Study On Commutation Failure Identification And Line Protection In UHVDC System

With the implementation of the national strategic policy of "west-to-east power transmission,south-to-north mutual supply and national interconnection",it has become a trend to accelerate the construction of UHVDC projects with voltage levels of ±800k V and ±1100k V.Commutation failure at inverter-side and transmission line fault are the two most common faults in DC system.In this paper,UHVDC system is taken as the research object and the simulation model is established.By combining theoretical analysis with verification in PSCAD/EMTDC and starting from AC and DC side faults,two kinds of commutation failure identification methods and two kinds of DC line protection principles are proposed.Firstly,based on the fault characteristics of AC side of UHVDC system,two identification methods of commutation failure in UHVDC system are studied.The influence of harmonics on commutation failure is analyzed and the Commutation Failure Risk Index(CFRI)is put forward,and CFRI amplitude can be used to judge whether the commutation failure occurs in UHVDC system.Then,in order to further obtain the information of frequency,duration and failure valve of commutation failure,an improved commutation failure identification method based on valve voltage or valve current is proposed.Finally,effectiveness of the two methods is verified by PSCAD/EMTDC simulation.Secondly,the fault characteristics of the dc side of the uhvdc system are discussed,and two protection principles of UHVDC lines are proposed.The relationship of initial traveling wave on both sides of lines is analyzed,then the novel protection principle based on difference between the actual and calculated backward traveling wave at the side away from the fault point is proposed.This method has the advantages of faster action time,better resistance to high resistance fault,better pole selection,good adaption to lightning interference and lower sampling frequency.Different types of faults with different transition resistances are set in PSCAD/EMTDC,proving that the protection principle can operate correctly and reliably.Thirdly,in order to further reduce the transmission burden and solve the problem that the previous method needs to change the setting value in different projects,an improved pilot protection based on traveling wave tuning energy is proposed.This method has the advantages of low communication burden and good universality in different DC projects,and its threshold setting is independent on simulation.But its sampling rate is high,and it is affected by lightning interference.The above scheme is simulated and verified in PSCAD/EMTDC,which proves that the protection schemes can act correctly and quickly under different fault resistances and different types of faults.Finally,the main research contents and results were summarized.