Research On The Protection Principle For VSC-HVDC Transmission Line Considering Frequency-dependent Parameters

The voltage source converter based high voltage direct current(VSC-HVDC)transmission technology has the advantages of economy,flexibility and environmental protection.However,due to the small impedance of DC lines,a short-circuit fault in the VSC-HVDC transmission line will lead to a rapid rise of current and a sudden drop of voltage,which may pose a serious threat to the safe operation of the system and the equipment.It is of great practical significance to study the protection principle that can enable fast,reliable,and selective fault clearing to ensure the security and stability of the VSC-HVDC transmission system.In this paper,the propagating characteristics of voltage traveling wave after a fault occurs in the VSC-HVDC transmission line were analyzed,and then a voltage traveling wave differential protection principle and a distance protection principle considering the frequency-dependent line parameters were further studied.The main research work and achievements are as follows:The calculation method of frequency-dependent transmission line parameters using equivalent geometrical parameters derived by fixed-frequency parameters was studied.Based on the analysis of the calculation of self-impedance and mutual impedance using Carson formula,the expressions of the equivalent geometrical parameters of the line using the line impedance were proposed.On this basis,if both the impedance and capacitance of the transmission line are given,the method for improving the accuracy of the equivalent geometric parameters was studied.The voltage traveling wave differential protection principle for VSC-HVDC transmission line was studied.The propagating characteristics of voltage traveling wave after a fault occurs in the VSC-HVDC transmission line were analyzed.A protection principle using the differential voltage traveling wave based on the frequencydependent transmission line model in time domain and a data out-of-sync detection method were proposed.The causes of operation delay were further analyzed and the delay time was estimated.The distance protection principle for detecting the pole-to-pole fault of VSC-HVDC transmission line was studied.The relationship between the travel time of the fastest waves and the distance of the line was analyzed.The algorithm of recursive convolution considering the frequency-dependent parameters was studied.On this basis,a distance protection using the minimum variance sum of recursive coefficients to solve the fault distance was proposed.The influences of the numbers of the zeros and poles,the fault impedance and the sampling frequency were analyzed.