Research On Dual-PWM Converter For Variable-speed Constant-frequency Doubly-fed Wind Generation System

As an effective solution to energy crisis and environment pollution , wind power generation is developing rapidly worldwide and also coming into a period of rapid development in China. Due to its advantages such as adjustable power factor, high efficiency and frequency conversion devices with small capacity, doubly-fed wind power generation technology has become the mainstream technology of wind power generation.In this paper, the doubly-fed wind power generation excited by a dual PWM converter is set as the study object, and control technology for dual PWM converter has been analyzed and researched mainly in depth. Taking the idea of modular modeling, doubly-fed wind turbine is divided into wind-turbine, doubly-fed induction generator and double PWM converter three sub-modules, then the simulation model of the three sub-modules was established in Matlab / Simulink environment based on their own mathematical model, finally the simulation model of the entire system was integrated by the three sub-modules simulation model .The achievement mechanism of variable-speed constant-frequency for double-fed wind turbine and four different stages corresponding to different levels of wind speed which are the four stages of starting, biggest wind energy capturing, constant rotor-speed operation and constant power operation is introduced. Doubly-fed wind turbine have different control objectives in different stages, which are primarily realized through the control of rotor converter and the pitch angle. The vector control strategy of rotor-side converter adopts stator flux orientation, whose inner-loop is used to control the rotor current, outer-loop of speed is used to realize of the largest wind energy capturing and the constant rotor-speed operation of the third phase , outer-loop of stator reactive power is used to achieve per unit power factor of stator side. The constant power operation of the fourth phase is realized by the pitch control . The main function of grid-side converter is to maintain the stability of DC-link capacitor voltage. Simulation results show the control objectives of various stages are achieved, also proves that modeling and control strategy is correct.As voltage imbalance is the most common fault, adopting symmetrical component method ,the operational characteristics of DFIG under the conditions of voltage imbalance is studed, also the mathematical model of DFIG in the positive and negative synchronous rotating coordinate and the expression of the instantaneous power and electromagnetic torque is derived. The definition of the degree of pulsation of the stator active power and the electromagnetic torque is introduced, and the relations between the degree of pulsation and voltage imbalance under conventional control strategy are discussed. Finally, in order to eliminate electromagnetic torque pulsation and active power pulsation respectively , positive and negative sequence separation control strategy for rotor side controller is designed, through simulation the correctness of separation control strategy is proved.