Research On Fault Line Selection Method Of Distribution Network In Small Current Grounding System

There are many kinds of grounding methods for distribution network's neutral points,including arc suppression coil grounding system for neutral points,small resistance grounding system for neutral points,direct grounding system for neutral points,high resistance grounding system for neutral points and ungrounded system for neutral points.Different grounding modes require different fault monitoring devices,overvoltage protection conditions,insulation level of equipments,grounding devices,etc.Therefore,after single phase grounding fault occurs in the distribution network system,according to whether the system needs fault monitoring device,the neutral grounding mode is divided into large current grounding mode and small current grounding mode.The large current grounding mode includes two kinds of grounding modes: direct grounding of neutral point and grounding of neutral point with small resistance;The small current grounding mode includes three kinds of grounding modes: ungrounded neutral point,grounded neutral point by arc suppression coil and grounded neutral point by high resistance.When the system adopts a effectively grounding mode,the circuit breaker disconnects the faulty device immediately after the fault,so this grounding mode does not need to be installed with a monitoring device,and the fault line is obvious.However,the excessive fault grounding current also interferes with the communication system,so it generally exists in extra-high-voltage power systems;When the system adopts the small current grounding mode,the system can run with single-phase grounding fault for 1-2 hours,and the frequency of such fault accounts for 85% of the total number of faults.Therefore,non-effectively grounded system is widely used in distribution network in China.However,the weakness of fault current using this grounding method has also become its disadvantage,the fault is not easy to be detected.If the fault cannot be removed accurately and timely,it will easily cause damage to electrical equipment and further expand the fault area.Therefore,when non-effectively grounded system is used and a single phase grounding fault occurs in the distribution network,the maintenance personnel can cut out the fault line as soon as possible according to the signal sent by the monitoring device,which can greatly shorten the recovery time of power supply,which is of great significance to accelerate the automation process of the distribution network.In this paper,the line selection method of single phase grounding fault in distribution network of non-effectively grounded system was studied deeply.The main contents are as follows:First,In view of the distribution network neutral point of small current grounding under three different grounding modes,when single phase grounding fault occurs,When a single phase grounding fault occured,the fault characteristics of each line were analyzed in detail from two aspects of transient characteristics and steady-state characteristics,which laid a theoretical foundation for the determination of subsequent line selection methods.Furthermore,this paper selected the fifth harmonic method based on the zero sequence current steady state quantity,the wavelet analysis method and the transient energy method based on the zero sequence current transient quantity,and analyzed the principles of these three line selection methods.Then,the simulation model of small current grounding system was built by using MATLAB/simulink,and The bus of the system has three outgoing lines.The single-phase grounding fault is caused by setting the parameters of the system,and the three wire selection methods mentioned above are used for the simulation analysis of each type of grounding system.The simulation results show that the selected wire selection method has a good experimental effect.The realization of the fault line selection method cannot be separated from the design of the line selection device.On the basis of the determination of the line selection method,the STM32 finally was selected as the main control chip,and a set of fault line selection devices for the distribution network of small and medium-sized enterprises was designed.The line selection device mainly included the design of hardware circuits and software systems.In terms of hardware circuit design,it mainly included the design of five parts: data acquisition circuit,analog-digital conversion module,operation control unit,keyboard,and LED display module.The power supply board and the PCB board of the data acquisition module were produced.In terms of software system design,the main program,analog-digital conversion module program,line selection program,display module program were designed.