Control And Optimization For The Full-scale Converter Of Wind Turbine With Induction Generator

Due to environment pollution and energy shortage, people is looking for cleanrenewable energy. Wind energy is a widely existing, clear and renewable energy, so thatthe research of wind energy application develops rapidly. Recently, many countries makefull development and utilization of wind power, and the wind power technology hasbecome very sophisticated. Of all kinds of wind power generation systems, thesquirrel-cage induction generator (SCIG) is widely applied, especially in offshore windpower generation. The full-scale converter wind turbine with SCIG has many advantages,such as simple structure, high reliability, low maintenance cost. However, there are stillsome drawbacks in its traditional control strategy, such as control stability, systemperformance, efficiency and reliability.This is a direct cause for this paper to investigate full-scale converter with SCIG inwind power generation system. A series of key issues are considered and several optimaland effective control laws are presented to increase the efficiency and reliability of windturbines in this paper. The main contributions are as follows.1. In order to improve stability and increase robustness of the grid-side LCL voltagesource converter, a grid-side control strategy based on sliding mode technique is proposed.As a base of the design of sliding mode current controller, multiple order error equationsare established. Cascade control structure that consists of PI regulator for the DC linkvoltage in the outer loop and sliding mode current controller in the inner loop is designedfor the grid-side converter with LCL filter. Simulation and experimental results validatethe control performance and robustness of the proposed controller.2. In order to improve the control performance, increase reliability and reducehardware cost, observation technique of the induction generator where speed sensor is notused is studied. A novel closed-loop rotor flux observer with discrete input is proposed toensure the asymptotical stability of observer error through LMI constraints. Projectimplementation about generator speed estimation based on MRAS method is studied, andDC bias suppression is described completely. Simulation and experimental results validate the correctness of flux observer design and the effectiveness of speed sensorlessimplementation strategy.3. In order to improve the dynamic response speed and reduce the DC linkcapacitance, optimization of the DC link voltage fluctuation suppression is studied. Thewhole unit Euler-Lagrange model is built, and a combined controller which consists ofconverters both in generator-side and grid side is designed based on passivity theory. Theoptimal setting of the equilibrium point of error system can improve the system dynamicperformance. An L2gain suppression method is proposed to deal with the problem ofmodel parameter uncertainty which may lead to passivity control performance degradation.By analyzing system constraints, a method using power slope control is proposed tofurther optimize the dynamic performance. Simulation and experimental results show theeffectiveness of the proposed method.4. To improve the generation efficiency and reduce the system losses, an optimizationmethod based on system loss model of full-scale wind power generation system withSCIG is proposed. A loss model of induction generator which includes iron loss and powerconverter is considered. In different wind speeds, the optimum operating points of speed,torque and rotor flux are established and applied to the vector control of generator-sideconverter. Simultaneously, system loss and energy saving performance under differentwind speed is analyzed. The experimental results verify the correctness of the theoreticalapproach.5. To suppress low-frequency power fluctuations under steady-state operatingconditions and to improve the low voltage ride through capability under fault conditions,supercapacitor energy storage system is considered. Parameters design method ofsupercapacitor energy storage system is given. And, coordinated control strategies areproposed to suppress low-frequency power fluctuations under steady state and to achievelow voltage ride through, respectively. Control algorithms of each converter andswitchover process for each converter are studied in detail. Simulation results validate thecorrectness of proposed parameter design strategy and coordinated control strategy.