Research On Influence Of Harmonics On The Failure Of UHVDC Transmission

With the rapid development of Chinese economy,the contradiction between the energy resources in the West and the demand for electricity in the eastern part of the country is becoming increasingly prominent.HVDC(high voltage direct current)technology has become an important energy channel connecting the West and the East.UHVDC(ultra-high voltage direct current)transmission technology has also been unprecedented development in China.However,commutation failure in HVDC systems is still one of the difficult problems yet to be solved.The commutation failure,which is mainly caused by voltage drop and voltage distortion,seriously affected the normal operation of UHVDC.At present,there is much research on commutation failure caused by voltage reduction,and the latter is less studied.Harmonic is an important cause of system voltage distortion,and there are abundant harmonic sources in UHVDC.Therefore,it is necessary to further investigate the effect of harmonic commutation on commutation failure.In this paper,the influence of harmonics on commutation failure is discussed from the aspects of the mechanism of commutation failure and harmonic generation?electromagnetic transient simulation and analysis calculation..The main work is as follows:The first,the model of ± 800 kV UHVDC transmission is set up in PSCAD / EMTDC,and the voltage and current waveforms of the rectifier side and the inverter side,the waveform of DC voltage and current are simulated and analyzed.Simulation results verify the correctness of the simulation model.The second,harmonics are numerous,complex and diverse.In order to study the influence of harmonics on the commutation failure of UHVDC transmission,this paper introduces the harmonic injection method and builds the harmonic injection model.In the UHVDC,single-phase harmonic source?negative sequence and positive sequence harmonics were injected into the DC transmission line and the rectifier station AC side respectively.The simulation results show that In the system,the lower the harmonic number and the higher the amplitude,the greater the commutation process will affect the converter.Many simulations show that negative sequence harmonic has a great influence on the commutation failure than positive sequence harmonic.The third,The fault conditions of bipolar balanced operation and unipolar-ground operation are simulated,and verify the harmonics produced by DC bias aggravate the failure of commutation.In order to further explore the influence of harmonics on the commutation failure of DC transmission,the method of analyzing the time and area of harmonic commutation voltage is adopted.The negative influence of harmonics exceeds a certain value,the commutation failure phenomenon will appear in the converter,and the harmonics increase the possibility of its occurrence.