A New Principle Of Traveling Wave Differential Protection For HVDC Transmission Lines

Since the advent of high voltage direct current(HVDC)transmission technology in 1954,due to its advantages of large transmission capacity,long transmission distance and excellent controlling performance.HVDC transmission technology has been widely applied to long-distance large-capacity transmission,urban power grid expansion,asynchronous power grid interconnection,isolated island power supply,new energy generation connection to grids and other fields.It plays an increasingly important role in the power system.However,HVDC transmission lines have the following characteristics: long transmission distance,poor working conditions and high failure rate.Therefore,it is of great significance to study the protection of HVDC transmission lines to ensure the safe and stable operation of the power grid.This paper introduces the basic structure,control system and control strategy of HVDC transmission system.Based on PSCAD/EMTDC,the simulation model of HVDC transmission system is built and the simulations of the normal working conditions and faults of HVDC transmission system are realized.In this paper,the distribution parameter model is used to derive the hyperbolic function transmission equation of the transmission lines.The current and voltage of each frequency are decomposed into two parts: the forward traveling wave and the backward traveling wave.The physical meaning and relationship of the forward and backward traveling waves at both ends of the transmission lines are described.Based on the forward traveling wave current and the backward traveling wave current of the direct current component,through the hyperbolic function transmission equation,the calculated value of the opposite side is obtained by using the measured value of this side.And then the calculated value of the opposite side is subtracted from the measured value of the opposite side to form the forward traveling wave differential current and the backward traveling wave differential currents.So,a new principle of traveling wave differential protection for HVDC transmission lines based on the forward and backward traveling wave currents is proposed.The phase-mode transformation is used to decouple the electric quantity of the bipolar HVDC transmission lines.The forward traveling wave differential current and the backward traveling wave differential current based on the line mode of direct current component constitute the selected-area criterion of the bipolar HVDC transmission lines.By analyzing the relationship between the ground mode current on the installation site of the protection device and the ground mode current at the fault point after fault,the ground mode component of direct current is used to constitute the selected-fault-pole criterion.On this basis,the principles of setting calculation for the selected-area criterion and the selected-fault-pole criterion of the protection are given.The protection proposed in this paper theoretically solves the unbalanced current problem of current amplitude attenuation caused by distributed parameter in the transmission lines transmission process.The simulation results show that the proposed traveling wave differential protection can correctly distinguish fault from inside or outside and select the fault pole,which is suitable for backup protection of HVDC transmission lines.