Fault Line Selection Method Of Single Line To Ground Fault For Ungrounded Neutral Distribution Network

Single line to ground fault accounts for 80% of the distribution network’s faults.Ungrounded neutral distribution network’s obvious advantage is good power-supply reliability because it can operate continuously with fault.However,there are two problems of intermittent arc threatening at the grounded position and low accuracy of selecting fault-line for ungrounded neutral distribution network.To solve the problems,fault-line selection method of single-line to ground that can not only select the line accurately but also extinguish the arc stably is studied.And a new fault-line selection method based on grounded branches to shunt is proposed by this paper.Set up a group of grounded branches on the three-phase bus bar and operate it after the fault phase has been selected.In this way can reduce excessive fault-current of grounded positon and change the fault-current distribution of all lines to produce obvious increment-characteristics to select falut line.It is verified that the method proposed can improve the accuracy of fault-line selection under high and low transition resistance effectively and extinguish arc of grounded position stably.The specific research results are as follows:(1)A fault-phase selection method for single-phase to ground is proposed based on the positive and negative superposition characteristics of phase voltage amplitude difference.The correct operation of grounded branch set up on the bus bar is determined by selecting the fault phase accurately,so the fault phase selection method must have the ability to adjust all kinds of transition resistance.Considering the effect of the transition resistance on the amplitude of phase voltage amplitude,a mathematical model of phase voltage amplitude ratio and transition resistance is established,and the law between phase voltage amplitude and transition resistance is found.In order to improve the speed of fault phase selection,the obtained law is equivalently converted into the positive and negative characteristics of phase voltage amplitude difference.And a fault-phase selection method is proposed based on the positive and negative superposition characteristics of phase voltage amplitude difference.Build a distribution network model composed of overhead lines and cables and make the calculation and verification under different transition resistances for the fault phase selection method.The conclusion is that the fault phase can still be selected correctly by the method with the transition resistance less than 10Ω or greater than 2000Ω.It is verified that the method has good ability against transition resistances,which can ensure the correct operation of grounded branch set up on the bus bar.(2)This paper proposes a fault-current shunt method using grounded branch which can be operated on demand.Fault-current of single-phase to ground of the ungrounded neutral distribution network increases significantly,which leads to the problem of arc extinction.Considering both fault line selection and arc extinction,it is proposed to set up grounded branch on the bus bar composed of arc extinguishing switch and adjustable resistor.It is operated after the fault phase has been selected,which can reduce fault current of grounded positon and produce fault-current increment characteristics of all lines.(3)A fault-line selection method of single line to ground is proposed based on the increment characteristics of fault current of all lines with the grounded branch on the bus bar operated.Considering that ground fault occurs on the feeder or bus bar,it is divided into metal ground,arc ground and impedance ground to extract the increment characteristics of zero-sequence current and capacitance current for all lines after grounded branch on the bus bar has been operated.And the fault line is determined according to the fault-current difference results of each two lines for per outgoing line.A hybrid distribution network model composed of overhead lines and cables is built based on the conclusion of theoretical research.And the proposed method is verified and compared from feeder metal ground,bus bar metal ground,feeder high-resistance ground and bus bar high-resistance ground,respectively.The conclusion is that the proposed method can accurately identify fault position that occurs on the feeder or the bus bar under the metal ground and high-resistance ground up to 2000Ω.Moreover,the fault current can be reduced by 80% when the metal ground occurs through operating grounded branch with arc-extinction switch.The fault current can be reduced by 76.3% when the high-resistance ground of 2000Ω occurs through operating grounded branch with arc-extinction switch and adjustable resistor,and the reduction can be changed flexibly to meet demand by adjusting the resistance parameter of grounded branch,and the fault current can be reduced by 100% when the arc ground occurs through operating grounded branch with arc-extinction switch.It is verified that the proposed method has good ability against transition resistance and good performance to extinguish arc synchronously.This paper is guided by the problem of both fault line selection and arc extinction for single-line to ground fault in the ungrounded neutral distribution network.And the fault line selection method based on comparing the fault-current difference results of each two lines for per outgoing line is proposed under selecting fault-phase accurately and setting up grounded branch on the bus bar artificially.The proposed method can adapt to single-line to ground fault with different transition resistances and can effectively reduce the fault current of the grounded position by operating the grounded branch set up on the bus bar.The problems of both high rate of single-line to ground fault and increasing probability of arcing in the ungrounded neutral distribution network are better solved by the proposed method.The reliability of power supply in the ungrounded neutral distribution network is further improved.The research result can provide theoretical basis and method reference for the study of fault-line selection and arc extinction synchronously in the ungrounded neutral distribution network.