Research On Accurate Simulation Technique Of DC Transmission Line Fault

High voltage DC transmission technology based on voltage source converter(VSC-HVDC)was firstly proposed by Ooi Boon-Teck and others in 1990,the voltage source converter(VSC)which is composed of wholly-controlled power electronic device is used to replace the current source converter which is based on semi-controlled thyristor device.As a new generation of DC transmission technology,VSC-HVDC can effectively overcome some disadvantages of traditional HVDC.It applied to wind power integration,Island power supply,Asynchronous interconnection of AC systems,interconnection of distributed generation,MTDC,expansion of urban distribution network and other fields.Application of VSC-HVDC in long distance and large capacity overhead line transmission is the future trend of the development of power grid technology.Compared with DC cable,overhead lines are generally long and have a poor transmission environment.So the failure rate of overhead lines is higher than other transmission equipment.Line protection plays an important role in DC system.How quickly to identify the fault is an important problem of DC protection.In this paper,the problem of VSC-HVDC line protection is studied.First of all,the operation principle and modulation mode of two level VSC are summary analyzed and simulation models of two-terminal VSC-HVDC and VSC-MTDC are established in PSCAD/EMTDC which with overhead lines of distributed parameters.Study on the operating and fault characteristics of VSC-HVDC according to the simulation of various circuit faults.Secondly,applying the principle of superposition to study the propagation characteristics of single non-loss transmission line.Because of the low reliability of traditional traveling wave protection,The effect of sampling rate on improving the reliability is studied.By comparing the sampling rate of 10k~100kHz range denotes that the improvement of sampling rate can make protection identify fault more accurately.Finally,the sensitivity of traditional traveling wave protection is relatively low when there is high resistance grounding fault.This paper presents a new traveling wave protection scheme for VSC-HVDC transmission lines according to the propagation characteristics of fault wave.This protection using wavelet transform modulus maximum to identify a fault internal or external,using the pole wave difference value to detect the fault pole.The simulation analysis of a two-terminal VSC-HVDC on PSCAD/EMTDC platform validates that this new wave protection can accurately identify transmission line fault within 1ms and its adaptability to earth resistance are not less than 1000?,it has good sensitivity and speed.