Research On New Strategy Of Relay Protection For EHV Power Network With Sampling Abnormality

Traditional power grids are developing toward smart grids,and traditional substations are also moving toward digital and intelligent substations.In order to adapt to the development of the power gird,its sampling terminal equipment is also developing from conventional transformer to electronic transformer.However,due to the poor antiinterference ability of the electronic transformer and unstable operation,it can not completely replace the status of the conventional transformer.Therefore,there is a situation in which two kinds of sampling terminao devices,such as electronic transformer and conventional transformer,coexist in the power gird.But no matter conventional transformers or electronic transformers,there is a similar problem,that is,the protection information of the transformer sampling is abnormal.Conventional tansformers are prone to saturation,causing the seceondary current transfer abnormalities;Electronic transformers are susceptible to environmental impact and lead to abnormal sampling data.In addition,the disconnection of the transformer during actual operation will also cause the protection information to be abnormal.To this end,this paper analyzes the impact of protection information anomaly on relay protection,in detail,and then proposes some new countermeasures,which can be divided into the following three aspects:(1)Analysis of the impact of protection information anomalies on relay protection from different scenarios such as conventional transformer saturation,sampling data anomalies and transformer disconnection.Firstly,the influence of the protection filtering algorithm is analyzed from the theoretical point of view.Then the PSCAD simulation model is established to analyze the influence of various information anomaly scenes on the current differential protection/distance protection action behavior,which lays a foundation for the subsequent new countermeasures.(2)Vulnerable to the transformer disconnection and the abnormity of protection data,the pilot differential protection for UHV transmission line is prone to maloperation or even failure.In order to solve this problem,a novel UHV transmission line protection scheme,on basis of the characteristics of Bergeron model and the Hausdorff algorithm,is proposed.The criterion,which is designed by the Bergeron model and the Hausdorff algorithm,can identify the internal and external fault accurately by comparing the Hausdorff distance with the setting threshold.And the Hausdorff distance is calculated between the observed values and the deductive values through the criterion.This eliminates the influence of the capacitive current of the UHV transmission line and improves the protection sensitivity while enhance the anti-interference ability of the criterion.On this basis,the redundant data groups of voltage and current are constructed,and In order to solve the problem of the transmission line pilot protection which is subject to the breaking of CT or CVT,a reliable protection scheme,which is fully adapt to extreme conditions such as breaking of CT or CVT,is proposed.Theoretical analysis and simulation results indicate that the criterion proposed in this paper have high sensitivity and ability to tolerate data abnormality.(3)In order to solve the problem of busbar protection misoperation caused by the extreme fast saturation of CT outside the busbar area,this paper introduces for the first time the Superimposed Distance Protection(SDP)principle to implement a CT saturation blocking and reopening scheme which is suitable for bus differential protection.After busbar protection is started,this scheme,firstly,uses the phaselet algorithm of a quarter-cycle data window to calculate the action of SDP corresponding to each CT connected with busbar.Considering the bus fault and line fault can not occur simultaneously,when the action of SDP,which is corresponding to a certain line current transformer,is detected,therefore the current transformer of this line is considered to be saturated,and then the busbar protection locking signal is issued,as the data window of SDP is smaller than the data window of the busbar protection,the busbar protection can be successfully locked before it operates.Besides,according to the action of SDP in any one cycle under different fault types,the busbar protection reopening criterion is proposed to realize the busbar protection’s rapid opening within a cycle wave when an external fault transfers to internal.Theoretical analysis and simulation results show that the blocking criterion can clearly distinguish internal and external faults and CT saturation from each other,even with the CT super-fast deep saturation,the locking criterion can rapidly and reliably block busbar protection;And,the reopening criterion can also trip rapidly when an external fault transfers to internal,allowing the busbar protection to respond quickly to evolved fault.