Research On The Relay Protection Method Of Radial Medium Voltage DC Distribution Network

DC power distribution network has incomparable advantages over AC power distribution network in power quality,stability,transmission capacity and other aspects,and gradually occupies the highland of distribution network development.However,the research on the protection method of the DC distribution network is not perfect and needs further exploration.This paper mainly analyzes and studies this problem,and the main work is as follows:The working principle and control method of the VSC converter and DC/DC transformer are analyzed,and the ±12kV radial medium-voltage DC distribution network model is built in PSCAD and the correctness is verified.Based on the established simulation model,the fault characteristics of distribution lines are analyzed,and the polarity of fault current components and the difference characteristics of 0-mode power between normal operation of DC line and occurrence of single-pole fault are studied.Then,the fault identification criteria are set.And simulation results show that the method is fast and effective.For various fault conditions of the DC line,the difference characteristics of amplitude of 1-mode power transmitted by DC lines under normal working conditions and fault conditions are analyzed,and the identification criteria of internal and external faults are obtained by combining the positive and negative of 1-mode power increment of sub-feeders.Meanwhile,the difference characteristics of the transient energy of the positive and negative current of the DC line under the fault occurring are analyzed to set the fault pole selection criterion.Subsequently,the simulation results show that this method can solve all kinds of faults of DC line,and has good performance in resisting transition resistance and noise interference.The current-limiting inductor is introduced as boundary element of the DC distribution line in the established DC distribution network,and its high-frequency transient energy is extracted by wavelet packet transform.The difference of the sum of transient energy in high frequency band is compared and the fault area is identified by combining the positive and negative voltage of current-limiting inductor of the main feeder and sub-feeder lines.Then,the Pearson correlation coefficient is introduced to calculate the correlation of high frequency transient energy before and after distribution line fault to identify single-pole and pole-to-pole faults and fault poles.The simulation results show that this method is better in the tolerance of transition resistance and also has the ability to tolerate noise interference.Finally,the performance of the three protection methods proposed in this paper is compared,and the configuration of the overall protection scheme of DC distribution network is obtained.And the adaptability of the proposed protection method is verified for DC distribution network based on MMC converter,and the results confirm the strong adaptability of the proposed protection method.