Transmission Line Protection For VSC-HVDC Based On Time-frequency Analysis Method

When faults of DC transmission line of High Voltage Direct Current based on Voltage Source Converter(VSC-HVDC)occur,the short-circuit current rises rapidly.The short-circuit impulse current will burn down the power electronic elements of converter in a few milliseconds.Therefore,VSC-HVDC requires extremely high speed of main protection for DC transmission lines.Compared with pilot protection,single-terminal protection need not interact with information by channel.Besides,there are natural characteristics of ultra-high-speed for traveling-wave protection principle.Therefore,the single-terminal traveling-wave protection of DC transmission line is studied in this thesis.This kind of protection principle needs to detect arrival time of traveling-wave accurately.However,time resolution of present time-frequency analysis methods,including wavelet transform,are generally low.Therefore,the improved Transient Extraction Transform(TET)with extremely high time resolution is introduced to detect the arrival time of traveling-wave for the single-terminal traveling-wave protection accurately.Firstly,a complete main protection scheme based on single-terminal traveling-wave is proposed in this thesis.The basis of the proposed scheme includes the basic structure of VSC-HVDC and the theory of traveling-wave transmission.The proposed scheme includes the fault start-up criterion based on the instantaneous change of aerial-mode current,the fault-pole selection criterion based on the existence of zero-mode and identification between external faults and internal faults.Among them,the identification between external faults and internal faults is studied emphatically.Single-pole protection principle and double-pole protection principle are proposed after analyzing the propagation process of traveling-wave when single-pole faults and double-pole faults occur,respectively.The single-pole protection principle is based on the difference of the arrival time between the initial reverse aerial-mode traveling-wave and theinitial reverse zero-mode traveling-wave.The double-pole protection principle is based on the difference of the arrival time between the initial reverse aerial-mode traveling-wave and the second reverse aerial-mode traveling-wave.Single-pole protection criterion and double-pole protection criterion are constructed by the above protection principles,respectively.Secondly,TET is introduced into the proposed protection scheme,because the proposed protection scheme needs to detect the time when traveling-wave arrives at the protection accurately.The original TET method is improved after fully considering the fault traveling-wave characteristics of DC line.As a result,the time resolution is improved greatly,and arrival time of traveling-wave is detected more accurately.The simulation results indicate that the improved TET can detect the abrupt-change time of step abrupt-change signal like traveling-wave accurately.In addition,the improved TET is more superior than wavelet transform that is traditional time-frequency analysis method.Finally,the improved TET is used to detect the arrival time of traveling-wave in the single-pole protection scheme and double-pole protection scheme.The performance of the proposed protection scheme based on the improved TET is verified by the PSCAD/EMTDC simulation software.The simulation results indicate that single-pole protection scheme and double-pole protection scheme based on the improved TET can distinguish internal faults and external faults accurately.There is better ability of resisting transition resistances for single-pole protection scheme and double-pole protection scheme,and it is not affected by DC boundary component.In addition,the simulation results also indicate that performance of single-pole protection scheme based on the improved TET is better than that based on wavelet transform.