Research On Protection Of Hybrid Double-end HVDC Transmission Lines

It has a very broad application prospect and has become the current research hotspots in the field of DC transmission.In order to ensure the safe operation of the hybrid double-ended HVDC transmission system,it is necessary to study the new principle of hybrid DC transmission line protection.The topology and mathematical model of the hybrid double-ended DC transmission system are analyzed and introduced.The principle of selecting the main component parameters in the model is studied,and the control methods of the converter stations at both ends of the system are given.A ± 500kV hybrid double-ended DC transmission system model is built in PSCAD.Aiming at the research on the fault characteristics of the hybrid double-ended DC transmission system after a fault occurring on the DC line,the protection method of the hybrid double-ended DC transmission line is studied.A pilot protection method based on the correlation coefficient of hybrid double-terminal HVDC transmission line is proposed.The protection method uses the characteristic that the correlation coefficient can reflect the degree of linear correlation between the two sets of data,and it uses the absolute value of the correlation coefficient obtained from the calculation of the correlation coefficient of the current at both ends of the system as the criterion to identify the fault type and to select the fault pole.Simulation results show that the protection method can quickly and effectively identify internal fault in various situations,and can also distinguish high-resistance ground faults.The transient protection scheme based on the improved local mean decomposition(LMD)decomposition is studied.This protection scheme analyzes the blocking characteristics of the boundary elements at both ends of the hybrid double-terminal DC transmission line and extracts the high-frequency components of the transient voltage signal using the improved LMD decomposition.Then,the PF component obtained by decomposition is calculated for time-frequency distribution energy and information entropy.According to the difference of the high-frequency component content,the instantaneous energy protection criterion and the information entropy protection criterion are proposed.The ptotection scheme can identify the fault pole without the extra pole selection criterion.It uses the instantaneous energy protection as main protection,and uses the information entropy protection as the backup protection.The simulation results verify that all the criteria of the protection scheme can accurately identify all the internal or external faults on the DC transmission line,the ability to resist the transition resistance is strong,and no malfunction will occur.