Reaserch On Low Voltage Ride Through Of Doubly-fed Wind Power Generation System Using SDBR

Low voltage ride through capacity(LVRT)has gone to be one of the basic requirements for grid-connected wind farms.Doubly fed induction wind power generators have become currently popular because of its many advantages,such as low cost,light weight,variable speed operation,and high control ability.However,since the stator of the doubly-fed induction generator is directly connected to the grid and the capacity of the rotor excitation converter is small,the doubly-fed wind power generation system is easily affected by grid failure and the improvement of its LVRT capacity is limited.The LVRT capacity of doubly-fed wind power generation system has become a major subject for the academic and engineering research.In this paper,researches on the LVRT capacity of doubly-fed wind power generation system are done continually.Firstly,basic structure and operating characteristics of doubly-fed wind power generation system are analysed,and its modular models include aerodynamic system,pitch control system,doubly-fed wind generator and inverter are established.Among them,the DFIG model in static three-phase coordinates and in rotary two-phase coordinates is studied.The control strategy of rotor side converter based on stator flux orientation and grid side converter based on grid voltage orientation are confirmed.So the variable speed constant frequency operation and decoupling control of active power and reactive power are achieved in doubly-fed wind power generation system.The electromagnetic transient characteristics of doubly-fed wind turbine are analysed theoretically when the grid voltage drops sharply.The main causes of over-current in the rotor and DC bus over-voltage in doubly-fed wind power generation system when the faults occur are discussed.The current status of research on LVRT in doubly-fed wind power generation system,the current LVRT standard at home and abroad,and the current methods for improving LVRT capability are studied and analyzed.When symmetric three-phase serious fault occurs in the power grid,the LVRT method using Crowbar circuit will lose control of the power.In view of this situation,the impact of this method on the system was analyzed through simulation and the problems that existed were analyzed.Based on this,a method for synthesizing the series Dynamic Braking Resistor(SDBR)on the rotor side and using a chopper circuit on the DC side and introducing over-ride control of grid side converter is proposed.The principle and the control strategy of LVRT method for coordinated control of SDBR are further studied.Finally,a simulation model of doubly-fed wind power generation system based on PSCAD/EMTDC simulation software is created.A comprehensive control strategy is used for simulation in both super-synchronous and sub-synchronous cases under severe three-phase symmetric and asymmetry grid faults.The simulation verified the effectiveness and superiority of the proposed LVRT scheme.For the influence of different parameter sizes on the system,the parameters of the system are further designed through the time domain simulation analysis method.