Study On The Influence Of Harmonic On Commutation Failure Of UHVDC In Split Connection To AC Grid

Commutation failure is a typical fault of DC transmission system.In order to enhance the voltage support capacity of the AC power grid at receiving end,the current UHVDC project adopts the method of layered access to the AC power grid at receiving side,which also leads to more complex commutation failure problem.Under some conditions,the traditional strategy to prevent commutation failure in UHVDC transmission can not meet the operation requirements.Based on the commutation failure in the actual Xitai project,this paper studies the mechanism and control strategy of commutation failure caused by the waveform distortion of commutation voltage and the interaction of high and low end valve groups in UHVDC system in split connection to the AC grid.Firstly,for studying the commutation failure of the practical project,it is necessary to build a simulation model consistent with the actual project.Taking Xitai UHVDC project as the research object,this paper studies the mathematical model and simulation model of UHVDC receiving end hierarchical access system,establishes the simulation model of Xitai UHVDC project in PSCAD/EMTDC,and verifies the consistency of the model and the project through the verification of steady-state and transient characteristics.Secondly,based on the commutation failure during the commissioning of Xitai DC project,the influence of harmonics on commutation failure is analyzed,and the calculation method of turn off angle considering the influence of harmonics is proposed.This paper first introduces the specific commutation failure problems in the commissioning of Xitai DC project,and analyzes that the harmonic is the main reason for the commutation failure when the valve group is put into online test.Based on the commutation mechanism,this paper analyzes the influence of harmonics on commutation process,and considers that the harmonics not only interfere with the local valve group,but also interact with the other valve group in the same pole under the layered access mode,so as to obtain the calculation method of high and low commutation related breaking angle considering harmonic amplitude,harmonic phase angle and harmonic number under the harmonic interference.Finally,according to the mechanism analysis of commutation failure caused by harmonics,an improved strategy of preventing commutation failure based on harmonic distortion rate is proposed.On the basis of conventional commutation failure prevention control,the detection module of harmonic distortion rate is added,and the forward trigger angle is adjusted according to harmonic distortion rate to prevent commutation failure caused by harmonics.Secondly,considering the mutual influence of the same high and low end converters in the receiving end hierarchical access mode of DC system,in order to avoid commutation failure of non fault layer valve group caused by commutation failure of fault layer valve group,the control level is adjusted.In this chapter,the trigger angle control is adjusted according to the DC current,and then the DC current criterion is added in the fault detection module to improve the overall control structure after the fault.After the fault,the high and low end prevent the commutation failure and coordinate the control,so as to suppress the continuous commutation failure of the DC system under the hierarchical access mode.