Research On The Key Technologies Of Low Voltage Ride Through For Doubly-fed Wind Power Generation System

Many wind farms in China and other countries are located in the placewith poor conditions, sparse population and the access of the wind farms arefar away from the load center, so these grids are comparatively weak. Forsome weak grids, they are vulnerable to be fluctuated; moreover, the voltageof the grids may drop sharply. When the grids are fluctuated, the fluctuationwill cause huge affect on the stability of the wind turbines. To ensure thestability of the grids, nowadays, for the application of the large scale of windpower grid integration, when the grids are faulted, the methods thatdisconnecting the wind turbines with grids will never be allowed by the newgrid rules. Our countries have established the rules of the low voltageride-through (LVRT), and applied the rules to the every wind farms. Thus,the research on the LVRT of the wind turbines has important theoretical andpractical meanings.This paper mentioned the present conditions of wind power at ourcountries and abroad, and included the conditions and correspondingmethods of the LVRT of doubly-fed wind turbine. Moreover, this papercompared the merits and shortcomings of these methods. By strictlytheoretical analysis, this paper deduced the computing values of the stator’sflux linkage and rotor voltage of the doubly-fed induction generator(DFIG)when the grids are faulted, and supplied theoretical basis for the protectingstrategies.Applying non-linear control method based on state feedbacklinearization and depending on the mathematical dynamic modeling of thedoubly-fed wind turbine, the control strategy of the LVRT is designed.Firstly, we selected the output functions according to control target; then, by using linearization the state feedback and proper coordinate transforming,we converted this nonlinear system to the linear and controllable system;finally, through the optimized linear control strategy we designed the controlstrategy of the control system. This control strategy was only depended onthe parameters of the wind turbines, and strengthened the robust of controlstrategies.This paper analyzed that one of the most effective methods fordoubly-fed induction generator to deal with the severe LVRT is connectingCrowbar to rotor side to protect the doubly-fed induction generator andinvertors. By using the vector methods and stable circuit methods, the paperanalyzed that the Crowbar of AC sides will cause the inverse vibration of thegenerators torque. This paper found the equation of the electromagnetictorque for the parameters optimization of the Crowbar to decrease thevibration of the generator and protect the shaft system of the generator. Atthe same time, this paper analyzed the issue of reactive power absorption ofthe doubly-fed induction generator using Crowbar, and set up the reactivecompensation strategy based on the maximal current methods.This paper designed a7.5KW variable speed constant frequencydoubly-fed wind power system laboratory platform composed by virtualwind farm, wind resource imitation system, doubly-fed induction generator,main controller, virtual instruments, converter systems and LVRT units. Onthis laboratory platform, some basic theories and operation characteristics ofthe doubly-fed wind power system are demonstrated. It will be helpful forthe deeper understanding of the theories of the doubly-fed wind power andthe optimization of the control strategy. From the theory deduction andcorresponding experiments of the LVRT of doubly-fed induction generator,the experiment results are expected to show good agreement with thetheoretical analysis.