Research On Fault Identification And Location Technology Of MMC-HVDC Transmission Lines

With the advantages of large capacity and long distance transmission,flexible direct current(VDC)transmission technology has been widely used in the field of HVDC transmission system.Although the technology development of MMC-HVDC system has been relatively mature,it still faces many problems in large-scale application.Due to the low damping,low line resistance and many nonlinear components of the DC system,the fault current rises fast,the peak value is large and the fault transient process is complex,which brings challenges to the DC fault protection.Therefore,reliable fault identification and fault location methods are needed to quickly and accurately detect the fault location,provide a basis for rapid fault removal,and ensure the safe and stable operation of the DC system.This thesis first analyzes the basic structure and operation principle of the MMC-HVDC system,and adopts a Thevinan equivalent MMC integral modeling method to solve the problems that the simulation model runs slowly and the system debugging can not effectively simulate,which most of the steady-state and transient characteristic curves of the converter station.An efficient simulation model of double-terminal 101-level MMC-HVDC system was built in PSCAD/EMTDC,and the main fault types of AC side and DC side were analyzed.Secondly,a fault identification method based on EWT energy entropy is proposed to solve the DC fault identification problem.Based on the energy entropy theory of empirical wavelet transform,this method takes the EWT energy entropy value as the criterion to identify the unipolar fault and the bipolar fault,and uses the positive and negative voltage coefficients to judge the fault line polarity when the unipolar fault occurs.The analysis results show that this method can reliably identify the fault poles of DC line.Finally,on the basis of fault identification,the fault location method of MMC-HVDC DC line based on empirical mode decomposition is studied.Based on the EMD algorithm in HHT,which has the ability to detect signal mutation points,the transient signal characteristics of transmission lines are extracted,and the fault location scheme of DC lines is proposed in combination with the principle of double-terminal traveling wave location algorithm.The simulation analysis of single-pole grounding fault and bipolar short circuit fault of HVDC transmission line under different transition resistances shows that the method is suitable for different fault situations,and the ranging accuracy is not affected by the transition resistances,so it has good adaptability and accuracy.Through the above theory and simulation experiment analysis,the algorithm used in the simulation experiment can get good results of pole selection and ranging,and the results have a certain theoretical reference value.