asymmetric situation such as a grounding fault occurs in the distribution network, it will cause the neutral-point potential of the system to shift, resulting in the appearance of zero-sequence voltage, which induces the corresponding zero-sequence voltage in the auxiliary secondary winding. When a single-phase grounding fault occurs, the zero-sequence voltage will change significantly and is not zero. The significant increase in the zero-sequence voltage is a direct sign of the grounding fault. By detecting the zero-sequence voltage, the fault type can be judged more accurately.

[0028] When a single-phase grounding fault occurs, the fault current forms a loop through the neutral-point resistor, generating a significantly increased zero-sequence current. The magnitude and direction of the zero-sequence current can assist in judging the faulty phase. The zero-sequence current can be directly measured by a zero-sequence current transformer (CT), or the three-phase currents can be measured separately and then added vectorially to obtain the zero-sequence current. In a distribution network with a neutral point grounded through a small resistor, when a single-phase grounding fault occurs, the zero-sequence current of the faulty phase will flow from the fault point to the neutral-point grounding location. If the flow direction of the zero-sequence current in each phase line can be determined, the faulty phase can be roughly judged.

[0029] In an embodiment of the present application, the processing device of the present application further includes a current-limiting device. The first end of the current-limiting device is connected to the second ends of the single-phase circuit breakers Ka, Kb, and Kc, and the second end of the current-limiting device is grounded. As Figure 1As shown, in an embodiment of the present application, the current limiting device includes a single-phase reactor L. Preferably, the power frequency inductive reactance Z of the single-phase reactor L is in the range of 10 Ω to 100 Ω. This is because too small an inductive reactance cannot limit the short-circuit current when the wrong phase is closed, while too large an inductive reactance cannot transfer the ground fault current at the fault point. Specifically, the single-phase reactor L has the following functions: suppressing the amplitude and rising rate (di / dt) of the high-frequency current at the moment of closing, weakening the impact of the sudden change current, and achieving the purpose of protecting the equipment; when the wrong phase is closed, it can limit the short-circuit current between the fault phase and the closing phase, so that tripping does not occur. It should be noted that when transferring the ground fault current at the fault point with a reactor, the transfer current must be detected to determine whether the closing phase is correct. Therefore, the single-phase reactor L can achieve a balance between suppressing high-frequency current, limiting two-phase short-circuit current, and transferring the ground fault current at the fault point.

[0030] In an embodiment of the present application, the current limiting device further includes a single-phase circuit breaker Kl connected in parallel with the single-phase reactor L. The single-phase circuit breaker Kl is electrically connected to the controller. When the closing is correct, the controller will instruct the single-phase circuit breaker Kl to close within the time t. Preferably, the time t is in the range of 20 ms to 30 ms. After the single-phase circuit breaker Kl closes with a delay, the grounding impedance in the processing device can be made lower than the grounding impedance at the fault point. Since the current always chooses the path with the smallest impedance to flow, the current can be effectively transferred from the fault point to the device. As Figure 2 shown, assuming that a ground fault occurs at point G in phase A, the ground current provided by the neutral point small resistor flows back through the fault point, and the current flow schematic diagram is the dotted part. As Figure 3 shown, after the controller closes the single-phase circuit breakers Ka and Kl, the ground fault current at the fault point has been transferred into the processing device, and the current at the fault point has been effectively cut off, and the arc grounding phenomenon extinguishes naturally.

[0031] Specifically, after the controller detects a ground fault and determines that it is a ground fault in phase A, it can instruct the single-phase circuit breaker Ka corresponding to phase A to close. In order to ensure that the total time from the occurrence of the ground fault to the closing of the single-phase circuit breaker Ka does not exceed 30 milliseconds, the opening and closing time of the single-phase circuit breaker should not exceed 10 milliseconds. Therefore, on the one hand, the sampling frequency of the controller should be at least 9.6 kHz to ensure that the ground fault can be quickly identified and the fault phase can be determined, and on the other hand, a single-phase vacuum circuit breaker with a fast operation time can be selected.

[0032] It should be noted that a delay of 20 ms to 30 ms is sufficient to stabilize the current of the single-phase reactor L, avoid the impact of high-frequency current on the equipment, and at the same time determine whether the closing phase is a faulty phase. If not, there is enough time to trip. If the delay is too long, the current may not be transferred in time, affecting the efficiency of fault handling. Moreover, the controller needs to complete operations such as signal sampling and fault judgment during this period. If the delay is too short, the controller may not be able to complete all operations.

[0033] As Figure 4 shown, in various embodiments of the present application, the controller may include a signal sampling module, an operation module, and an input / output module. In another embodiment of the present application, the controller may further include a human-machine interaction module and a communication module.

[0034] The signal sampling module is used to collect electrical quantity data of the medium-voltage distribution network in real time and transmit the electrical quantity data to the operation module. Specifically, the signal sampling module may collect three-phase voltages (voltages of phase A, phase B, and phase C), zero-sequence voltage (vector sum of three-phase voltages), and neutral point current flowing through a small resistor, and the neutral point current can reflect the ground fault current.

[0035] The operation module is used to identify the faulty phase and output a control instruction to the input / output module when it is determined that a single-phase ground fault has occurred based on the electrical quantity data of the medium-voltage distribution network collected in real time. Specifically, the operation module can judge whether a ground fault has occurred based on the zero-sequence voltage amplitude and the neutral point current magnitude, and at the same time can identify the faulty phase by combining the phase shift of the three-phase voltages and the phase relationship between the zero-sequence current and voltage. After determining that a ground fault has occurred and identifying the faulty phase, a circuit breaker opening / closing instruction can be generated according to a preset algorithm (such as fuzzy logic, threshold judgment). In addition, the operation module can also monitor the status of itself and external devices (such as abnormal reactor, circuit breaker jamming), and trigger an alarm or locking when necessary.

[0036] The input / output module is used to receive the control instruction and the circuit breaker status, and output a driving signal to the circuit breaker to control the opening and closing operations of the circuit breaker. Specifically, the input / output module can send a driving signal (opening / closing signal) to the circuit breakers (Ka, Kb, Kc, Kl) according to the control instruction issued by the operation module, and collect the position signal (opening / closing status), operation completion signal, etc. of the circuit breaker, and can trigger an alarm or locking in case of an abnormality (such as opening failure, refusal to operate).

[0037] The communication module is used to realize the information interaction between the device and the external system, and supports remote monitoring and fault management. Specifically, the communication module can upload fault information (such as fault phase, occurrence time, current waveform) to the background monitoring system, receive remote control instructions (such as device parameter settings, reset commands), etc. This communication module can support multiple communication protocols (such as Modbus, CDT, IEC61850, etc.) to adapt to different scenario requirements.

[0038] The human-machine interaction module is used to provide device status display and human-machine interaction interface, which is convenient for operation and maintenance personnel to operate and maintain. Specifically, the human-machine interaction module can display information such as system voltage, current, and fault status in real time through the liquid crystal screen or indicator lights, and supports setting device parameters (such as delay time, reactor switching threshold) by buttons or touch screens, and can alarm for faults or device abnormalities through sound and light or text prompts.

[0039] In summary, the controller in the processing device of this application is mainly responsible for real-time monitoring of the system status, quickly judging the grounding fault and identifying the fault phase, controlling the circuit breaker to operate to transfer the fault current, ensuring the safe and stable operation of the system, and realizing remote monitoring and data transmission through communication with external devices.

Claims

1. A processing device for temporary grounding faults in a medium-voltage distribution network, where the medium-voltage distribution network adopts a neutral point grounded through a small resistor, characterized in that, The processing device is located inside the substation and is connected in parallel to the busbar of the medium-voltage distribution network. The processing device includes: Single-phase circuit breakers, including single-phase circuit breaker Ka, single-phase circuit breaker Kb, and single-phase circuit breaker Kc, whose first ends are respectively connected to the A-phase, B-phase, and C-phase of the medium-voltage distribution network and whose second ends are grounded. A controller, which is electrically connected to single-phase circuit breaker Ka, single-phase circuit breaker Kb, and single-phase circuit breaker Kc. Based on the electrical quantity data of the medium-voltage distribution network collected in real time, when it is judged that a single-phase grounding fault has occurred, the controller identifies the faulty phase and instructs the single-phase circuit breaker corresponding to the faulty phase to close, so as to transfer the grounding current at the fault point into the substation.

2. The processing device for temporary grounding faults in a medium-voltage distribution network according to claim 1, characterized in that The electrical quantity data of the medium-voltage distribution network includes the three-phase voltage, zero-sequence voltage of the medium-voltage distribution network, and the neutral point current flowing through the small resistor.

3. The processing device for temporary grounding faults in a medium-voltage distribution network according to claim 1, characterized in that, The sampling frequency of the controller is at least 9.6 kHz.

4. The processing device for temporary grounding faults in a medium-voltage distribution network according to claim 1, wherein, It further includes a current-limiting device, and single-phase circuit breaker Ka, single-phase circuit breaker Kb, and single-phase circuit breaker Kc are grounded through the current-limiting device.

5. The processing device for temporary ground faults in a medium-voltage distribution network according to claim 4, characterized in that, The current-limiting device includes a single-phase reactor L, and the power-frequency inductive reactance Z of the single-phase reactor L is in the range of 10 Ω to 100 Ω.

6. The processing device for temporary grounding faults in a medium-voltage distribution network according to claim 5, wherein The current-limiting device further includes a single-phase circuit breaker Kl connected in parallel with the single-phase reactor L, and the single-phase circuit breaker Kl is electrically connected to the controller; the controller instructs the single-phase circuit breaker Kl to close within the time t after closing the single-phase circuit breaker corresponding to the faulty phase.

7. The processing device for temporary grounding faults in a medium-voltage distribution network according to claim 6, wherein The time t is in the range of 20 ms to 30 ms.

8. The processing device for temporary ground faults in a medium-voltage distribution network according to claim 1, wherein, The circuit breaker is a vacuum circuit breaker.

9. The processing device for temporary ground faults in a medium-voltage distribution network according to any one of claims 1 to 8, characterized in that, The controller includes a signal sampling module, an operation module, and an input / output module. The input / output module is connected to the single-phase circuit breaker. Among them, The signal sampling module is used to collect the electrical quantity data of the medium-voltage distribution network in real time and transmit the electrical quantity data to the operation module. The operation module is used to identify the faulty phase based on the electrical quantity data of the medium-voltage distribution network collected in real time when it is judged that a single-phase grounding fault has occurred, and output a control instruction to the input / output module. The input / output module is used to receive the control instruction and the circuit breaker status, and output a driving signal to the single-phase circuit breaker to control the opening and closing operations of the single-phase circuit breaker.

10. The processing device for temporary ground faults in a medium-voltage distribution network according to claim 9, characterized in that, The controller further includes a human-machine interaction module and a communication module. Among them, The communication module is used to realize the information interaction between the processing device and the outside. The human-machine interaction module is used to provide device status display and a human-machine interaction interface.

Images