Underground power distribution network single-phase grounding protection system and method based on signal injection method

Abstract

The invention relates to the technical field of power system protection, in particular to an underground power distribution network single-phase grounding protection system and method based on a signal injection method. The system comprises a signal acquisition module, a measurement and control protection module and an online monitoring module. According to the invention, by injecting the independent direct current signal and combining a zero-sequence fundamental wave time sequence identification algorithm and a traveling wave distance measurement method, rapid detection, high-precision positioning and selective protection of the single-phase earth fault of the underground low-voltage power distribution network are realized, and the method is suitable for complex environments such as coal mines and metal mines; through insulation value on-line monitoring and electric leakage protection, the ground insulation value level of a line is reflected to the voltage change of an additional direct current signal and a reactor neutral point. A zero-sequence fundamental wave time sequence identification model is adopted for electric leakage fault judgment so as to judge the fault type. By outputting a relay signal, on-off control is performed on a protected branch, so that a fault line can be quickly cut off when a fault occurs, and equipment and a bus are protected from being damaged by the fault.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power system protection, and particularly relates to an underground power distribution network single-phase grounding protection system and method based on an injection signal method. BACKGROUND

[0002] In China, 10Kv power distribution networks usually use neutral point resonance grounding. Under this grounding mode, short-circuit faults become one of the main reasons for power distribution network faults, and 80% of the faults are caused by single-phase grounding. When a single-phase grounding fault occurs in a line, the voltage of the non-fault phase will be much higher than that during normal operation due to the weak zero-sequence current of the fault phase, and the line voltage between the three phases will remain symmetrical. Therefore, in order to ensure the safety of the system, it is stipulated that the neutral point resonance grounding mode can continue to operate for 1-2h in the fault state. However, if the fault is not timely eliminated, the system will run at high voltage for a long time, which will easily cause equipment breakdown and cause more serious faults in a larger range. Therefore, when a single-phase grounding fault occurs in a power distribution network, we should timely identify the fault feeder and cut off the fault feeder to ensure the normal operation of the line and provide help for the repair of the fault line.

[0003] Nowadays, coal mines are mostly mechanized and highly automated modern coal mines, which leads to an increase in the power consumption of coal mines. Therefore, the safety of the power supply system is increasingly important for the production of coal mines. In the underground 10Kv low-voltage power distribution network, the environment is harsh, dark, and humid. During the operation of the cable, partial discharge may occur due to mechanical damage, operating overvoltage, and the gradual penetration of moisture, which will cause the insulation resistance to decrease and age. Leakage fault is the most common fault type, accounting for more than 70% of all faults. The related fault characteristics of leakage faults are not obvious enough in actual working conditions, and the research difficulty coefficient is high, which is easy to cause safety accidents. Therefore, the leakage protection system is very important for the underground power supply protection. When a fault occurs, cutting off the fault branch can not only reduce the power outage range during the fault, but also provide convenience for further finding and processing the fault point.

[0004] ​In the current actual project, the topology structure of 10Kv distribution network is complex, there are many branch lines, and the neutral point resonance grounding mode is adopted. When single-phase grounding fault occurs, the fault current is very small, and the fault characteristics are weak. Therefore, when the 10Kv distribution network line fails, the main working direction is to select the fault feeder line in the complex branch structure and locate it, and improve the inspection efficiency of the power grid workers. In the underground 10Kv mine distribution network, the decrease of cable insulation resistance can cause leakage current, which can not only cause electric shock, but also form single-phase grounding, and further develop into inter-phase short circuit. The fault information of leakage is extremely small, and due to environmental interference, the fault signal is easy to be submerged by noise. Therefore, in the underground 10Kv mine cable protection, the main is leakage protection, which monitors the insulation parameters in real time, and when the insulation parameters rise to a certain threshold, the leakage cable is cut off, and the fault cable is selected to protect the safety of workers and the property safety of enterprises.

[0005] The Chinese patent document with publication number CN117175513B discloses a kind of active distribution network single-phase grounding protection method and system, wherein the method includes: distribution terminal real-time acquisition line voltage and current data, and calculates the active power of line;Record and store the active power of line, form active power curve;According to active power curve statistical data;Based on statistical data, calculate direction compensation coefficient and zero sequence current compensation coefficient;Detect whether single-phase grounding fault occurs in line, when single-phase grounding fault occurs in line, through single-phase grounding protection action direction compensation coefficient, transient zero sequence direction and steady-state zero sequence direction are compensated, or, through zero sequence current compensation coefficient, zero sequence current is compensated, when compensation result meets action condition, single-phase grounding fault is judged, tripping or alarm is carried out;From this, it can be seen that the existing single-phase grounding protection technology has deficiencies in reliability, selectivity, sensitivity or applicability, especially in complex underground environment with large capacitive current, harsh environment and high safety requirements. SUMMARY

[0006] Therefore, the present application provides an underground distribution network single-phase grounding protection system and method based on injection signal method to overcome the problem of low positioning accuracy of underground distribution network fault point due to lack of multi-modal fusion processing in the prior art.

[0007] To achieve the above-mentioned purpose, on the one hand, the present application provides an underground distribution network single-phase grounding protection system based on injection signal method, comprising,

[0008] The signal acquisition module is used for acquiring the insulation parameters of the underground distribution network, including insulation resistance value;

[0009] The measurement and control protection module is connected with the signal acquisition module, is used for judging single-phase grounding fault type based on the insulation resistance value and zero sequence fundamental wave timing discrimination model, positioning fault point based on traveling wave distance measurement method, and outputting control instruction including leakage protection control instruction;

[0010] Wherein, the independent DC power supply is injected between the three-phase power grid and the ground to form a loop, the loop current change is monitored to calculate the power grid ground insulation resistance, and the fault type is determined based on the zero sequence fundamental wave timing discrimination model;

[0011] The online monitoring module is connected with the measurement and control protection module through a communication interface, is used for real-time display of power grid operation state, storage of data and remote management.

[0012] Further, the measurement and control protection module includes a fault type monitoring unit, a fault point positioning unit and a leakage protection unit, wherein,

[0013] The fault type monitoring unit is used to determine the phase relationship between zero sequence voltage and zero sequence current according to the zero sequence fundamental wave timing discrimination model when the insulation resistance value is less than the resistance value set threshold, so as to judge the single-phase grounding fault type;

[0014] The fault point positioning unit performs traveling wave signal decoupling and feature extraction based on the Kelvin-Bell transformation, and determines the fault point position based on the zero modulus wave speed dynamic compensation factor;

[0015] The leakage protection unit is used to trigger the leakage protection control instruction based on the single-phase grounding fault current effective value.

[0016] Further, the fault type monitoring unit includes a cooperative signal processing subunit and a dynamic correction threshold subunit, wherein,

[0017] The cooperative signal processing subunit is used to synchronously acquire the fundamental wave components of power grid zero sequence voltage and zero sequence current, and extract the modulus maximum value features of fault transient signals through wavelet transform;

[0018] The dynamic correction threshold subunit is used to dynamically correct the phase difference threshold range of zero sequence voltage and zero sequence current.

[0019] Further, the dynamic correction threshold subunit dynamically corrects the phase difference threshold range of zero sequence voltage and zero sequence current, including,

[0020] According to the independent DC power supply to form a monitoring loop;

[0021] The injected current amplitude and the injected current change rate in the loop are acquired in real time;

[0022] The fault type is determined based on the injected current change rate and the phase difference between the zero sequence voltage and the zero sequence current, including low resistance grounding fault and intermittent grounding fault.

[0023] Further, the monitoring loop formed by the independent DC power supply comprises,

[0024] Injecting the independent DC power supply of the preset frequency between the neutral point of the three-phase power grid and the ground to form a monitoring loop;

[0025] Wherein, the DC power supply is connected between the neutral point of the three-phase reactor and the ground;

[0026] The ground insulation resistance of the power grid is inversely calculated by the current value of the monitoring loop;

[0027] When the insulation resistance is lower than the preset threshold, a relay is triggered to cut off the fault line.

[0028] Further, the fault point positioning unit comprises a feature extraction subunit and a wave speed compensation factor adjustment subunit, wherein,

[0029] The feature extraction subunit separates the line modulus and the zero modulus by the Kramers-Kronig transformation, extracts the wave head mutation characteristics by the wavelet transform, and records the time stamps corresponding to the line modulus and the zero modulus respectively;

[0030] The wave speed compensation factor adjustment subunit calculates the distance of the fault point from the M end, corrects the zero modulus wave speed according to the zero modulus wave speed compensation factor, and corrects the value range of the wave speed compensation factor according to the fault type.

[0031] Further, the feature extraction subunit records the time stamps corresponding to the line modulus and the zero modulus respectively, performs cross-modal correlation verification on the wave head time stamps of the line modulus and the zero modulus, and determines whether the current traveling wave is an effective fault traveling wave.

[0032] Further, the wave speed compensation factor adjustment subunit corrects the value range of the wave speed compensation factor according to the fault type, which comprises,

[0033] According to the wave head polarity and the wavelet energy distribution, the metallic grounding fault and the high resistance grounding fault are distinguished;

[0034] The value range of the wave speed compensation factor is corrected according to the fault type.

[0035] Further, the single-phase grounding fault current effective value calculation formula is,

[0036]

[0037] Wherein, Ijd is the single-phase grounding fault current effective value trigger value, Uo is the zero sequence voltage, Rg is the grounding resistance, w is the angular frequency, C is the ground capacitance, and L is the reactor inductance value.

[0038] In another aspect, the application also provides a downhole power distribution network single-phase grounding protection method based on an injection signal method, which is applied to the downhole power distribution network single-phase grounding protection system based on the injection signal method and comprises,

[0039] In step S1, an independent DC power supply is injected between a neutral point of a three-phase power grid and the ground to form a monitoring loop, and a loop current signal is collected in real time, and a ground insulation resistance value of the power grid is calculated reversely;

[0040] In step S2, when the insulation resistance value is lower than a set threshold, a fault discrimination mechanism is started, including zero sequence fundamental wave timing discrimination and traveling wave distance positioning, to judge the fault type and locate the fault point, and a fault branch identification is generated;

[0041] In step S3, a communication interface is connected with the measurement and control protection module, which is used for real-time display of power grid operation state, storage of data and remote management;

[0042] The remote management includes control of the corresponding relay to cut off the fault line, and uploading of fault information to a monitoring platform.

[0043] Compared with the prior art, the application has the beneficial effects that, by injecting an independent DC signal in combination with a zero sequence fundamental wave timing discrimination algorithm and a traveling wave distance positioning method, rapid detection, high-precision positioning and selective protection of single-phase grounding faults of a downhole low-voltage power distribution network are realized, and the application is suitable for complex environments such as coal mines and metal mines; by using an additional DC power supply method in combination with a wavelet transform, the detection precision is improved, and the application has high scalability due to support of parallel monitoring of multiple branch lines.

[0044] Further, insulation value online monitoring and leakage protection are realized to realize the reaction of the voltage change of the additional DC signal and the neutral point of the reactor to the ground insulation value level of the line; a zero sequence fundamental wave timing discrimination model is used for leakage fault discrimination to determine the fault type according to the phase of the zero sequence voltage and zero sequence current signal waveforms. A relay signal is output to control the on-off of the protected branch line, so that the fault line is quickly cut off when a fault occurs, and the equipment and bus are protected from damage caused by the fault. BRIEF DESCRIPTION OF DRAWINGS

[0045] Figure 1 FIG. 1 is a low-voltage insulation monitoring and leakage protection system total structure diagram of an embodiment of the application, which shows the connection relationship of a signal acquisition module, a measurement and control protection module and an online monitoring module and a DC injection loop principle;

[0046] Figure 2 FIG. 2 is a hardware overall structure diagram of an embodiment of the application, which shows the hardware composition of the system and the connection relationship between the parts;

[0047] Figure 3 FIG. 3 is a signal acquisition board actual object diagram of an embodiment of the application, which shows the actual appearance and internal structure of the signal acquisition board.

[0048] Figure 4 It is the physical diagram of the circuit module of the measuring and control protection device of the embodiment of the application, which shows the actual appearance and connection mode of each circuit module in the measuring and control protection device;

[0049] Figure 5 It is the flow chart of the current signal collection of the embodiment of the application, which shows the flow of the current signal collection;

[0050] Figure 6 It is the program design flow chart of the measuring and control protection device of the embodiment of the application, which shows the working principle and function description of the measuring and control protection device;

[0051] Figure 7 It is the software program flow chart of the low-voltage insulation monitoring and leakage protection system software of the embodiment of the application, which shows the working flow of the low-voltage insulation monitoring and leakage protection system software;

[0052] Figure 8 It is the power grid parameter model graph when single-phase grounding fault occurs of the embodiment of the application, including the equivalent resistance of the fault point, the zero sequence voltage and the current direction diagram;

[0053] Figure 9 It is the graph of the direction and size relationship of the zero sequence current of each branch of the embodiment of the application, which shows the direction and size relationship of the zero sequence current of each branch of the system when single-phase grounding fault occurs;

[0054] Figure 10 It is the principle diagram of the additional DC power supply method, which shows the leakage protection principle of the additional DC power supply;

[0055] Figure 11 It is the system simulation model graph of the embodiment of the application, which shows that the system simulation model about the additional DC power supply method for measuring the ground insulation resistance value of the power grid is established by using the Simulink component of MATLAB;

[0056] Figure 12 It is the waveform graph when the power grid normally operates of the embodiment of the application, which shows that the branch zero sequence current is 0, the zero sequence voltage and insulation value signal of the additional DC power supply part are 0, and the corresponding simulation waveform;

[0057] Figure 13 It is the single-phase leakage waveform graph of the branch A phase of the embodiment of the application, which shows the branch 2 zero sequence current waveform of the single-phase leakage of the A phase in the power grid branch 2 and the zero sequence voltage and insulation value signal waveform of the additional DC power supply part. DETAILED DESCRIPTION

[0058] In order to make the objects and advantages of the present application more clear, the present application will be further described below in conjunction with embodiments; it should be understood that the specific embodiments described herein are only used to explain the present application and not to limit the present application.

[0059] The preferred embodiments of the present application will be described below with reference to the accompanying drawings. Those skilled in the art should understand that these embodiments are only used to explain the technical principles of the present application and not to limit the protection scope of the present application.

[0060] It should be noted that, in the description of the present application, the terms of direction or position relationship such as "upper", "lower", "left", "right", "inner", "outer" and the like are based on the direction or position relationship shown in the drawings, which is only for the convenience of description and does not indicate or imply that the device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present application.

[0061] In addition, it should also be noted that, in the description of the present application, unless otherwise explicitly specified and limited, the terms "mounting", "connecting", "connection" should be understood broadly, for example, it can be fixed connection, or detachable connection, or integrally connected; it can be mechanical connection, or electrical connection; it can be directly connected, or indirectly connected through intermediate medium, or the communication inside two elements. Those skilled in the art can understand the specific meaning of the above terms in the present application according to the specific circumstances.

[0062] Please refer to Figure 1 As shown in the figure, it is a total structure diagram of a low-voltage insulation monitoring and leakage protection system according to an embodiment of the present application, and the present application provides a downhole distribution network single-phase grounding protection system based on an injection signal method, which comprises,

[0063] A signal acquisition module is used to acquire current, voltage signals and insulation parameters of the downhole distribution network, including insulation resistance value;

[0064] A measurement and control protection module is connected with the signal acquisition module and is used to process the acquired signals and judge the single-phase grounding fault type, and output control instructions;

[0065] An online monitoring module is connected with the measurement and control protection module through a communication interface and is used to display the power grid operation state in real time, store data and remotely manage;

[0066] Among them, the system forms a loop by injecting an independent direct current power between the three-phase power grid and the ground, monitors the loop current change to calculate the insulation resistance of the power grid to the ground, and judges the fault type based on a zero sequence fundamental wave timing discrimination model to realize selective leakage protection.

[0067] A self-diagnosis unit is used to monitor the running state of the hardware module in real time and record fault logs;

[0068] Anti-aliasing filter, pre-processing of analog signals to eliminate high-frequency noise;

[0069] Data storage unit, using TF card to store historical data and fault information.

[0070] In this embodiment, by injecting independent DC signal combined with zero sequence fundamental wave timing discrimination algorithm and traveling wave distance measurement method, the fast detection, high precision positioning and selective protection of single-phase grounding fault of underground low-voltage distribution network are realized, which is suitable for complex environments such as coal mines and metal mines; By using the method of additional DC power supply combined with wavelet transform, the error is ≤3%, and the detection accuracy is improved, the fault removal time is ≤100ms, and the response speed is improved; Strong anti-interference ability, stable operation in noise ratio ≥20dB environment; Through supporting multiple branch line parallel monitoring, the expansibility is high.

[0071] In this embodiment, the insulation monitoring adopts the method of additional DC power supply, realizes the leakage protection through online monitoring of insulation value, and realizes the response of the voltage change of the additional DC signal and the neutral point of the reactor to the line-to-ground insulation value level; The leakage fault discrimination adopts the zero sequence fundamental wave timing discrimination model, and the phase of the zero sequence voltage and zero sequence current signal waveform when leakage is used to determine the type of fault. Through the output of the relay signal, the on-off control of the protected branch is carried out, so as to quickly remove the fault line when a fault occurs, and protect the equipment and bus from damage. The total structure diagram of the low-voltage insulation monitoring and leakage protection system in this embodiment is shown in Figure 1 .

[0072] The circuit design in this embodiment is,

[0073] (1) Controller circuit design

[0074] The data storage circuit has two important functions: one is to back up the data in real time to ensure data security and avoid data loss; When the monitoring terminal fails, the running state of the fault point should be recorded immediately to quickly restore the normal running state when the system restarts. The TMS320F28335 controller provides strong support for the data storage circuit, and the data is stored through the TF card, thereby improving the storage efficiency.

[0075] (2) Communication circuit design

[0076] The SP485R chip is the core part of the communication circuit, which has excellent performance and can connect multiple devices on the same transmission bus without causing signal attenuation.

[0077] The voltage sensor and the current sensor are the core components of the current and voltage acquisition system, which are responsible for collecting voltage and current data. In addition, an anti-aliasing filter is needed to process the analog signal, and a level converter is also needed to convert the voltage signal, so as to realize accurate measurement of the current. Through the A / D conversion circuit, the complex analog signal is converted into a high-precision digital signal.

[0078] (3) Current acquisition unit circuit design

[0079] The current acquisition unit detects the current through a high-precision sensor and converts it into a voltage signal, thereby realizing accurate measurement of the current. Its current signal acquisition flow chart is shown in Figure 5

[0080] The measurement and control protection device controls the grid branch switching by means of FPGA, collects data by STM32F103, and realizes message generation and analysis, log data reading and writing, and panel display interaction through data processing and statistical analysis processes. The program design flow chart of the measurement and control protection device is shown in Figure 6 The online monitoring software receives data through Modbus communication, displays main parameter waveforms, presents data panel in real time, saves reports, manages equipment uniformly, and performs user account login management and other operations, which together ensure the stable operation of the power system. The measurement and control protection device is a field monitoring terminal that can be installed in the switch cabinet and completes the corresponding actual functions through the built hardware platform. The main control program of the system is responsible for monitoring the running state of the system and performing parameter initialization and analog-to-digital conversion. It can also set up a general timer for the event manager to count, detect and control, in addition to on-site debugging and parameter setting.

[0081] Online monitoring software design

[0082] According to the actual production conditions and corresponding fault handling requirements of the downhole working conditions, the monitoring software is designed, and its main functions are as follows:

[0083] (1) Real-time waveform display of voltage value and insulation resistance value data waveform, real-time saving of data report, measurement value display, and other parameter value panel display.

[0084] (2) Warning for faults and writing into the database.

[0085] (3) Operation log and fault information record table, automatic access of database information.

[0086] (4) Setting the time, voltage level, action voltage resistance, etc. of the measurement and control protection device.

[0087] (5) Data maintenance of each device and unified management of device groups.​

[0088] (6) User account permission management.

[0089] The software program flow chart of the low-voltage insulation monitoring and leakage protection system software is shown in Figure 7 When the software is started by double-clicking, the account login interface pops up, the user selects his own account and inputs the password to log in to the main interface. After that, the main interface of the system starts to initialize. The software starts to poll whether there is a signal generated and whether the device is successfully connected. If there is an event generated, the polling is exited and the corresponding signal slot function is directly jumped to process; if it has been successfully connected to the device, the corresponding software starts to receive data, visualize data, synchronize data to the database and other steps. If the user does not click the operation of closing the software, and the communication has been successfully connected, the software will be in a default continuous running state, realizing the online centralized management and monitoring of the low-voltage switch cabinet.

[0090] The system is composed of a signal acquisition board, a measurement and control protection device and an online monitoring software. The overall structure of the hardware is shown in Figure 2 After the bus of the low-voltage switch cabinet in the mine passes through the signal acquisition board of the data acquisition layer to perform voltage, rectification, filtering and other processing on the current and voltage signals, it is connected to the monitoring and protection device of the data processing layer through the corresponding interface, the data is processed and analyzed, and the low-voltage switch cabinet circuit breaker is directly switched according to the insulation resistance data, the leakage protection and short-circuit protection are performed. At the same time, the processed data and result information are transmitted to the data monitoring layer, i.e. the online monitoring software on the computer end through Modbus communication, realizing real-time visual monitoring of the data and centralized management of the low-voltage switch cabinet equipment.

[0091] By continuously measuring the insulation resistance, the trend of the insulation resistance of the power grid line to ground is obtained, the misoperation during instantaneous short circuit or lightning strike can be avoided, the action threshold and action time of the system leakage protection can be adjusted according to requirements, combined with the online monitoring software, the line operation of the entire power distribution system can be remotely monitored online through Modbus communication, and the line parameters can be set to realize centralized management of the low-voltage switch cabinet equipment. The system can measure the insulation resistance value of the low-voltage switch cabinet branch online, realize selective leakage protection, use FPGA to directly judge the leakage fault type and switch the power grid line circuit breaker, improve the rapidity of the leakage protection, and use the online monitoring software to realize centralized online monitoring and management of the low-voltage switch cabinet equipment.

[0092] Referring to Figure 3 The signal acquisition board is mainly composed of a reactor, a choke coil, a capacitor, a resistor and a transformer. Its working principle is to collect the current and voltage signals of the equipment, and then measure the phase and amplitude through the corrected harmonic analysis to obtain the insulation parameters.

[0093] Specifically, the signal acquisition module comprises:

[0094] An electric reactor is used for neutral point resonance grounding.

[0095] A choke coil is used for suppressing high-frequency interference.

[0096] A capacitor is used for filtering signal noise.

[0097] A high-precision sensor is used for collecting power grid current and voltage signals.

[0098] An insulation signal processing circuit is used for filtering the collected insulation value voltage signal and transmitting it to the measurement and control protection module.

[0099] Specifically, the measurement and control protection module comprises:

[0100] A signal detection board is used for providing multiple DC power supplies and processing insulation signals.

[0101] An MCU board is used for realizing data sampling, communication and human-computer interaction based on an STM32F103VET6 microcontroller.

[0102] A logic processing board is used for realizing electric leakage fault discrimination and relay control by using an FPGA chip EP4CE6E22C8N.

[0103] An output control board is used for containing 12 relay circuits and performing line on-off operations.

[0104] An IO interface board is used for integrating a liquid crystal display, a key and an indicator light and providing a human-computer interaction interface.

[0105] The core function of the measurement and control protection device is to measure and protect the line from electric leakage hazards and to interact with remote monitoring equipment through an RS485 bus. The circuit structure is the basis of the detection terminal, which is composed of a sensor, a signal filter, an ADC digital-analog converter, a controller, a communication system and a power supply system.

[0106] The measurement and control protection device is mainly composed of a signal detection board, an MCU board, a logic processing board, an IO interface board and an output control board, as shown in FIG. Figure 4 This is a measurement and control protection device developed to meet the needs of on-line monitoring of electrical equipment insulation. The device can measure the insulation resistance of the line on site.

[0107] The signal detection board is composed of a DC 48V power supply, a DC 5V 2A power supply, a DC ±12V 0.5A power supply, and an insulation signal processing circuit. The DC 48V power supply is obtained by directly reducing three-phase power to 48V and then rectifying, and the transformer capacity is required to be 25VA; the DC 5V power supply is provided by converting AC 220V to 5V, and is used to power the MCU, FPGA, and keys and liquid crystals; the DC ±12V power supply is provided by converting AC 220V to ±12V, and is used to power the operational amplifier and other parts; the insulation signal processing circuit filters and reversely processes the insulation value voltage signal accessed from the outside, and provides the signal to the MCU for AD sampling, so as to obtain the insulation value.

[0108] The MCU core board uses an STM32F103VET6 microcontroller, and interfaces such as keys, liquid crystals, and indicator lights are led out through the interface, and the board has an AD sampling circuit. The board has the functions of key input, liquid crystal display data, state indicator light, 485 communication, and insulation signal processing identification.

[0109] The logic processing board uses an EP4CE6E22C8N (FPGA), has 12 zero sequence current detection circuits and 1 zero sequence voltage detection circuit, and has 12 control signal outputs to the output control board. The board has the functions of leakage fault type discrimination and 12 relay on-off control.

[0110] The output control board has 12 relay control circuits, and the control signals are provided by the logic control board.

[0111] The IO interface board has a 5-inch dot matrix liquid crystal display screen, 9 keys, and 6 indicator light circuits, and is a human-computer interaction interface of the system.

[0112] Specifically, the signal detection board comprises:

[0113] The DC 48V power supply circuit is generated by three-phase voltage reduction rectification;

[0114] The DC 5V power supply circuit is used to power the MCU and peripheral devices;

[0115] The DC ±12V power supply circuit provides operating voltage for the operational amplifier;

[0116] The insulation signal processing circuit reversely and filters the insulation value voltage signal.

[0117] Specifically, the logic processing board realizes the determination of the fault type and the cooperative processing of the injection signal method through the following steps:

[0118] a. injecting a preset frequency independent DC signal source between the neutral point and the ground of the three-phase power grid to form a monitoring loop, and obtaining the injection current amplitude and the change rate in the loop in real time;

[0119] b. Combined with the zero sequence fundamental time discrimination algorithm, the fundamental components of the power grid zero sequence voltage U0 and zero sequence current I0 are synchronously collected, and the modulus maximum value characteristics of the fault transient signal are extracted through wavelet transform;

[0120] c. Based on the inverse relationship between the injected current amplitude and the insulation resistance value, the phase difference threshold range of the zero sequence voltage and the zero sequence current is dynamically corrected, wherein:

[0121] When the injected current rate of change ΔI / Δt exceeds the set threshold and the zero sequence voltage leads the zero sequence current phase difference θ ∈ (85°, 95°), it is determined as a low resistance grounding fault;

[0122] When the injected current amplitude continues to decrease and is accompanied by the zero sequence current leading the zero sequence voltage phase difference θ ∈ (45°, 75°), it is determined as an intermittent grounding fault dominated by distributed capacitance;

[0123] d. The fault type discrimination result is logically associated with the fault branch identification output by the traveling wave distance measurement module to generate a differentiated protection strategy:

[0124] For low resistance grounding fault, trigger the relay to cut off the fault line within ≤80ms;

[0125] For capacitive grounding fault, start the insulation resistance adaptive re-measurement mechanism, and perform delay removal after detecting that the insulation value is lower than the safety threshold for three times.

[0126] Specifically, the online monitoring module is connected with the measurement and control protection module through the Modbus communication protocol, and the specific functions include:

[0127] Real-time waveform display, dynamically displaying the voltage and insulation resistance value change curve;

[0128] Fault alarm and record, triggering sound and light alarm and storing fault information to the database;

[0129] Parameter setting, remotely adjusting the leakage protection threshold, action time and voltage level;

[0130] Equipment management, supporting centralized monitoring of multiple switch cabinets and user permission hierarchical management.

[0131] Specifically, the specific implementation of the injected independent DC power supply is:

[0132] A DC power supply is connected between the neutral point of the three-phase reactor and the ground;

[0133] The ground insulation resistance of the power grid is calculated inversely through the monitoring loop current value;

[0134] When the insulation resistance is lower than the preset threshold, the relay is triggered to cut off the fault line.

[0135] Specifically, the traveling wave distance measurement method realizes high-precision positioning of the fault point through the following multi-modal fusion processing:

[0136] a. Traveling wave signal decoupling and feature extraction:

[0137] The traveling wave of three-phase coupling fault is decoupled by using the Karhunen-Loeve transform, and the line mode (α mode) and zero mode (0 mode) are separated, eliminating the interference of inter-phase electromagnetic coupling;

[0138] The line mode and zero mode after decoupling are respectively subjected to wavelet transform modulus maximum value detection, the mutation features of the traveling wave head are extracted by Daubechies wavelet basis function, and the first arrival time stamps t1 (line mode) and t2 (zero mode) are recorded;

[0139] b. Double-end cooperative distance measurement algorithm:

[0140] Synchronous clock modules are deployed at both ends (M end and N end) of the transmission line, and microsecond-level time synchronization is realized based on IEEE 1588 Precision Time Protocol (PTP);

[0141] The time difference Δt1 = |t1M-t1N| of the line mode traveling wave arrival at both ends is calculated, combined with the line mode wave speed vα and the total length L of the line, vα is 98% of the speed of light, through the formula:

[0142]

[0143] The precise position of the fault point from the M end is determined;

[0144] The zero mode wave speed dynamic compensation factor k is introduced, the zero mode wave speed v0 is corrected according to the type and aging degree of the downhole cable, and the following is met:

[0145] v0=k·v α (k∈[0.25,0.35])

[0146] The wave head misjudgment caused by environmental noise is reduced through redundancy check;

[0147] c. Error suppression and adaptive optimization:

[0148] The sliding time window algorithm is used to statistically analyze the historical distance measurement data, and an error distribution model is established;

[0149] When the deviation between the single distance measurement result and the model predicted value exceeds 1.5%, the parameter self-tuning mechanism based on genetic algorithm is triggered, and the k value and vα coefficient are dynamically adjusted, so that the double-end distance measurement error is stably controlled within the range of-2.3% to 2.6%;

[0150] d. Fault location visualization mapping:

[0151] The ranging result is integrated with the GIS coordinate system of the underground roadway to generate a fault point identifier in a three-dimensional topological map, and a confidence interval range is marked.

[0152] The specific implementation of the Cattell transform and the wavelet transform modulus maximum value detection in the traveling wave distance measurement method includes the following innovative steps:

[0153] a. Multi-source signal coupling and decoupling optimization:

[0154] Based on the dynamic selection of the Cattell transform matrix from the cable parameter library, the decoupling coefficient is adaptively adjusted for cables with different cross-sectional areas (35-95 mm 2 ) and insulation materials (EPR / XLPE), eliminating the modal aliasing caused by mutual inductance;

[0155] A high-frequency pulse signal (frequency 1-10 kHz, amplitude ±50V) is superimposed and injected before the Cattell transform, and by comparing the propagation time delay difference between the injected signal and the fault traveling wave, the separation degree of the line mode and the zero mode is enhanced;

[0156] b. Noise suppression and wave head precise extraction:

[0157] The decoupled line mode signal is decomposed by 5 layers using the Db6 wavelet basis, and the effective frequency band (1-100 kHz) is selected by the energy entropy threshold method to eliminate the broadband noise interference such as underground frequency converter and motor start-stop;

[0158] An adaptive notch filter is introduced to the zero mode signal to dynamically track the power frequency harmonic components (50 Hz and its integer multiples), and suppress the influence of the low-frequency oscillation of the grounding current on the wave head detection;

[0159] Based on the Lipschitz index analysis of the singularity of the modulus maximum value, a multi-level dynamic threshold is set:

[0160]

[0161] Where σ is the noise standard deviation, N is the signal length, and Wj(k) is the jth layer wavelet coefficient, which realizes the precise distinction between the fault wave head and the reflected wave;

[0162] c. Time stamp fusion calibration:

[0163] The wave head time stamps of the line mode and the zero mode are cross-modally correlated and verified, and if the time difference Δt satisfies:

[0164]

[0165] Then it is determined as an effective fault traveling wave, otherwise the redundant detection mechanism is triggered to reacquire the signal;

[0166] Combining GPS disciplined clock and oven-controlled crystal oscillator, the timestamp synchronization error is controlled within ±0.1 μs;

[0167] d. Fault type correlation analysis:

[0168] According to the wave head polarity characteristics (positive / negative jump) and wavelet coefficient energy distribution, the metal grounding (energy concentrated in the high frequency band) and high resistance grounding (energy diffuses to low frequency) faults are distinguished.

[0169] The fault type characteristics are embedded into the traveling wave distance measurement model, and the value range of the wave speed compensation factor k is dynamically optimized (k=0.28±0.02 for metal fault, k=0.32±0.03 for high resistance fault).

[0170] Specifically, the leakage protection logic of the measurement and control protection module includes:

[0171] When the single-phase grounding fault current effective value satisfies the following formula, the protection action is triggered:

[0172]

[0173] Wherein, Uo is the zero sequence voltage, Rg is the grounding resistance, w is the angular frequency, C is the capacitance to ground, and L is the inductance value of the reactor.

[0174] Specifically, the application conditions in the downhole environment include:

[0175] The environmental temperature range is -20℃ to 60℃;

[0176] The relative humidity is ≤95%;

[0177] The anti-vibration level meets the GB / T 2423.10 standard;

[0178] The protection level is not less than IP65.

[0179] Specifically, the measurement and control protection device adopts modular design, and each functional board is connected through the following interfaces:

[0180] SPI interface, used for high-speed data exchange between MCU and FPGA;

[0181] RS485 bus, realizing remote communication with the upper computer;

[0182] GPIO interface, controlling the relay and receiving the key input signal.

[0183] Specifically, the software flow of the online monitoring module includes:

[0184] User login verification;

[0185] Device connection state polling;

[0186] Data reception and real-time visualization;

[0187] Fault event interruption processing;

[0188] Database synchronization and report generation.

[0189] Specifically, the embodiment also provides a downhole power distribution network single-phase grounding protection method based on an injection signal method, applied to the downhole power distribution network single-phase grounding protection system based on the injection signal method, and including,

[0190] Step S1, injecting an independent DC power supply between a three-phase power grid neutral point and the ground to form a monitoring loop, collecting a loop current signal in real time, and inversely calculating a power grid ground insulation resistance value;

[0191] Step S2, when the insulation resistance value is lower than a set threshold, starting a fault discrimination mechanism, including zero sequence fundamental wave timing discrimination and traveling wave distance positioning, to judge a fault type and locate a fault point, and generating a fault branch identification;

[0192] Step S3, connecting with the measurement and control protection module through a communication interface, for real-time display of a power grid operation state, storage of data, and remote management;

[0193] The remote management includes controlling a corresponding relay to cut off a fault line, and uploading fault information to a monitoring platform.

[0194] Specifically, the specific implementation of the traveling wave distance positioning method includes:

[0195] Performing a Kelvin-Bell transformation on the fault traveling wave to extract a line mode and a zero mode;

[0196] Calculating a wave speed difference and an arrival time difference of the line mode and the zero mode;

[0197] Determining a fault distance in combination with a double-end distance positioning formula:

[0198]

[0199] Wherein, L is a total length of a line, u is a traveling wave speed, t1 and t2 are arrival times of wave heads at two ends.

[0200] Specifically, the zero sequence fundamental wave timing discrimination algorithm includes:

[0201] Collecting fundamental wave components of a zero sequence voltage U and a zero sequence current Io;

[0202] Calculating a phase difference θ of the two;

[0203] If θ∈(80°, 100°), it is determined as a metallic grounding fault;

[0204] If θ is in (40°, 80°), it is determined as high resistance grounding fault.

[0205] Specifically, the relay control logic comprises:

[0206] 12-way PWM control signals are generated by using FPGA;

[0207] The relay action is driven by optical coupling isolation;

[0208] After fault removal, the insulation resistance re-measurement process is automatically triggered.

[0209] The line model of 10Kv is mainly a lumped parameter circuit model, but in actual engineering, the inductance and admittance of the line are uniformly distributed along the line, and there is also an equivalent distributed capacitance and conductance between the line and the ground. Therefore, when fault location is considered, a distributed parameter model is used as the basis for calculation. Assuming that the line is a rational model, the line has a unit length inductance (L0), capacitance (C0), conductance (G0) and resistance (R0). According to the theory of calculus, the following equation can be obtained:

[0210]

[0211] For an ideal circuit, the values of R0 and G0 are 0, so (1.1) can be simplified as:

[0212]

[0213] (1.2) is the wave equation of a single segment lossless line. By differentiating x and t and solving the second-order wave equation, the D'Alembert solution is obtained as:

[0214] u(x,t)=u f (x-vt)+u b (x+vt)

[0215] i(x,t)=i f (x-vt)+i b (x+vt)

[0216] Where v is the wave speed, u f (x-vt) is the forward propagation voltage wave, u b (x+vt) is the backward propagation voltage wave, i f (x-vt) is the forward propagation current wave, and i b (x+vt) is the backward propagation current wave.

[0217] In the distributed parameter circuit model, the transmission line is formed by the superposition of individual elements. When each element is in an ideal state, the wave impedance encountered by the traveling wave when propagating along the transmission line is constant. When a single-phase ground fault occurs on a transmission line, the stability parameters of the transmission line will be affected, resulting in a change in wave impedance. At the same time, the circuit parameters of the bus and other connections will also be affected, and the fault point is the so-called discontinuity point. Therefore, when the traveling wave propagates, the original propagation state will be changed when it encounters a fault point or a line connection point, and the traveling wave no longer propagates along the transmission line in the previous manner. When a portion of the traveling wave passes through a series of distortion points, it will maintain the original trajectory, but another portion will undergo refraction, thereby forming a new trajectory, i.e., the so-called reflection of the traveling wave or the refraction of the traveling wave. When a wave propagates from one location to another, the wave is referred to as an incident wave, and when it passes through a fault point, it will undergo refraction. The refraction coefficient M(ε) of the voltage traveling wave and the reflection coefficient N(ε) of the voltage traveling wave are respectively:

[0218]

[0219] The refraction coefficient M(ε) of the current wave and the reflection coefficient N(ε) of the current wave are respectively:

[0220]

[0221] wherein the refraction coefficient M(ε) of the voltage wave ranges from 0 to 2, the reflection coefficient N(ε) of the voltage wave ranges from -1 to 1, and 1+N(ε) = M(ε) is satisfied between the two.

[0222] In the actual operating environment of the transmission line, there is coupling between the three-phase lines due to mutual inductance and other factors. When a fault occurs, the fault phase will generate a fault traveling wave, and the non-fault phase will also generate a corresponding traveling wave due to the influence of the transient electromagnetic change. If the electromagnetic transient process after the fault is regarded as a transient process of each cable independent of each other, it will cause a large error. Therefore, when analyzing the three-phase circuit, the phase-mode transformation is used to decouple the three-phase lines, so that they are converted into independent modules.

[0223] When a single-phase ground fault occurs on the line, the wave equation of the line is as follows:

[0224] (1.6)

[0225] wherein wherein L s is the self-inductance per unit length, L m is the mutual inductance per unit length, and C sC is the per unit length ground capacitance m C is the per unit length inter-phase capacitance. In the formula, [L] and [C] are symmetric matrices, and the coefficient matrix of three-phase current and three-phase voltage fluctuation equation is the same, but the coefficient matrix is not a diagonal matrix, which brings great difficulty in solving (3.6), therefore, in the solving process, by using matrix transformation, [L] and [C] are converted into diagonal matrices, and the vectors that exist in space are converted into independent and uncoupled modules, so that a new set of voltage and current variables is obtained, and the relationship between each module value and each phase value is as follows:

[0226] [u]=[S][u m ]

[0227] [i]=[Q][i m ] (1.7)

[0229] By matrix transformation, it is decoupled into α module, β module and 0 module, and the three module components are as follows:

[0230]

[0231]

[0232] Wherein, L0 is the per unit length zero sequence inductance, L1 is the per unit length positive sequence inductance, C0 is the per unit length zero sequence capacitance, C1 is the per unit length positive sequence capacitance, and the wave impedance of the line module α module, β module and 0 module is:

[0233]

[0234] The wave speed of the line module component is:

[0235]

[0236] The commonly used phase-module transformation includes Kelvin-Bell transformation and Clark transformation, wherein the Kelvin-Bell transformation is:

[0237]

[0238] The inverse transformation matrix is:

[0239]

[0240] The Kelvin-Bell transformation converts the three phase components into module components, i.e. α line module component, β component and zero module component. The output of the line module component is usually a loop for the cable, and its propagation characteristics make its wave speed almost not affected by any external interference, and the zero module component takes the ground as a loop, and the wave speed is slow.

[0241] In fault location, different mathematical methods are used to analyze distance measurement, allowing the most appropriate method to be determined based on the actual situation. Distance measurement methods can be divided into two categories: two-ended and single-ended. The former measures the required fault information at both ends of the line, while the latter measures fault information at only one end.

[0242] Through the single-ended traveling wave ranging method, fault ranging can be divided into two types according to the different collected data: one is to use the time difference between the forward traveling wave and the reflected wave, as well as the time difference between the line mode wave head and the zero modulus wave head to achieve fault ranging; the other is to determine the fault location by the time difference between the line mode refraction and reflection.

[0243] Method 1: When a single-phase ground fault occurs in the line, two propagation waves are generated. One reaches the M terminal for the first time, and the other is reflected from point F for the second time. When the speed of the traveling wave is determined, the time difference between the two times can be used to calculate the distance from the M terminal to the fault point as follows:

[0244]

[0245]

[0246] Method 2: As we know from the previous article, when a fault occurs in the transmission line, the traveling wave generated at the fault point F, the line modulus and the zero modulus are transmitted to the M end at different propagation speeds. Since the speeds of the line modulus and the zero modulus are different, the time they arrive at the M end is also different. Let the wave speed of the line modulus be v1, the wave speed of the zero modulus be v0, and the time it takes for the line modulus to reach the M end be t M1 , the time when the 0 modulus reaches the M end is t M0 , let the time when the fault occurs be t, from this we can get:

[0247] Linear module component:

[0248] x=v1(t M1 -t) (1.15)

[0249] For the 0-mode component:

[0250] x=v0(t M0 -t)(1.16)

[0251] Combining equations (1.15) and (1.16) yields:

[0252]

[0253] From the second method, the second method only needs to collect the linear modulus and the 0 modulus of the first wave head, effectively avoiding the attenuation of the traveling wave in the reflection process, and it is easy to determine the time when the wave head reaches the endpoint. However, the wave impedance of the 0 modulus is larger than the linear modulus, so the 0 modulus is easily affected by external factors during propagation, and therefore is not stable enough.

[0254] Compared with the single-end traveling wave distance measurement method, the double-end positioning method has higher positioning accuracy and is basically not affected by fault forms and fault conditions, and the wave head that first reaches the endpoint is the most obvious. Therefore, the double-end traveling wave distance measurement method has good applicability and robustness. The F point in the transmission line MN has a fault at time t, traveling wave checkpoints are set at the two ends M and N of the transmission line, and the wave speed of the linear modulus is v, the total length of the transmission line is l, and the time when the linear modulus of the fault traveling wave reaches the traveling wave checkpoints at the M end and the N end is t M and t N , the distance from the M end to the fault point F is x, and the distance from the N end to the fault point F is l-x, so the distance from the fault point to the two ends can be expressed as:

[0255] For the M end: x = v1(t M -t)(1.18)

[0256] For the N end: l-x = v1(t N -t)(1.19)

[0257] Solving equations (1.18) and (1.19) gives:

[0258]

[0259] The wavelet transform analysis method uses a set of specific functions, such as a basic wavelet, and a series of transform coefficients, which are compared with the original signal to obtain more accurate results. The core idea of this method is to fit a set of specific functions, such as a basic wavelet, to a specific signal through parallel movement and scale stretching, so as to obtain more accurate results.

[0260] The core concepts and basic principles of wavelet transform theory include basic wavelets, continuous wavelets, and wavelet basis functions, and the specific main theories are as follows:

[0261] (1) Basic wavelet

[0262] Assume that ψ∈L 2 (R), and its Fourier transform is ψ(ω). If then ψ is called the wavelet basis of wavelet transform, and the basic wavelet has the following four characteristics:

[0263] 1) Band-pass, when ω→0, The amplitude of the signal at zero frequency is zero, so the mother wavelet can be considered as a high-pass filter.

[0264] 2) Zero mean, when ω = 0, ψ(ω) = 0, and

[0265]

[0266] Therefore, the DC component of the wavelet function is zero.

[0267] 3) Localization, the mother wavelet is selected, but it is not required Its energy will be concentrated in the time-frequency plane.

[0268] (2) Wavelet base function

[0269] For the wavelet base function under the window, scaling and translation, etc. Processing can get the wavelet base function, which is defined as:

[0270]

[0271] In (1.22), ψ a,t (t) is the wavelet base, a and τ are the scale factor and translation factor respectively, Can make the function stable after scaling transformation.

[0272] (3) Continuous wavelet transform

[0273] In order to increase the efficiency of the algorithm operation, the integral kernel needs to be discretized and converted into an orthogonal basis. Therefore, the continuous wavelet transform proposed by Morlet et al. is based on the change of oscillation function ψ(t), instead of the traditional window function, which greatly improves its mathematical performance.

[0274] Assume that the function f(t) is any function in the space L 2 (R), and use the wavelet base to expand f(t), then this expansion method is called the continuous wavelet transform of f(t), and its expression is:

[0275]

[0276] In equation (1.23), a is the scaling factor and τ is the translation factor.

[0277] Therefore, the wavelet transform is to map a one-dimensional function f(t) to the time-scale plane. From the functional point of view, it is to filter f(t) with a filter, and the impulse response of the filter φ a (x) is analyzed from the time-frequency point of view, let

[0278]

[0279] The wavelet transform can be regarded as the Fourier transform. The wavelet coefficient of f(t) at scale a and translation position τ actually functions to characterize the filtered frequency component with a center frequency of ω0 / a and a bandwidth of Δω / a at τ, aΔt. As time goes by, its center frequency and bandwidth will change. Therefore, when processing transient signals of distribution line faults with a low proportion of high-frequency signals, the wavelet transform has better applicability and robustness.

[0280] If a function f(t)∈R has a singularity at a point, it means that the function has infinite derivatives within the entire defined range. For a signal with a mutation point, the mutation point is singular, so the singularity principle is used to detect signal mutation points. Signal singularity detection theory is defined as using the singularity exponent Lipischitz to characterize the detected signal mutation point.

[0281] The singularity of the function f(t) at a certain point is characterized by the Lipischitzα index. Let 0≤α≤1, and there can be a constant K at point t0, so that for the neighborhood t of t0, the following formula holds:

[0282] |f(t)-f(t0)|≤K|t-t0| α (1.25)

[0284] Then f(t) is said to be lipischitzα at t0. If α = 1, then f(t) is said to be differentiable at t0, that is, f(t) has no singularity. The larger α is, the closer f(t) is to being flat; conversely, the smaller α is, the greater the singularity of f(t) at t0.

[0285] Therefore, the singularity of the function and signal can be expressed by Lipischitzα, and its value can be obtained by the modulus maximum operation of the wavelet transform. The definition of its modulus maximum is:

[0286] When the signal scale is 2 j When t in the neighborhood δ of t0 is:

[0287]

[0288] In wavelet transform, the value of t0 that satisfies formula (1.26) is the modulus maximum point of wavelet transform. The singularity detection theory is used for signals with mutations, and describes the time and change degree of mutations. Since the signal can be localized in time-frequency domain, the width of time-frequency window can be automatically adjusted according to the actual situation of the signal, and therefore the maximum point of the modulus of the transformed coefficient of the signal of the fault point is more obvious.

[0289] After the relevant current data at the two ends of M and N are measured, the obtained data is subjected to Karhunen-Loeve transformation to obtain the bus alpha modulus. At the M end, the first is the initial fault traveling wave, and the polarity is positive, and the second wave head is the traveling wave reflected at the fault point, and the polarity is negative. At the N end, the first is the initial fault traveling wave, and the polarity is positive, and the second wave head is the traveling wave reflected at the fault point, and the polarity is negative.

[0290] The transformed signal is obtained, the wavelet maximum value of each segment is obtained by setting a threshold, in order to compare the single-end and double-end distance measurement methods, and thus make a selection criterion for the distance measurement method under various fault conditions, and therefore multiple simulations under different fault conditions are set. The data of the simulation results show that in the application of the traveling wave distance measurement, the overall error range of the double-end method is between-2.3% and 2.6%, and the error of the single-end method is mainly between-4% and 6%, and therefore the double-end method has a better precision advantage than the single-end method.

[0291] In the 10Kv underground low-voltage distribution network, the relevant fault characteristics of the line fault in the actual working condition are not obvious enough, which can easily endanger personal safety, affect normal production, and reduce economic benefits. Therefore, the leakage protection system is very important for power supply protection. The low-voltage insulation monitoring and leakage protection system is designed to meet the actual needs of safe operation of the underground low-voltage power grid. It uses the method of additional DC power supply, and applies DC power supply between the power grid and the ground. When the insulation resistance between the power grid and the ground changes, the current of the additional circuit will change, so that the insulation resistance to the ground can be indirectly calculated, and the single-phase ground fault can be detected, and the fault feeder can be selected.

[0292] The system can continuously measure the insulation resistance, obtain the insulation resistance change trend of the power grid line to the ground, avoid misoperation in the case of instantaneous short circuit or lightning stroke, adjust the action threshold and action time of the leakage protection system according to the requirements, combine the online monitoring software, remotely monitor the line operation of the entire distribution system through Modbus communication, set the line parameters, and centrally manage the low-voltage switch cabinet equipment.

[0293] As Figure 8As shown, when a single-phase grounding fault occurs in the low-voltage distribution network, it can be equivalent to the case of Rg=0. At this time, a single-phase grounding current is generated at the grounding point, and the grounding current forms a loop with the insulation resistance R and the ground capacitance C of the other two phases;

[0294] Suppose that a single-phase grounding fault occurs in phase A, the ground voltage of which is 0 V, but the phase-to-phase voltage between the three phases remains unchanged, and the system can continue to operate. However, to prevent other serious faults caused by single-phase grounding, power supply should be stopped immediately.

[0295] Since the voltage of each phase to the ground is no longer symmetrical, the three-phase voltage vector is not zero, and zero-sequence voltage Uo, zero-sequence current Io and single-phase grounding current Ijd will be generated. Combining the above formula, Rg=0 can be obtained:

[0296]

[0297] The effective value of the single-phase grounding current is:

[0298]

[0299] The current leads by an angle , and its expression is:

[0300]

[0301] When a single-phase grounding fault occurs, the directions and size relationships of the zero-sequence currents of each branch of the system are as shown in Figure 9 , wherein the size of the zero-sequence current flowing through the L1 line start end is:

[0302]

[0303] The L1 line start end zero-sequence current is the sum of the non-fault branch zero-sequence current sizes, and the size of the L2 start end zero-sequence current of the non-fault branch is:

[0304]

[0305] The size of the L3 start end zero-sequence current of the non-fault branch is:

[0306]

[0307] The direction of the L2, L3 zero sequence current is opposite to the direction of the zero sequence current of the fault line L1. The direction characteristic of the zero sequence current can be used as an important feature of selective leakage protection line selection. However, in the 10Kv underground distribution network, due to the low voltage level and the small amplitude of the zero sequence current caused by the environment, it is difficult to detect, so the single use of this selection method based on the direction relationship between the branches is unreliable, the error is serious, and the voltage and load of the underground network cannot be absolutely symmetrical. When the network is unbalanced or the network has fluctuations, misjudgment is easy to occur, which reduces the reliability of the judgment.

[0308] An independent power supply is added between the three-phase power grid and the ground to form a loop. When a cable grounding fault occurs, the voltage difference between the loops will generate a current, and the corresponding resistance can be calculated through this current, so that the change of the insulation resistance of the power grid can be obtained from the side. Therefore, by using the characteristics of the additional loop current method, we can set the direct current in the circuit to a threshold value, and once the threshold value is exceeded, the switch will automatically trip, thereby realizing leakage protection.

[0309] The principle of additional DC power leakage protection is shown in Figure 10 , in which Zj is a DC relay, kΩ is a 1,000 ohm meter obtained by modifying an ammeter according to Ohm's law. In the figure, SK is a three-phase reactor, LK is a zero sequence reactor, R A , R B , and R C are ground insulation resistors, and C A , C B , and C C are ground distributed capacitances.

[0310] When the low-voltage power grid is operating normally, the direct current formed by the additional DC power supply first flows from the positive pole of the additional power supply through the ground, then passes through the insulation resistors R A , R B , and R C of the power grid. Finally, it flows through the three-phase reactor SK, LK, ZJ, and the 1,000 ohm meter before returning to the negative pole of the power supply.

[0311] A system simulation model for measuring the ground insulation resistance of the power grid using the additional DC power supply method is established using the Simulink component of MATLAB. The additional DC power supply method for measuring the ground insulation resistance of the power grid is verified by the Simulink model simulation, which provides a corresponding theoretical basis for the subsequent system design. The additional DC power supply system simulation model is shown in Figure 11 .

[0312] When the system is in a normal operating state, the branch zero sequence current is 0, the zero sequence voltage and insulation value signal of the additional DC power supply part are 0, and the corresponding simulation waveform is as followsFigure 12 (a) and (b) in FIG. 1.

[0313] The waveform diagram of the zero sequence current of the A-phase single-phase leakage of the power grid branch 2 is shown in (a) of FIG. 2, and the waveform diagram of the zero sequence voltage and the insulation value signal of the additional DC power supply part is shown in (b) of FIG. 2. Figure 13 Figure 13 Due to the single-phase leakage of the power grid branch, the three-phase current vector sum of the branch is not 0, resulting in the zero sequence current, and the zero sequence voltage and the insulation value signal of the loop of the additional DC power supply are not 0.

[0314] The technical scheme of the present application has been described in combination with the preferred embodiments shown in the drawings, but it is easy for those skilled in the art to understand that the protection scope of the present application is obviously not limited to these specific embodiments. Those skilled in the art can make equivalent changes or replacements to the related technical features without departing from the principles of the present application, and the technical scheme after the changes or replacements will fall within the protection scope of the present application.

[0315] The above description is only the preferred embodiments of the present application and is not used to limit the present application; the present application can have various changes and variations for those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.​

Claims

1. A single-phase grounding protection system for underground distribution network based on injection signal method, characterized in that: include, Signal acquisition module, used to collect insulation parameters of the underground distribution network, including insulation resistance value; a measurement, control and protection module connected to the signal acquisition module, configured to determine the type of single-phase grounding fault based on the insulation resistance value and the zero-sequence fundamental wave time series identification model, locate the fault point based on the traveling wave ranging method, and output control instructions, including leakage protection control instructions; Among them, an independent DC power supply is injected between the three-phase power grid and the ground to form a loop, and the loop current change is monitored to calculate the insulation resistance of the power grid to the ground. The fault type is determined based on the zero-sequence fundamental wave time series identification model; The online monitoring module is connected to the measurement, control and protection module through a communication interface and is used to display the power grid operation status in real time, store data and perform remote management.

2. The single-phase grounding protection system for underground distribution network based on the injection signal method according to claim 1 is characterized in that: The measurement, control and protection module includes a fault type monitoring unit, a fault point location unit and a leakage protection unit, wherein: The fault type monitoring unit is used to determine the phase relationship between the zero-sequence voltage and the zero-sequence current according to the zero-sequence fundamental wave time sequence identification model when the insulation resistance value is less than the resistance value set threshold, so as to judge the type of single-phase grounding fault; The fault point location unit performs traveling wave signal decoupling and feature extraction based on Karen Bell transform, and determines the fault point location based on the zero modulus wave velocity dynamic compensation factor; The leakage protection unit is used to trigger the leakage protection control instruction based on the effective value of the single-phase grounding fault current.

3. The single-phase grounding protection system for underground distribution network based on the injection signal method according to claim 2 is characterized in that: The fault type monitoring unit includes a coordinated signal processing subunit and a dynamic correction threshold subunit, wherein: The collaborative signal processing subunit is used to synchronously collect the fundamental components of the zero-sequence voltage and zero-sequence current of the power grid, and extract the modulus maximum value characteristics of the fault transient signal through wavelet transform; The dynamic correction threshold subunit is used to dynamically correct the phase difference threshold range between the zero-sequence voltage and the zero-sequence current.

4. The single-phase grounding protection system for underground distribution network based on the injection signal method according to claim 3 is characterized in that: The dynamic correction threshold subunit dynamically corrects the phase difference threshold range between the zero-sequence voltage and the zero-sequence current, including: A monitoring circuit is formed based on an independent DC power supply; Real-time acquisition of the injected current amplitude and injected current change rate in the loop; The fault type is determined based on the injection current change rate and the phase difference between the zero-sequence voltage leading the zero-sequence current, including low-resistance grounding fault and intermittent grounding fault.

5. The single-phase grounding protection system for underground power distribution network based on the injection signal method according to claim 4 is characterized in that: The monitoring circuit formed based on the independent DC power supply includes: An independent DC power supply of a preset frequency is injected between the neutral point of the three-phase power grid and the ground to form a monitoring loop; Wherein, a DC power supply is connected between the neutral point of the three-phase reactor and the ground; Reversely calculate the insulation resistance of the power grid to the ground by monitoring the loop current value; When the insulation resistance falls below a preset threshold, the relay is triggered to cut off the fault line.

6. The single-phase grounding protection system for underground distribution network based on the injection signal method according to claim 2, characterized in that: The fault point location unit includes a feature extraction subunit and a wave speed compensation factor adjustment subunit, wherein: A feature extraction subunit is used to separate the line modulus and zero modulus through Karen Bell transform, extract the traveling wave head mutation feature using wavelet transform, and record the timestamps corresponding to the line modulus and zero modulus respectively; The wave velocity compensation factor adjustment subunit is used to calculate the distance between the fault point and the M end, correct the zero modulus wave velocity according to the zero modulus wave velocity compensation factor, and correct the value range of the wave velocity compensation factor according to the fault type.

7. The single-phase grounding protection system for underground power distribution network based on the injection signal method according to claim 6, characterized in that: The feature extraction subunit records the timestamps corresponding to the line modulus and the zero modulus respectively, so as to perform cross-modal correlation verification on the wave head timestamps of the line modulus and the zero modulus, so as to determine whether the current traveling wave is a valid fault traveling wave.

8. The single-phase grounding protection system for underground power distribution network based on the injection signal method according to claim 7, characterized in that: The wave speed compensation factor adjustment subunit modifies the wave speed compensation factor according to the fault type. The value range includes: Distinguish metallic ground faults and high-resistance ground faults based on wave head polarity and wavelet energy distribution; Modify the value range of the wave velocity compensation factor according to the fault type.

9. The single-phase grounding protection system for underground power distribution network based on the injection signal method according to claim 2, characterized in that: The calculation formula for the effective value of single-phase ground fault current is: Where, Ijd is the effective value trigger value of the single-phase ground fault current, Uo is the zero-sequence voltage, Rg is the ground resistance, w is the angular frequency, C is the capacitance to ground, and L is the inductance of the reactor.

10. A single-phase grounding protection method for underground distribution network based on injection signal method, applied to the single-phase grounding protection system for underground distribution network based on injection signal method according to any one of claims 1 to 9, characterized in that: include, Step S1: inject an independent DC power supply between the neutral point of the three-phase power grid and the ground to form a monitoring loop, collect the loop current signal in real time, and reversely calculate the insulation resistance value of the power grid to the ground; Step S2: When the insulation resistance value is lower than the set threshold, the fault identification mechanism is activated, including zero-sequence fundamental wave timing identification and traveling wave ranging and positioning, to determine the fault type and locate the fault point, and generate a fault branch identifier; Step S3, connecting to the measurement, control and protection module via a communication interface to display the grid operation status in real time, store data and perform remote management; Remote management includes controlling the corresponding relay to cut off the fault line and uploading the fault information to the monitoring platform.

Images