Study Of Pilot Protection Algorithm On T Type Network Based On Longitudinal Impedance

With the increasing complexity of power system,and from saving corridor or other investment cost considerations,along with the objective limited conditions,the application of T type circuit in high voltage power network is becoming more and more widely.However,these lines are often connected with power plants or other power systems,once the fault occurs,the protection is required to make judgement rapidly and clear fault in the least time reliably,to minimize economic losses.In various kinds of protection of T type circuits,current differential protection is widely applied,due to its simple principle and advantages of high sensitivity,fast response speed.Although the current differential protection has many advantages,there are still compatibility issues between the reliability sensitivity and external fault of current differential protection in the area of fault,among which the saturation of the current transformer is the most serious.Therefore,based on the deficiency of the differential protection,a study of pilot protection algorithm on T type network based on longitudinal impedance is proposed in this paper.Through the theoretical analysis,pilot protection algorithm on T type network based on longitudinal impedance has the effect of solving the reliability of the external fault and the sensitivity of the internal fault in the area.And the longitudinal impedance has different fault features in internal and external areas,the method of using the amplitude characteristic of longitudinal impedance to distinguish fault zone,is simple and convenient.In three phase circuit R-L model,the decoupling of longitudinal impedance is used to reduce the influence of inter phase coupling,which is consistent with single-phase circuit model.Through the analysis of the longitudinal impedance of current transformer under transient saturation,the longitudinal impedance has strong resistance to current transformer transient saturation,which will not be affected by the transient saturation of current transformer.In this paper,the corresponding circuit models are built in ATP-EMTP simulations,and MATLAB software is used to simulate the data processing.The simulation results show that in the T type line model,when external fault occurs,the amplitude of longitudinal impedance is far greater than the value of the positive sequence impedance,it has strong reliability.When internal fault occurs,the impedance is less than the value of the positive sequence impedance,it has high sensitivity.In the model of transient current transformer saturation,the longitudinal impedance has strong resistance to transient saturation of current transformer capacity.Especially when external fault occurs,no matter how transient saturation of current transformer is,theamplitude of longitudinal impedance is larger than the threshold of protection action,which will not make the protection misoperate.Therefore,the simulation results verify the correctness and effectiveness of longitudinal protection theory longitudinal protection in T networks.The algorithm has certain engineering application value.