Fault Analysis And Distance Protection Research Of High Voltage Transmission Lines Connected To Wind Farms

At present,wind farm transmission lines still use the traditional protection scheme,without considering the particularity of wind power systems,which leads to frequent wind power accidents.Doubly-Fed Induction Generators have been widely used in existing wind farms.However,their fault characteristics are different from that of synchronous generators,which has a great impact on the applicability of traditional relay protection,especially traditional distance protection.Therefore,it is necessary to further study the fault characteristics of the wind farm integrated into power system and the applicability of distance protection.This paper studies the composition of a Doubly-Fed Induction Generator,the principle of variable-speed constant-frequency operation,mathematical model,control strategies for rotor-side and grid-side converters,and the working principle of the Crowbar circuit.On this basis,the expressions of the positive and negative sequence short-circuit currents provided by the Doubly-Fed Induction Generator are deduced.It is found that the stator short circuit current consists of a decaying DC component,a decaying AC component at rotor speed frequency,and a forced AC component at synchronous frequency.The paper describes the composition and equivalent method of the wind farm integrated into power system and uses Matlab/Simulink to build the simulation model according to the actual parameters.This paper analyzes three real fault cases of DFIG-based wind farms.The first one is a single-phase ground fault occurs on the transmission line connected to a DFIG-based wind farm without LVRT capability.Another two are single-phase ground and phase-to-phase faults occur on the transmission line connected to a DFIG-based wind farm with LVRT capability,respectively.Based on the theory,fault cases and simulations,the characteristics of impedance,weak-feedback and frequency deviation,when faults occur on transmission lines connected to wind farm,are deeply studied.Finally,the applicability of the distance protection for the transmission line is studied.It is pointed out that the traditional distance protection based on the Fourier algorithm is no longer reliable.A time-domain distance protection method combined with least-squares method is proposed.The method is not affected by the frequency and can calculate the inductance of the fault point to the protection installation so that fault distances can be accurately and quickly identified when faults occur on the transmission line connected to a DFIG-based wind farm.The efficacy of the proposed method is validated with real fault cases and simulation.The research in this paper helps to improve the understanding of the fault characteristics of the wind farm integrated into power system and ensure the reliability of the system.