| Many electromagnetic observation stations undertake the important task of monitoring and predicting seismic activity.Among them,geoelectric field observation is one of the important means of electromagnetic precursor monitoring.A number of High Voltage Direct Current(HVDC)transmission projects have been put into operation nationwide with the implementation of the strategy of "transmission of electricity from west to East,exchange of electricity between south and north,and the interconnection of grids",which causing increasingly interference on geoelectric field observation.In this paper,the characteristics of influence and mechanism of HVDC on geoelectric field observation are studied,which will provide a basis for further identification and judgment of such interference.First of all,this paper introduces the HVDC transmission system and the characteristics of geoelectric field observation data,and describes the interference on geoelectric field observation when the HVDC transmission system fails or testing.By collecting typical interference events,we found characteristics of the interference,and the typical interference patterns are summarized statistically,including square wave,pulse and square wave+pulse,among which square wave is the main one,accounting for about 65.5%.Secondly,based on the working principle of HVDC transmission system and geoelectric field observation instrument,we discuss the causes and mechanism of the interference,which may include the earth-fault current,line induction electric field and line total electric field,etc.,and further study makes clear that the main factors are the earth-fault current and line induction electric field.In addition,the intensity formula of the additional geoelectric field caused by the earth-fault current and the numerical variation law of the line induction electric field are analyzed,and the interference models are established.Thirdly,combined with the influence mechanism,the causes of different interference forms are discussed.It is believed that in the area near the ground pole,the interference is mainly caused by the earth-fault current,which is in the form of square wave;in the central area of the HVDC system,the interference is mainly caused by the line induction electric field,which is in the form of pulse;in the coincidence area,the interference is affected by the combination of the earth-fault current and the line induction electric field,which is in the form of square wave and pulse.Then,by comparing different interference events of the same HVDC system,we analyze the relationships between the interference characteristics and the distance from the electrode,the earth-fault current and the underground structure of the area affected.At last,by carrying out the actual observation experiment of the station,it is shown that both the earth-fault current and the line induction electric field will affect the observation of the geoelectric field.The accuracy of the interference models is verified by comparing the measured data and the theoretical results.Besides,the discriminant method of multiple HVDC systems in the same area is preliminarily studied,and the discriminant flow chart of 4 HVDC systems in 4 stations in Jiangsu Province is given,which can provide reference for the discrimination of HVDC transmission systems in the whole country. |
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