Study On Low Voltage Ride Through Of Wind Turbine With Doubly-Fed Induction Generator

With the progress of productivity and technology since the new century, people’s energy demand is increasing quickly and energy crisis is becoming more and more serious, at the same time, the excessive development of traditional energy sources such as oil, coal will cause the deterioration of environment pollution, so how to better development and utilization of renewable energy and new energy has become a problem which urgent need to solve, as a result, under the current international society wind power has become one of the hot topic in the forefront area of clean energy. Due to the installed capacity of wind turbine has been increasing continuously in the transmission system, when the grid voltage dips wind turbines tripping will be very adverse impact to the stability of the operation of power grids, thus many countries in the world have introduced the corresponding regulations for wind power connecting to the system. According to the standard wind turbines should have a certain ability of low voltage ride through (LVRT) and can provide power to the grid during network fault.Doubly-fed induction generator (DFIG) as a mainstream model of the wind turbines, this paper first describes its running mechanism and structure characteristics in detail, then based on mathematical models of DFIG are established and expressed in two-phase rotating reference frame under ideal conditions, the mathematical models of DFIG’s grid-side converter & rotor-side converter are expressed when the strategies of stator flux oriented control (SFO) & grid voltage orientation control (VOC) are adopted. Simulation studies of DFIG’s running state are carried out in this paper with Matlab/Simulink during grid failure, the simulation results show that DIFG’s ability of LVRT is effectively improved when adopting vector control during the grid voltage dips is not serious.Mathematical models of flux linkage & current for stator & rotor are established in detail when DFIG is running in steady state or transient state, the transient mathematical model of DFIG’s physical quantities are expressed, and the oscillation mechanism of DC-link voltage are analyzed during network fault. Based on the transient mathematical model of DFIG, DFIG’s equivalent circuit model are established when adopting Crowbar circuit, then the factors affecting the effects of Crowbar circuit protection are analyzed, and Crowbar circuit resistance are improved, meanwhile, quitting time of Crowbar circuit is optimized. DFIG using Crowbar circuit is simulated during the grid voltage dips is serious, simulation results show that Crowbar circuit could guarantee the safe and stable operation of DFIG’s unit when grid voltage drop seriously.This paper introduces the concept of LVRT and its standardization, when the grid voltage dips the characteristics of DFIG are analyzed, and the implementation of wind power generation system based on DFIG are summarized and evaluated from tow aspects of control method and hardware, finally the development trend of LVRT technology is previewed.