Distribution And Suppression Strategy For Ground Currents In Transformer Neutral Points

Since there is a long distance between the location of energy resources and that of demand for power loads,long-distance power transmission is needed.With the advantages of longdistance and high-power transmission,high voltage direct current systems have been widely constructed in China.However,when a high voltage direct current system is in monopole ground return mode,the operation currents will flow to the earth.The ground currents will invade power systems through the transformer neutral points,which will have bad effects on the stability of power systems.However,due to the complex electrical connection among transformers,the suppression of one transformer’s neutral point ground current may aggravate the direct ground current intrusion of other transformers passively.Thus,there is a high requirement for the overall suppression of regional ground currents of multi transformer neutral points.At the same time,with the development of economy,the scale of cities expands continuously.Therefore,conventional public transportations cannot meet people’s needs for fast travel.Thus,in recent years,rail transit with the characteristics of safety and large passenger capacity has developed rapidly in China.However,when rail transits are in operation,they will also produce ground currents.The changes are more frequent and complex than those produced by high voltage direct current systems.Therefore,it is necessary to study the distribution of ground currents at the neutral points of transformers and the influence of related factors,and propose the corresponding suppression strategy.Firstly,based on the field circuit coupling theory,the equivalent resistance model of direct ground currents in transformer neutral points produced by high voltage direct current systems is established.Then,the influence of the transmission current,the surface area of the grounding electrode,the surface area of grounding nets,the type of transformers,the type of transmission lines,the distance between substations,the distance between the substation and high voltage direct current system,and soil resistivity on the distribution of ground currents is analyzed.At the same time,based on the model of rail,drainage nets,and earth,a neutral point ground current distribution model considering multi trains and traction substations rather than single train in conventional methods is proposed.The influence of the rail longitudinal resistance,the longitudinal resistance of drainage nets,the transition resistance between rails and drainage nets,the transition resistance between drainage nets and earth,the distance between traction substations,and train operation conditions on stray currents is analyzed.Aming at the disadvantage of ignoring the measured data in conventional methods,a method of soil resistivity calculation based on the measured data and particle swarm optimization is proposed to improve the accuracy of the model.Secondly,aiming at the disadvantage of aggravating the direct ground current intrusion of transformers in the outside region,an optimal configuration and switching strategy of blocking devices based on non-dominated sorting genetic algorithm Ⅱ for suppressing region ground currents produced by high voltage direct current system and rail transit is proposed.More specifically,the high voltage direct current system switching strategy is determined based on the working condition of high voltage direct current systems.Besides,the rail transit switching strategy is determined by analyzing the possible relationship between ground currents and passenger loads and the change of passenger loads.Finally,a suppression strategy for ground currents with harmonic and direct components in transformer neutral points is proposed.The mathematical model,the selection of main circuit parameters,and the design of control strategy are explained.At the same time,the simulation results show that the strategy can suppress the harmonic current,the direct current,multiple harmonic currents,and multiple harmonic currents superimposed direct current effectively.