A New Principle Of AC Transmission Line Protection Based On Wave Characteristic Impedance

With the continuous expansion and development of interconnected power grid in China,the system capacity is increasing,the voltage level is getting higher and higher,and the structure of electrical network has been more and more complex,which requires higher selectivity of relay protection.However,the existing protection of transmission line,such as longitudinal differential protection and distance protection,is affected by factors such as ground capacitance current,current transformer saturation and transition resistance.Therefore,it is of great theoretical significance and practical value to study the new principle of transmission line protection in order to meet the development needs of the power system under the new form and ensure the safe and stable operation of the power grid.In this paper,the distribution parameter model is used to derive the hyperbolic function transmission equation of the transmission lines.On this basis,the voltage and current of each point on the transmission line are decomposed into two parts: the forward traveling wave and the backward traveling wave.This paper analyzes the transmission characteristics of the forward traveling wave and the backward traveling wave of voltage and current between the head and the end of transmission line.Based on the decay characteristics of voltage fault component at the beginning of the line and differential current between two ends of the line,the forward wave characteristics impedance and the backward wave characteristics impedance are constructed.The characteristics and difference between the forward wave characteristic impedance and the backward wave characteristic impedance under internal faults and external faults are analyzed.The criterion of protection action and the principle of setting calculation based on the wave characteristic impedance protection are proposed.The practical criterion of the protection in three-phase system is also studied.This paper establishes the simulation model of bilateral-power-supplied transmission line based on the SIMULINK module of MATLAB,and realizes the simulation of various kinds of faults in AC transmission system.The simulation results show that the proposed protection based on the wave characteristic impedances can accurately identify the internal and external faults of transmission lines,has clear selectivity,can protect the full length of transmission lines,and is not affected by short-circuit point transition resistance.This protection is suitable for 110 k V and higher voltage transmission lines as the main protection.