Active line selection method, device and system for high-resistance grounding fault

Abstract

The embodiment of the invention discloses a high-resistance grounding fault active line selection method, device and system, relates to the technical field of power distribution network relay protection, and aims to solve the problems of low sensitivity and poor reliability of high-resistance grounding fault line selection. The method comprises the following steps: when fault starting is monitored, controlling a switch assembly connected with a damping resistor in a neutral point grounding loop to act so as to change the access state of the damping resistor; collecting bus zero-sequence voltage and zero-sequence current of each feeder line before and after the action of the switch assembly; calculating the zero-sequence active power variation of each feeder line before and after the action; and finally, comparing the zero-sequence active power variation of each feeder line to determine a fault feeder line. According to the invention, excitation is actively injected and the power variation is analyzed, so that the fault characteristics are enhanced, and the method has the beneficial effects of high sensitivity and high reliability.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of power distribution network relay protection, in particular to a high-impedance ground fault active line selection method, device and system. BACKGROUND

[0002] In medium-voltage distribution networks, such as 35kV power grid, with the wide application of cable lines, the system-to-ground capacitance current is increasing. To solve this problem, the neutral point through arc suppression coil and resonance grounding mode has become the mainstream technology. This mode can effectively compensate the ground capacitance current, suppress the arc overvoltage generated during single-phase ground fault, and improve the power supply reliability. However, when a high-impedance ground fault occurs in the distribution network, i.e. there is a transition resistance of thousands of ohms or even higher at the fault point, the current flowing through the fault point will become extremely weak. This leads to the amplitude of fault characteristic quantities in the entire zero sequence network, such as zero sequence voltage and zero sequence current, being very small, almost being submerged by the unbalance and background noise during normal operation of the system.

[0003] Currently, some passive line selection methods mainly rely on steady-state zero sequence voltage, zero sequence current amplitude or power direction to make judgments. In the above-mentioned weak signal scenario of high-impedance ground fault, the sensitivity of these methods is seriously insufficient, it is difficult to set a fixed threshold that can reliably start without misjudgment, often leading to the protection device refusing to act or malfunctioning. The existing research generally ignores the transient characteristics of controllable switches (such as anti-parallel thyristors) during the switching of damping resistors, and does not fully utilize the zero sequence transient response signals generated by this action as a source of active excitation, thus missing the opportunity to enhance the fault recognition ability. In addition, although some transient line selection methods can be implemented in 10kV or low-voltage systems, in 35kV distribution networks, due to larger capacitance current, complex noise environment, and significant changes in system impedance, their reliability and adaptability are significantly reduced, making it difficult to meet the stability and accuracy requirements of engineering applications. SUMMARY

[0004] The main purpose of the present application is to provide a high-impedance ground fault active line selection method, device and system, which can solve the problem of low sensitivity and poor reliability of traditional line selection methods caused by weak characteristic signals of high-impedance ground fault in existing resonance grounding distribution networks, and is prone to refusal to act or malfunction. At the same time, it also aims to improve the existing active injection method, and provide a more stable and more adaptable technical solution by utilizing more comprehensive electrical quantity information.

[0005] To achieve the above-mentioned purpose, the first aspect of the present application provides a high-impedance ground fault active line selection method applied to a distribution network with neutral point through arc suppression coil and damping resistor grounding, the method comprising: monitoring a zero sequence network parameter of the power distribution network, and starting a line selection process when the zero sequence network parameter meets a preset fault starting condition; controlling a switch component electrically connected to a damping resistor in a neutral point grounding loop of the power distribution network to change an access state of the damping resistor in a zero sequence loop; collecting bus zero sequence voltages of the power distribution network and zero sequence currents of each feeder before and after the switch component is actuated; calculating zero sequence active power variation of each feeder before and after the switch component is actuated based on the bus zero sequence voltages and the zero sequence currents of each feeder; comparing the zero sequence active power variation of each feeder with a preset fault determination condition to determine a fault feeder.

[0006] The second aspect of the application provides a high-resistance grounding fault active line selection device, comprising: a fault starting module configured to monitor a zero sequence network parameter of the power distribution network, and start a line selection process when the zero sequence network parameter meets a preset fault starting condition; a switch control module configured to control a switch component electrically connected to a damping resistor in a neutral point grounding loop of the power distribution network to change an access state of the damping resistor in a zero sequence loop; a signal collection module configured to collect bus zero sequence voltages of the power distribution network and zero sequence currents of each feeder before and after the switch component is actuated; a data processing and calculation module configured to calculate zero sequence active power variation of each feeder before and after the switch component is actuated based on the bus zero sequence voltages and the zero sequence currents of each feeder; a fault determination module configured to compare the zero sequence active power variation of each feeder with a preset fault determination condition to determine a fault feeder.

[0007] The third aspect of the application provides a high-resistance grounding fault active line selection system, comprising: a neutral point grounding device comprising an arc-extinguishing coil, a damping resistor and a switch component electrically connected to the damping resistor; and The high-resistance grounding fault active line selection device of the second aspect, wherein the switch control module of the device is connected to the switch component.

[0008] The fourth aspect of the application provides a computer readable storage medium, wherein the computer program is executed by a processor to make the processor execute the steps of the first aspect and any possible implementation manner thereof.

[0009] The technical scheme provided by the application has the following beneficial effects: 1. High sensitivity and high reliability, the change amount of the zero sequence active power caused by the change of the damping resistance state and the injection of the excitation signal is analyzed, the change amount takes into account the changes of the zero sequence voltage and the zero sequence current, the system disturbance can be more comprehensively reflected, the observability of the high resistance grounding fault characteristics is greatly enhanced, and the problems of insufficient sensitivity of the traditional passive method and insufficient stability of the method of only analyzing the change of the current component in voltage fluctuation are solved. 2. Strong anti-interference ability, the line selection criterion is based on the “change amount” before and after the switch action, which can effectively offset the influence of the inherent unbalance degree, background noise and other steady-state interference factors, so that the criterion is cleaner and more reliable. 3. Good robustness, since the criterion is based on the relative difference before and after the change of the system parameters, it is not sensitive to the system operation mode, load fluctuation, line parameter change and the like, and has good universality and adaptability. 4. Simple engineering implementation, the method can directly use the controllable damping resistance module commonly configured in the resonant grounding system, without the need for additional large or expensive hardware devices, and is convenient for upgrading and modification in the existing power grid, and has good economy. BRIEF DESCRIPTION OF DRAWINGS

[0010] In order to more clearly illustrate the technical schemes in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or the prior art description will be briefly introduced as follows. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0011] Among them: Figure 1 A typical power distribution network system topology diagram provided by the embodiment of the present application; Figure 2 A flowchart of the high resistance grounding fault active line selection method provided by the embodiment of the present application; Figure 3 A structure diagram of the high resistance grounding fault active line selection device provided by the embodiment of the present application; Figure 4 A single-phase grounding fault diagram of a series damping resonant grounding system provided by the embodiment of the present application; Figure 5 A diagram showing the relationship between the power frequency zero sequence current and the zero sequence voltage provided by the embodiment of the present application; Figure 6 Another flowchart of the high resistance grounding fault active line selection method provided by the embodiment of the present application; Figure 7 A single-phase grounding fault topology structure diagram of a 35kV resonant grounding system provided by the embodiment of the present application; Figure 8 A damping resistance action before and after the line frequency component change diagram provided by an embodiment of the present application; Figure 9 Another structure schematic diagram of a high-resistance grounding fault active line selection device provided by an embodiment of the present application. DETAILED DESCRIPTION

[0012] In order for those skilled in the art to better understand the scheme of the present application, the technical solutions in the embodiments of the present application will be described clearly and completely below in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0013] The terms “first”, “second”, and the like in the specification and claims of the present application and the above-mentioned drawings are used to distinguish different objects, not to describe a specific order. In addition, the terms “include” and “have” and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or device that includes a series of steps or units is not limited to the listed steps or units, but can optionally include steps or units not listed, or can optionally include other steps or units inherent to the process, method, product, or device.

[0014] In this document, the term “embodiment” means that the specific features, structures, or characteristics described in connection with the embodiment can be included in at least one embodiment of the present application. The appearance of this phrase in various places in the specification does not necessarily mean the same embodiment is referred to, nor does it mean that independent or alternative embodiments are mutually exclusive of other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0015] The 35 kV distribution network involved in the embodiments of the present application significantly increases the ground capacitance current as the cable ratio increases. The traditional ungrounded or small resistance grounding method is prone to arc extinction and overvoltage risk, and the industry has generally shifted to resonant grounding through arc suppression coils. When the system uses a pre-adjusted arc suppression coil with a damping resistor, the zero sequence quantity changes very weakly after single-phase grounding, and is more difficult to observe in the high-resistance grounding scenario, resulting in insufficient sensitivity of conventional methods that rely on steady-state quantities or power direction, and coexistence of miss operation and misjudgment risk.

[0016] At the same time, in order to balance operation and fault handling, thyristors are widely used in the field to quickly switch the damping resistor, which introduces a short-term non-steady-state process and a wide frequency response of near power frequency components in the zero sequence network, which provides a physical basis for constructing a new high-sensitivity criterion.

[0017] The embodiments of this application are described below with reference to the accompanying drawings.

[0018] Figure 1 This is a schematic diagram of a typical distribution network system topology used in the embodiments of this application. This distribution network system can be a medium-voltage distribution network with a neutral point that is not directly grounded, such as a 35kV resonant grounding system. Figure 1 As shown, the system includes a substation busbar and multiple feeders L1 to Ln extending from it. The neutral point of the system is connected to the earth through a neutral point grounding device. Specifically, this neutral point grounding device mainly includes an arc-suppression coil L for compensating for the system's capacitive current, and a damping resistor R connected in series with the arc-suppression coil L. It should be noted that the damping resistor R provides a certain active component when a ground fault occurs in the system, in order to suppress resonant overvoltage and provide a criterion for relay protection. To achieve the active excitation function of this application, a controllable switching component T is connected in parallel across the damping resistor R. As a preferred implementation, the switching component T can be a semiconductor switch with fast response and precise control, such as a pair of anti-parallel thyristors. A single-phase ground fault point Rf is also schematically marked on feeder Ln in the figure. This fault point is connected to the earth through a large transition resistance, constituting a typical high-resistance ground fault scenario.

[0019] The method provided in this application embodiment can be executed by a high-resistance ground fault active line selection device 100. Figure 3This is a block diagram of the internal functional structure of the device 100. The device 100 can be an independent relay protection device or a functional unit integrated into a substation integrated automation system. Logically, the device 100 can be divided into multiple functional modules, specifically including: a signal acquisition module 10, used to acquire the instantaneous value of the bus zero-sequence voltage U0 and the instantaneous value of the zero-sequence current of all feeders in real time through the zero-sequence voltage transformer connected to the bus side and the zero-sequence current transformers on each feeder L1 to Ln, and to provide the acquired analog signals to other modules within the device in the form of a digital sampling sequence after analog-to-digital conversion; a fault initiation module 20, whose core function is to determine whether a ground fault has occurred in the system in order to initiate the subsequent active line selection process. This module continuously receives the bus zero-sequence voltage U0 data from the signal acquisition module 10; and a switch control module 30, used to generate and output control signals to the switch assembly T installed at the neutral point grounding device after receiving the start command from the fault initiation module 20, to control its operation. The data processing and calculation module 40, as the core calculation unit of this technical solution, receives sampled data of the bus zero-sequence voltage U0 and the zero-sequence current of each feeder from the signal acquisition module 10 during two time periods before and after the operation of the switching component T. The fault determination module 50 is used to execute the final line selection decision logic based on the change in zero-sequence active power of each feeder calculated by the data processing and calculation module 40.

[0020] The following will combine Figure 2 The flowchart shown illustrates in detail the working process of the high-resistance grounding fault active line selection device 100 in this embodiment.

[0021] Firstly, the monitoring and starting step 201 is performed. In normal operation of the system, the fault starting module 20 in the high-resistance grounding fault active line selection device 100 continuously monitors the amplitude of the bus zero sequence voltage U0 provided by the signal acquisition module 10. It can be understood that due to factors such as line asymmetry, load imbalance and the like in the power distribution network, a small background zero sequence voltage, i.e. unbalanced voltage, also exists in normal operation. In order to improve the reliability of starting and avoid false starting caused by voltage fluctuations in normal operation, while ensuring the sensitivity to real faults, the fault starting condition in the embodiment adopts an adaptive voltage threshold. Specifically, the device 100 can pre-learn or statistically obtain the average unbalanced voltage U0_normal in normal operation of the system according to historical data. The preset fault starting condition can be set as: when the amplitude of the monitored bus zero sequence voltage U0 continuously exceeds a preset starting threshold Uset, it is determined that a grounding fault may occur in the system. The starting threshold Uset can be adjusted according to the formula Uset = k_rel * U0_normal, k_rel is a reliability coefficient, for example, it can be taken as 1.2 to 1.5, and in the embodiment it can be taken as 1.3. In addition, in order to prevent false judgment caused by transient disturbance (such as voltage surge caused by remote fault or switching operation), the starting condition can also include a small time delay element, for example, it is required that the state of U0 exceeding Uset lasts for more than 10 milliseconds. When the two conditions are met at the same time, the fault starting module 20 confirms that a fault occurs, and immediately issues an internal instruction to start the line selection process to the switch control module 30.

[0022] In the power system, in addition to real grounding faults, some operations or disturbances, such as large motor starting, lightning-induced overvoltage, line switching and the like, can also cause transient fluctuations in the bus zero sequence voltage, and even short-time exceed the starting threshold Uset set in the foregoing embodiment. If only the voltage amplitude and simple time delay are used for starting, the line selection device may be misstarted in the case of non-grounding fault, and unnecessary active excitation operation is performed.

[0023] In an alternative embodiment, in order to solve this problem, the starting logic of the fault starting module 20 is upgraded, and a double criterion based on zero sequence voltage and zero sequence active power is adopted. The structure and process are basically the same as those of the foregoing embodiment, but the algorithm inside the fault starting module 20 is more complex. The specific working process can include: In the monitoring and starting step 201, the fault starting module 20 not only monitors the bus zero sequence voltage U0, but also uses the real-time voltage and current data provided by the signal acquisition module 10 to synchronously and online calculate the total zero sequence active power P0 of the entire system. The total zero sequence active power P0 is the algebraic sum of the zero sequence active powers of all feeders L1 to Ln, i.e. P0 = ΣP_i.

[0024] The composite fault starting criterion in this embodiment contains two conditions, and both of them need to be satisfied simultaneously: Condition a): the amplitude of bus zero sequence voltage U0 exceeds a lower warning value. This warning value can be set lower than Uset in the previous embodiment, for example, set to 5% of the rated phase voltage, which serves to preliminarily determine that the system zero sequence loop has an abnormal disturbance. Condition b): the absolute value of the calculated total zero sequence active power P0 exceeds a small power threshold (for example, 100 watts), and this state lasts for a preset short delay (for example, 5 milliseconds).

[0025] It should be noted that the physical meaning of the above composite criterion is that for a real ground fault, no matter how large the transition resistance is, there will always be a path for energy consumption (i.e., the ground resistance Rf), so the total zero sequence active power P0 of the system will necessarily have a sustained, non-zero value. For transient voltage disturbances caused by non-ground faults, they may appear as voltage spikes or zero crossings, but usually do not accompany sustained active power consumption, and their P0 value is very small or only a transient pulse.

[0026] Therefore, only when conditions a) and b) are satisfied simultaneously, the fault starting module 20 finally confirms that the system has a real ground fault, and starts the subsequent active excitation step 201.

[0027] After confirming the fault, the active excitation step 202 is performed. The switch control module 30 immediately performs the active excitation operation upon receiving the starting instruction. In this embodiment, the switch component T is a pair of anti-parallel thyristors, which are in the off state during normal operation, and the damping resistor R is normally connected in series in the zero sequence loop. The operation of active excitation is that the switch control module 30 sends a strong trigger pulse signal to the gate of the thyristor pair to make it reliably conduct. After the thyristor is turned on, it is equivalent to short-circuiting the two ends of the damping resistor R with a very low impedance path. The effect of this operation is that the damping resistor R is temporarily bypassed from the zero sequence loop, causing a step change in the total damping of the zero sequence loop from R to nearly zero. This step change is equivalent to injecting an effective excitation signal into the entire zero sequence network, which will cause the redistribution of voltage and current in the zero sequence network, thereby exciting a characteristic response in the zero sequence current of each feeder. This operation of changing the state of the damping resistor in the zero sequence loop is the basis for subsequent differential analysis.

[0028] Next, the data acquisition step 203 is performed. In order to accurately capture the system state changes before and after the switching action, the signal acquisition module 10 opens a specific data recording window around the time point when the control instruction is issued by the switch control module 30. For example, the window can cover the time period of 50 milliseconds before the switch component T is turned on and 50 milliseconds after it is turned on. Within this 100-millisecond window, the signal acquisition module 10 synchronously acquires and stores the instantaneous sampling values of the bus zero-sequence voltage U0 and the zero-sequence currents of all feeders Li to Ln at a high sampling rate (e.g., 1000 samples per second). In this way, two key data sets reflecting the steady state (or quasi-steady state) before the switching action and the transient state and new steady state after the switching action, respectively, can be obtained.

[0029] Subsequently, the feature calculation step 204 is performed by the data processing and calculation module 40. The module 40 first divides the acquired data into two groups: "before action" and "after action". For each group of data, the module 40 performs a fast Fourier transform or other harmonic analysis algorithm on the sampling sequence of the bus zero-sequence voltage U0 and the sampling sequence of the zero-sequence current of each feeder i, respectively, to accurately extract the amplitude and phase angle of the power frequency (usually 50 Hz or 60 Hz) component of each electrical quantity.

[0030] The specific calculation process can be as follows: For the data before the action, the power frequency phasor of the bus zero-sequence voltage U0_before and the power frequency phasor of the zero-sequence current of the i-th feeder I_i_before are calculated. Then the zero-sequence active power of the feeder before the action is: P_i_before = |U0_before| * |I_i_before| * cos(φ_i_before) where φ_i_before is the phase angle difference between U0_before and I_i_before.

[0031] Similarly, for the data after the action, the power frequency phasor of the bus zero-sequence voltage U0_after and the power frequency phasor of the zero-sequence current of the i-th feeder I_i_after are calculated. Then the zero-sequence active power of the feeder after the action is: P_i_after = |U0_after| * |I_i_after| * cos(φ_i_after) where φ_i_after is the phase angle difference between U0_after and I_i_after.

[0032] After obtaining the zero sequence active power P_i_before and P_i_after of each feeder i before and after the action, the data processing and calculation module 40 further calculates the zero sequence active power variation ΔP_i of the feeder, and the calculation formula is: ΔP_i = |P_i_after - P_i_before| The absolute value of the variation can reflect the intensity of the active incentive operation on the active power disturbance of the feeder. After performing the above calculation on all feeders L1 to Ln, the module 40 will obtain an array containing the zero sequence active power variations of all feeders {ΔP_1, ΔP_2,..., ΔP_n}, and pass it to the fault discrimination module 50.

[0033] Finally, in the line selection discrimination step 205, the fault discrimination module 50 receives the zero sequence active power variation array of each feeder, compares it with the preset fault determination condition, and finally determines the fault feeder.

[0034] When a ground fault occurs, the bus zero sequence voltage U0 is generated. For a healthy feeder Ii, its zero sequence current is mainly the capacitive current generated by the line-to-ground capacitance, so its current phasor Ii is about 90 degrees ahead of the voltage phasor U0 in phase, and its active power component is very small. For the fault feeder Ik, its zero sequence current not only contains the capacitive current, but also contains the resistive current component flowing through the ground fault point Rf, so the angle between its total current phasor Ik and the voltage phasor U0 will be significantly smaller than 90 degrees, showing a significant active power component. When the system damping is changed by the action of the switching assembly T, the parameters of the zero sequence loop change, and the resistive current component on the fault feeder changes significantly, thereby causing the zero sequence active power of the fault feeder to also change significantly. In contrast, the zero sequence active power of the healthy feeder changes very little during this process.

[0035] Based on this principle, the embodiment can use a differential comparison fault determination condition. Specifically, the fault discrimination module 50 first finds the maximum value in the array {ΔP_1, ΔP_2,..., ΔP_n}. Assuming that the zero sequence active power variation ΔP_k of the kth feeder is the largest among all feeders. Then, the module 50 compares ΔP_k with the variations of all other healthy feeders. As a preferred criterion, if the zero sequence active power variation ΔP_k of the feeder k is greater than the product of the maximum value of the zero sequence active power variations of all other feeders (i≠k) and a preset reliability coefficient (for example, 1.2), that is, it satisfies the condition: ΔP_k>1.2 * max(ΔP_i) (i≠k), then the kth feeder is finally determined as the fault feeder. Once the fault feeder is determined, the device 100 will output the corresponding signal, such as sending a trip command to the circuit breaker of the feeder to remove the fault, or sending an alarm information to the monitoring system.

[0036] In an alternative embodiment, a cross-verification mechanism based on multi-dimensional information can also be introduced in the fault discrimination module 50. That is, on the basis of the main criterion, an auxiliary criterion is added to confirm the result of the main criterion.

[0037] Specifically, in the line selection discrimination step 205, the fault discrimination module 50 can execute the following decision logic of the combination of the main and auxiliary criteria: Main criterion: As in the foregoing embodiment, first find the maximum value in the zero sequence active power variation of all feeders, and determine the corresponding feeder k (i.e., ΔP_kmax) as the "suspected fault line" as the first step of screening.

[0038] Auxiliary criterion (cross-verification): After the suspected fault line k is determined, the fault discrimination module 50 calls the intermediate calculation results of the data processing and calculation module 40, i.e., the zero sequence current phasor I_k_after and the bus zero sequence voltage phasor U0_after of the suspected fault line k after the switching component T acts. Then, the phase angle difference φ_k between the two is calculated.

[0039] A reasonable phase angle range can be set as the criterion. For example, the auxiliary criterion can be set as: if the phase angle difference φ_k of the suspected fault line k satisfies -70°< φ_k< 70°, the auxiliary criterion passes, and it is considered that the line has obvious resistive characteristics.

[0040] Final decision: Only when a feeder k satisfies both the main criterion (its ΔP_k is the maximum, and satisfies the differential comparison condition) and the auxiliary criterion (its phase angle difference φ_k falls within the preset resistive characteristic range), the fault discrimination module 50 finally determines it as the fault line, and issues a trip or alarm instruction. If the suspected line selected by the main criterion fails to pass the verification of the auxiliary criterion, the device can determine that it is not faulty, disturbed, or the criterion is unreliable, and lock the output, waiting for the next judgment or issuing an "abnormal criterion" alarm.

[0041] By introducing the phase angle characteristic as the cross-verification, the embodiment double confirms the fault from two different physical dimensions of "dynamic variation amplitude" (ΔP) and "steady-state phase angle characteristic" (φ). This method enhances the anti-interference ability of the criterion, can effectively avoid the misjudgment caused by accidental jump of a single data point, and makes the line selection result have a very high confidence level under various difficult and adverse working conditions.

[0042] In the foregoing embodiment, the active excitation is realized by short-circuiting a single damping resistor R. The application also provides a scheme of changing the total damping resistance value, which can also achieve the purpose of generating a step disturbance in the zero sequence network.

[0043] In this embodiment, the system topology of the distribution network, the overall structure of the high-resistance ground fault active fault location device 100, and the other method steps except for the active excitation step 202 can all be kept consistent with the aforementioned embodiments. The difference lies in the specific construction of the neutral point grounding device and the control logic of the switch control module 30.

[0044] Specifically, instead of a single damping resistor, the neutral point grounding loop uses two resistors with different values: a first resistor R1 and a second resistor R2. The first resistor R1 is directly connected in series in the zero-sequence loop. The second resistor R2 is connected in parallel with it through a switching assembly T. This switching assembly T can be a thyristor as in this embodiment, or it can be another type of high-speed semiconductor switch, such as an insulated-gate bipolar transistor. The control terminal of the switching assembly T is connected to the switch control module 30 of the line selection device 100.

[0045] Figure 4 This is a schematic diagram of a single-phase grounding fault in a series damped resonant grounding system provided in an embodiment of this application.

[0046] like Figure 4 As shown, specifically, the system parameters are consistent with those of the parallel damped resonant grounding system, where T is the anti-parallel thyristor controlling the switching of the damping resistor. When no fault occurs in the system, the damping resistor operates in series with the arc suppression coil; when a single-phase ground fault occurs, the thyristor is triggered to conduct, forming a short-circuit bypass next to the damping resistor, and the current no longer flows through the damping resistor. Therefore, the damping resistor is still connected in series with the arc suppression coil.

[0047] When a single-phase ground fault occurs in the system, the thyristor T is triggered to short-circuit the damping resistor, enabling the arc suppression coil to achieve full or near-full compensation for the ground fault current. The resonant grounding device consists of an arc suppression coil and a damping resistor, which is controlled by rapid switching of the thyristor. When the thyristor is turned on, the zero-sequence circuit of the system is short-circuited, and the compensation current of the arc suppression coil exhibits a non-sinusoidal periodic function. Fourier decomposition of this current reveals that its dominant component is the power frequency fundamental component. The fundamental component of the compensation current can be obtained as:

[0048] In the formula, The phase angle of the fundamental current. This represents the effective value of the fundamental current.

[0049] Fundamental current RMS value It can be represented as:

[0050] After a ground fault occurs, the series damping resistor can be considered as being short-circuited. Therefore, the zero-sequence voltage of the bus and the fault current can be expressed as:

[0051] is the power frequency capacitive current of feeder i, is the power frequency zero sequence current of feeder i, and the bus zero sequence voltage only retains the power frequency component:

[0052] The above equations can be combined to obtain a single second-order non-homogeneous differential equation:

[0053] To characterize the compensation and damping effects at power frequency, the definitions of power frequency off-tune degree and damping rate are used:

[0054] The measurement and processing unit collects the bus zero sequence voltage and each feeder zero sequence current in real time, performs power frequency component extraction, power calculation, and criterion decision-making.

[0055] In a resonant grounding system with a series damping resistor, the presence of the damping resistor suppresses the rise of the neutral point voltage after a grounding fault, and especially in high resistance grounding faults, the traditional zero sequence voltage starting method is difficult to accurately detect the fault. Therefore, the adaptive threshold method based on the neutral point offset voltage during normal operation of the system is proposed. By monitoring the neutral point offset voltage caused by asymmetric operation during normal operation of the system, an adaptive threshold value that can avoid normal fluctuations is set to improve the detection sensitivity of high resistance grounding faults. The zero sequence voltage starting threshold should be between the normal offset voltage and the fault zero sequence voltage amplitude:

[0056] In the formula, k is the effective value of the three-phase asymmetry degree of the system.

[0057] When the threshold value is too low, although the detection sensitivity is high, it is easy to be disturbed by disturbances; otherwise, the sensitivity is reduced. In order to consider the sensitivity and reliability, the reliability coefficient is introduced:

[0058] In the formula, is the reliability coefficient. Generally, .

[0059] In a certain distribution network, the system-to-ground capacitance, damping rate and off-tune rate parameters remain basically unchanged before and after the fault, therefore, the selection of the threshold directly affects the high resistance detection capability. The relationship between the threshold value and the expected value of the high resistance fault detection transition resistance is: R THR ​

[0060] When the expected value is higher, the threshold value is smaller, and the system is more sensitive to interference. Therefore, the present application takes While ensuring sensitivity and retaining a certain redundancy, the final zero sequence voltage threshold can be expressed as:

[0061] The power frequency signals are independent of each other in the system, so the active and reactive power calculations can be performed separately. Define the power frequency steady-state zero sequence active and reactive power of feeder i as:

[0062] In the formula, is the zero sequence fundamental active power of feeder i, is the zero sequence fundamental reactive power of feeder i, is the power frequency zero sequence voltage amplitude coefficient, is the zero sequence current amplitude coefficient of feeder i, is the phase angle difference between the two.

[0063] Due to the introduction of the active component by the series damping resistance, the zero sequence current phasor of the fault feeder tends to direction. The greater the damping resistance, the closer the zero sequence current phase angle is to After detecting the fault, the thyristor is actuated to exit the damping resistance, and the phase of the zero sequence current of the fault feeder is further closer to the imaginary axis. Figure 5 A power frequency zero sequence current and zero sequence voltage relationship diagram is provided.

[0064] Therefore, the power frequency zero sequence voltage of the system after single-phase ground fault can be expressed as:

[0065] The power frequency zero sequence currents of the healthy feeder i and the fault feeder k are The expressions are respectively:

[0066]

[0067] From the above formula, it can be seen that the fault feeder and the non-fault feeder have significant differences in amplitude and phase angle, the amplitude coefficient is large and the phase angle is not in the same quadrant. Due to the change of the system damping rate after the thyristor exits the damping resistance, the damping rates before and after the damping exit are respectively:

[0068] After the damping resistance exits, the power frequency zero sequence active component of the fault feeder decreases. The active power of each feeder before exiting is:

[0069] The corresponding power change after exiting is:

[0070] Therefore, the zero sequence active power change of the fault feeder before and after the damping resistor exits is much larger than that of the healthy feeder. Based on this, a single-phase grounding protection line selection criterion is proposed:

[0071] In the formula: is the maximum zero sequence power change of the non-fault feeder. In order to leave a certain margin when selecting the grounding fault line, is set to is the reliability coefficient of the grounding fault line selection criterion, which can be taken as 1.2 in this application.

[0072] The protection logic and execution unit outputs the line selection result and issues a trip command to remove the fault feeder. The neutral point offset voltage is tracked during normal system operation, and an adaptive zero sequence voltage threshold is calculated. This threshold setting takes into account sensitivity and redundancy, which can significantly improve the starting ability of high resistance faults. After detecting a fault, the thyristor shorts the damping resistor, allowing the system to form a characteristic wide frequency response in a short time, while continuously measuring the bus zero sequence voltage and each feeder zero sequence current. The zero sequence active power and reactive power of the two time periods before and after the damping resistor exits are calculated by calculating the power frequency components of each measurement. The zero sequence active power change is used as the main criterion. The zero sequence active power change of the fault feeder before and after the damping resistor exits is significantly larger than that of the healthy feeder. By setting the reliability coefficient and the comparison threshold of the maximum non-fault change in the entire network, robust line selection is achieved. It can be supplemented by cross-checking the reactive power polarity or the difference in the phase angle quadrant of the voltage and current to improve the anti-interference performance. When the fault feeder is determined, the trip removes the fault line. The thyristor restores the cutoff and puts in the damping resistor after the fault disappears, limiting the overvoltage.

[0073] Figure 6 Another process flow diagram of the active line selection method for high resistance grounding faults provided by the embodiment of the application.

[0074] To ensure sensitive detection of faults and accurate selection of fault feeders, for the 35kV medium voltage distribution system transformed into a composite grounding operation mode of resonance grounding and arc extinction device, the overall process of the grounding fault protection designed based on the wide frequency zero sequence signal is as shown in Figure 6 The specific execution steps include: Step 1: Monitor and record the neutral point unbalanced voltage and each feeder zero sequence current during normal system operation, and calculate the adaptive sensitive fault detection threshold according to the neutral point unbalanced voltage. When the system is detected to have a fault, the protection is started.

[0075] Step 2: After the single-phase grounding fault occurs, the thyristor short-circuits the damping resistor, and the neutral point voltage and the zero-sequence current of each feeder in the power distribution network are continuously monitored, and the power frequency signals of the neutral point voltage and the zero-sequence current of each feeder are decomposed.

[0076] Step 3: According to the decomposed power frequency signals of the zero-sequence voltage and the zero-sequence current of each feeder, the power frequency active power characteristic quantity before and after the damping resistor is withdrawn is calculated.

[0077] Step 4: The fault feeder is determined. The power frequency active power before and after the damping resistor is withdrawn is calculated. If the difference between the power frequency active power before and after the damping resistor is withdrawn is greater than a set threshold, the fault feeder is determined, otherwise, the healthy feeder is determined. Finally, the line selection is completed and a trip command is given to remove the fault feeder, thereby realizing the grounding fault line selection.

[0078] The method provided by the application is particularly suitable for a 35kV power distribution network operating in a resonance grounding system (arc suppression coil + series damping), and uses the thyristor switching damping resistor to generate a controllable transient excitation for active line selection.

[0079] In some existing schemes, the damping resistor is not used as an "excitation source", but is used for: partition judgment, providing resistive current for high-resistance grounding to enhance the difference in zero-sequence current, judging transient or permanent fault, etc. The resistance change does not build a line selection quantity through controllable active excitation.

[0080] The innovation of the application lies in that the thyristor is used for actively exciting the zero-sequence network, specifically including: short-circuiting the damping resistor at the time of fault, introducing a primary active disturbance (controllable transient) by forcibly changing the zero-sequence loop damping rate; using the "self-excited disturbance" to form an observable wide-frequency response; the line selection quantity is not simply the zero-sequence amplitude, but the zero-sequence "active power change quantity" before and after the damping resistor is withdrawn; which is essentially to amplify the equivalent impedance difference of the grounding fault loop; That is, the application creatively regards the thyristor as an active excitation switch, rather than simply switching the compensation resistor, to realize the idea of "actively applying controllable excitation -> observing system response -> building power difference criterion".

[0081] In addition, the application proposes an active line selection system based on "power difference division", which uses the zero-sequence active power change ΔP or the zero-sequence reactive power change ΔQ (auxiliary criterion) before and after the damping resistor is withdrawn. This is essentially the "energy difference between the two steady states before and after excitation". The physical mechanism is that the damping resistor makes the zero-sequence current of the fault feeder "close to the real axis", and after the damping is withdrawn, the phase is "close to the imaginary axis". The difference between this double-state response in the fault line and the healthy line is very large, so Become strong criterion.

[0082] Simulation verification based on the foregoing method is given below.

[0083] Figure 7 A 35kV resonance grounding system single-phase grounding fault topology structure diagram is provided for the embodiments of the application.

[0084] In order to verify the correctness and effectiveness of the proposed 35kV distribution network high resistance grounding fault active line selection method based on control switch transient spectrum, a complete simulation model is established in the power system simulation software PSCAD / EMTDC as shown in Figure 7 , wherein the feeder simulation parameters are shown in Table 1. The model can truly reflect the electrical characteristics and fault conditions of the 35kV resonance grounding system.

[0085]

[0086] Table 1 In the simulation system, the neutral point is grounded through a pre-adjusted arc suppression coil and a damping resistance, and the damping resistance is controlled by anti-parallel thyristors, and the on and off characteristics of the on-site device are completely simulated. The bus is provided with a zero sequence voltage detection point, and each feeder is connected to a zero sequence current measurement unit, which constitutes the signal acquisition structure of the active line selection system.

[0087] The simulation conditions include multiple groups of transition resistance values, from 100Ω to 10kΩ, to simulate high resistance grounding faults of different intensities. After the system is in normal operation, the thyristor is triggered to turn on at the set time, so that the damping resistance is put into operation, thereby forming a clear transient broadband response in the zero sequence network. At this time, the bus zero sequence voltage and each feeder zero sequence current change significantly, and after filtering and fundamental component extraction, the zero sequence active power and reactive power change of each feeder before and after switching are calculated.

[0088] Figure 8 A graph of the change of the power frequency component of the feeder before and after the action of the damping resistance is provided for the embodiments of the application.

[0089] The feeder L3 has a 100Ω grounding fault at 0.1s, and the damping resistance is withdrawn at 0.2s, and the amplitude of the zero sequence current of each feeder and the zero sequence voltage of the neutral point is as shown in Figure 8 a), and the phase angle difference of the zero sequence current of each feeder and the zero sequence of the neutral point is as shown in Figure 8 b). The fault characteristics can be used to accurately identify the grounding fault feeder, and the power frequency active power difference of each feeder before and after the damping resistance is withdrawn and the fault feeder determination result under different grounding fault conditions are shown in Table 2.

[0090]

[0091] Table 2 The simulation results show that: (1) In the range of 100 Ω to 10 kΩ of the transition resistance, the method can accurately identify the fault feeder, and the line selection accuracy reaches 100%.

[0092] (2) The zero sequence active power variation of the fault feeder is obviously larger than that of other feeders, and the difference coefficient exceeds 8 times, and the criterion characteristics are clear.

[0093] (3) The transient duration is short, the bus voltage fluctuation amplitude is small, and it has almost no effect on the stable operation of the system.

[0094] In addition, through sensitivity analysis of various conditions such as changing system load, cable length and arc suppression coil parameters, the consistent verification results show that the method is not sensitive to changes in system structure and operating parameters, and has good adaptability and robustness.

[0095] In summary, the simulation results fully prove that the method has high sensitivity, high accuracy and engineering feasibility in the 35 kV resonant grounding system, and can effectively realize the active line selection of high resistance grounding fault. Figure 9 A structure diagram of a high resistance grounding fault active line selection device provided for an embodiment of the application. As shown in Figure 9 , the high resistance grounding fault active line selection device 900 includes: A fault starting module 910 for monitoring the zero sequence network parameters of the distribution network, and starting the line selection process when the zero sequence network parameters meet the preset fault starting conditions; A switch control module 920 for controlling the action of a switch component electrically connected to a damping resistor in the neutral point grounding loop of the distribution network to change the access state of the damping resistor in the zero sequence loop; A signal acquisition module 930 for acquiring the bus zero sequence voltage of the distribution network and the zero sequence current of each feeder before and after the action of the switch component; A data processing and calculation module 940 for calculating the zero sequence active power variation of each feeder before and after the action of the switch component based on the bus zero sequence voltage and the zero sequence current of each feeder; A fault discrimination module 950 for comparing the zero sequence active power variation of each feeder with the preset fault determination condition to determine the fault feeder.

[0096] It can be understood that the related content of each module in Figure 9 has been described in detail in the foregoing method embodiments, and the specific content can be referred to in the method embodiments; that is, Figure 9 The high resistance grounding fault active line selection device 900 provided in the method embodiments can perform any step in the embodiments shown in Figure 2 or Figure 6 , which will not be described here.

[0097] In one embodiment, a high-resistance ground fault active line selection system is also provided, comprising: a neutral grounding device, the neutral grounding device comprising an arc suppression coil, a damping resistor, and a switch assembly electrically connected to the damping resistor; and A high-resistance ground fault active line selection device is shown in Figure 9 The switch control module of the device is connected to the switch assembly.

[0098] The system can implement Figure 2 or Figure 6 Any step in the embodiments shown, which will not be described here.

[0099] In one embodiment, a computer-readable storage medium is also provided, which stores a computer program, the computer program being executed by a processor to cause the processor to perform any step in the above method embodiments.

[0100] A person of ordinary skill in the art can understand that all or part of the processes in the above-mentioned embodiments can be completed by a computer program instructing related hardware, and the program can be stored in a non-volatile computer-readable storage medium. When the program is executed, it can include the processes of the above-mentioned embodiments. Any reference to memory, storage, database or other medium used in the embodiments provided by the present application can include non-volatile and / or volatile memory. Non-volatile memory can include read-only memory (ROM), programmable ROM (PROM), electrically programmable ROM (EPROM), electrically erasable programmable ROM (EEPROM) or flash memory. Volatile memory can include random access memory (RAM) or external cache memory. As an illustration but not limitation, RAM is available in various forms, such as static RAM (SRAM), dynamic RAM (DRAM), synchronous DRAM (SDRAM), double data rate SDRAM (DDR SDRAM), enhanced SDRAM (ESDRAM), synchronous link (Synchlink) DRAM (SLDRAM), memory bus (Rambus) direct RAM (RDRAM), direct memory bus dynamic RAM (DRDRAM), and memory bus dynamic RAM (RDRAM), etc.

[0101] Each technical feature of the above embodiments can be combined arbitrarily. In order to make the description concise, all possible combinations of each technical feature in the above embodiments are not described, however, as long as the combination of technical features does not exist, it should be considered as the scope of the present disclosure.

[0102] The above-described embodiments are merely illustrative of several embodiments of the present application, which are described in more detail and in a specific and detailed manner, but should not be construed as limiting the scope of the patent of the present application. It should be noted that, for those of ordinary skill in the art, several modifications and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application should be subject to the appended claims.

Claims

1. A method for active line selection in the event of a high-resistance grounding fault, characterized in that, The method, applicable to distribution networks with a neutral point grounded via an arc-suppression coil and a damping resistor, includes: Monitor the zero-sequence network parameters of the distribution network, and initiate the line selection process when the zero-sequence network parameters meet the preset fault initiation conditions; The switching assembly electrically connected to the damping resistor in the neutral grounding circuit of the power distribution network is controlled to change the connection state of the damping resistor in the zero-sequence circuit. Collect the zero-sequence voltage of the bus and the zero-sequence current of each feeder of the distribution network before and after the operation of the switching assembly; Based on the zero-sequence voltage of the bus and the zero-sequence current of each feeder, the change in zero-sequence active power of each feeder before and after the operation of the switching assembly is calculated. The zero-sequence active power change of each feeder is compared with the preset fault determination conditions to identify the faulty feeder.

2. The active line selection method for high-resistance grounding faults according to claim 1, characterized in that, The preset fault initiation conditions include: The zero-sequence voltage of the distribution network bus exceeds a preset voltage threshold.

3. The active line selection method for high-resistance grounding faults according to claim 1, characterized in that, The switching assembly is a semiconductor switch; The change in the connection state of the damping resistor in the zero-sequence loop includes: The semiconductor switch is controlled to close to short-circuit the damping resistor.

4. The active line selection method for high-resistance grounding faults according to claim 1, characterized in that, The neutral point grounding circuit is provided with two resistors of different resistance values, and the switching assembly is connected to one of the resistors. The change in the connection state of the damping resistor in the zero-sequence loop includes: Controlling the switching assembly to change the total damping resistance value connected to the zero-sequence circuit.

5. The active line selection method for high-resistance grounding faults according to claim 1, characterized in that, The preset fault determination conditions are as follows: If the change in zero-sequence active power of a certain feeder is greater than the product of the maximum value of the change in zero-sequence active power of all other feeders and the preset reliability coefficient, then the feeder is determined to be a faulty feeder.

6. The active line selection method for high-resistance grounding faults according to claim 2, characterized in that, The preset fault initiation conditions also include: The total zero-sequence active power of the system exceeds the preset power threshold.

7. The active line selection method for high-resistance grounding faults according to claim 1, characterized in that, The method further includes: The feeder with the largest change in zero-sequence active power is subjected to phase angle verification. When the phase angle difference between its zero-sequence current and the zero-sequence voltage of the bus meets the preset resistive characteristic range, the feeder is determined to be a faulty feeder.

8. A high-resistance grounding fault active fault location device, characterized in that, include: The fault initiation module is used to monitor the zero-sequence network parameters of the distribution network. When the zero-sequence network parameters meet the preset fault initiation conditions, the line selection process is initiated. A switch control module is used to control the operation of a switch assembly electrically connected to a damping resistor in the neutral grounding circuit of the power distribution network, so as to change the connection state of the damping resistor in the zero-sequence circuit. The signal acquisition module is used to acquire the bus zero-sequence voltage and the zero-sequence current of each feeder of the distribution network before and after the operation of the switching component. The data processing and calculation module is used to calculate the change in zero-sequence active power of each feeder before and after the operation of the switching assembly, based on the zero-sequence voltage of the bus and the zero-sequence current of each feeder. The fault detection module is used to compare the change in zero-sequence active power of each feeder with preset fault detection conditions to identify the faulty feeder.

9. The high-resistance grounding fault active line selection device according to claim 8, characterized in that, The fault initiation module is specifically used for: Monitor the zero-sequence voltage of the bus in the distribution network. When the zero-sequence voltage of the bus exceeds a preset voltage threshold, determine that the preset fault initiation condition is met.

10. A high-resistance grounding fault active fault location system, characterized in that, include: A neutral point grounding device, the neutral point grounding device including an arc suppression coil, a damping resistor and a switching assembly electrically connected to the damping resistor; as well as The high-resistance grounding fault active line selection device as described in claim 8 or 9, wherein the switch control module of the device is connected to the switch assembly.

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