Study On Integral Based Travelling Wave Directional Unit Protection For Transmission Line

With the rapid growth of power load,power transmitted from western district to eastern district,power supplied by south and north mutual and implementing of power grid interlink project,an extra-huge scale power grid is forming gradually. For the staffs engaging in power field,the safity and stability of power transmitting with high capacity and remote distance in a huge scale power grid is a challenge.For extra / ultra high voltage transmission lines,the protective relay should has a more excellent performance,such as operation speed,reliability and sensitivity and so on. In the condition of operation speed approaching limit of the relay based on power frequency,travelling wave protections had been focused on owing to it has an ability of finding fault rapidly,but the shortcoming of lower reliability had always restricted development of device.Therefore,a high reliable travelling wave directional protection scheme is proposed,as well as retaining the ability of detecting fault rapidly.In the studying process,main results obtained are following:(1) A novel integral based travelling wave directional protection is proposed.After fault occurred,using the constant relationship of forward and backward travelling wave during a special period,the waveforms of forward and backward travelling wave are integrated respectively.According to their ratio,the direction of fault is discriminated.Simulation results from 500kV and 1000kV transmission lines show that the presented directional criterion can completely adapt any constructions of line or busbar,and conditions of fault do not have any influence on performances of criterion.Therefore,the lower reliability and sensibility of traditional travelling wave protection only detecting the polarity or complitude of initial travelling wave are solved.In addition,for these transmission lines of medium or short distance, protection has quick discrimination speed of fault direction.For long distance transmission lines,as a long integral window,discrimination speed is relative slow. Analysis verifies,for the romote distance lines,that discrimination speed can be improved through adjusting time-window(such as using half integral time). Simultaion results clarify protective criterion has not been affected using half integral time. (2) On the basis of analyzing transient waveform characteristics of lightning stroke without leading to fault,lightning stroke with resulting in fault and general short-circuit fault,a discrimination criterion of lightning stroke and fault based on waveform integral of current travelling wave is proposed.For different lightning strokes and faults,lots of simulations have been carried out.Simulation and analysis demonstrate that the criterion can quickly and correctly determined fault or lightning stroke,and it is a simple and liable discrimination method.In addition,the paper analyzes mechanism of travelling wave generated by breaker switching.According to the fact of PT located at the side of line in high voltage transmission system,a method determining direction of disturbance is proposed using the integral based travelling wave amplitude comparison directional criterion,at the same time, simulation of breaker switching is carried out.Simulation results verify that the criterion can accurately discriminate the direction of disturbance source.(3) Aming at the shortcoming that CVT can not effectively transduce transient high frequency signals,a method extracting voltage travelling wave signal is proposed.Using tap capacitance of capacitance transducer,series and shunt resonance circuit is designed.Using simulation model,output responses of extraction circuit to different frequency signals and different fault ground resistances and different fault inception angles are checked and analyzed respectively.Theoretic analysis and simulation results demonstrate that the method can effectively restrain power frequency signals,accurately reflect transient voltage travelling wave signals, and thus solve technical problem extracting voltage travelling wave for travelling wave protection.(4) A set of integral based travelling wave directional unit protection scheme is designed.The main components,such as fault start-up component,faulted phase selection component,fault direction discrimination component and lightning stroke and breaker switching and so on,are analyzed and simulated.Analysis of principle and simulation results demonstrate that the integral criterion do not be affected by conditions of fault and for the length and structure of line,it has a good applicability;the applicability and reliability and sensibility of travelling directional protection are enormously improved;lightning stroke criterion can accurately discriminate general line fault from other disturbances;direction of various breaker switching can also be correctly determined by integral critertion and the designed circuit can obtain transient voltage travelling wave correctly and effectively.