Grid-Connected Control Technology For Electricity Excitation Direct-driven Wind Power Generation

Direct-driven wind generation system is considered to be an important development tendency in wind generation technology area due to its superiorities of high reliability, high efficiency, good grid fault ride-through ability, etc. Recently, the price and reserves of permanent magnet materials has great influence on the cost of permanent magnet direct-driven wind generation system, and electricity excitation direct-driven wind generation system becomes another development direction. This dissertation gives theoretical study on grid-connected control technology for direct-driven wind generation system. The control strategies of grid-connected inverter (GCI) and electrically excited synchronous generator (EESG) are both discussed comprehensively. Simulations and experiments demonstrate the correctness of the research results. Following are the major research works of this dissertation.Firstly, modeling and current control of GCI have been discussed. The GCI dynamic modeling is established and expressed in the stationary reference frame and synchronous rotating reference frame. Two typical vector control strategies of GCI are realized, which is decoupled PI current control implemented in the synchronous reference frame, and PR current control in the stationary reference frame. Using complex vectors, the vector control system based on different current regulators are analyzed from three aspects of decouple control, tracking performance and disturbance rejection performance. The power decouple control in stationary frame is analyzed comprehensively, and a new method is provided for achieving the power decoupling. Simulation and experimental results verify that the improved PR current regulator can realize the power decoupling control and increase the dynamic performance.Secondly, the control strategy of GCI under unbalanced grid voltage has been emphasized. Dynamic modeling of GCI applied on unbalanced voltage conditions is established using symmetrical component method. To synchronize at the fault condition, a novel frequency-locked loop technology based on the reduced order resonant controller (ROR-FLL) is presented. The ROR-FLL can accurately and rapidly extract the positive-negative sequences and frequency from unbalanced voltage, and the simulation and experimental results show its feasibility and excellent transient performance. An unbalanced control strategy in stationary reference frame is presented using ROR-FLL and generalized reference current generation method. A novel controller, proportion integral plus reduced order resonant (PI-ROR) controller, is proposed, which can be used in the unbalanced control system to control the unbalanced current. The novel unbalanced control method based on PI-ROR controller can realize the unbalance control targets with excellent transient performances.Thirdly, modeling and flux observer of EESG have been discussed. The dynamic modeling of EESG is created and expressed in the stationary reference frame, dq synchronous rotating reference frame, and MT synchronous rotating reference frame, respectively. The different flux oriented methods are discussed, and the working principle and system design method of air-gap flux oriented vector control are analyzed. The flux observer in EESG vector control system is emphasized based on voltage model of EESG. Then, an improved second-order generalized integrator (ISOGI) which could eliminate the DC bias and integral windup is proposed to replace pure integrator, and a new flux observer based on ISOGI is achieved. Simulation and experimental results show the feasibility of ISOGI flux observer, and this method has excellent steady and transient performance in the frequency range of electrically excited direct-drive wind power system.Fourthly, the flux weakening control strategy of EESG under short-time overspeed in the wind power system has been discussed. The working principle of flux weakening control is analyzed, and the current trajectory of EESG is programmed in constant torque and the flux weakening operation region. Then, the performance of stator and rotor side compositive flux weakening control method of EESG based on dynamic stator flux current compensation is discussed and verified by simulation and experimental results. In order to achieve better flux weakening dynamic response, two improved stator and rotor side compositive flux weakening control methods based on virtual impedance and single current regulator has been proposed. Simulation and experimental results validate the feasibility of these control methods, and the wind power system can operate in wide speed range.