Study On Fault Location Methods For DC Power Distribution Cable

With the increasing number of distributed generators and electric vehicle loads, flexible DC power distribution technology is more and more used in the distribution network and the micro grid. DC power distribution line is mainly cable, because of the difference of operation mode, the fault characteristic of DC power distribution cable is obviously different from traditional AC line, so DC power distribution cable fault location problem has not been satisfactorily resolved. When any point on the DC power distribution cable is at fault, the fault current increases rapidly, resulting in a greater impact to the converter and power grid. The DC voltage is decreased rapidly, so that the use of the electric equipment is influenced. After the fault, DC circuit breaker at both ends of fault cable quick trip, the fault area is isolated. At this point, in order to quickly remove the fault and recovery power supply, quickly and accurately find the fault point is particular important.This thesis takes the 12kV and dual-terminal powered DC power distribution network cable as the research object. Using multiple fault location methods to locate the fault point of the cable which occur single pole ground fault and pole-to-pole short-circuit fault, and the effectiveness of the above method is verified by Matlab/Simulink simulation platform. The main research results are as follows:(1) 12kV and dual-terminal powered DC power distribution network model is established in Simulink simulation platform, the fault characteristics of single pole ground fault and pole-to-pole short-circuit fault are analyzed, and the fault line selection method based on fault current polarity is verified.(2) The two methods of single-ended fault location and dual-ended fault location based on RL equivalent model and Prony algorithm are analyzed. In this thesis, the parameter design of fault location module is improved, single pole ground fault and pole-to-pole short-circuit fault are simulated, single-end method and dual-ended method are used for fault location, the positioning results of before and after improvement are compared, the influence of transition resistance and fault distance on the positioning results are discussed. The RL equivalent cable model is adopted in the simulation, simulation results demonstrate that the positioning accuracy of the single-ended method and dual-ended method are not subject to the influence of transition resistance and fault distance; and the improved method reduces the attenuation coefficient of the loop and concentrates the frequency of all kinds of fault cases, it is helpful for the Prony algorithm to extract the characteristic parameters and reduce the fault location error.(3) The π model is adopted to simulate the actual cable, and single-ended fault location method and dual-ended fault location method based on RL equivalent model are used to locate fault, simulation results demonstrate that these two methods can only be applied to the small transition resistance, short cable length and long fault distance. In order to solve this problem, a novel single-end fault location method based on π equivalent model and apparent pseudo-impedance identification is presented, taking the single pole ground fault as an example, the principle and implementation steps are expounded.. The π equivalent cable model is adopted in the simulation, the influence of transition resistance and fault distance on the positioning results are discussed. Simulation results demonstrate that the phase difference between voltage and current changes from positive to negative along with the increase of the transition resistance and the decrease of the fault distance, that means the apparent pseudo-impedance changes from inductive to capacitive. It is proved that the fault location based on the RL cable model is not suitable for the occasion of large transition resistance and small fault distance. However the single-ended fault location method in this thesis is theoretically eliminates the influence of transition resistance and fault distance, so the location accuracy is high. The distribution capacitance of the cable is considered in this method, which is in line with the actual situation.