Active Control Strategies And Characteristics Of Dynamic Loads Of Wind Turbine Generator Systems

Wind power is becoming an important way to solve energy shortage, and it attracts more and more attention all over the world. With the increasing capacity of a wind turbine generator system(WTGS), its rotor diameter, tower height and the flexibility increase rapidly. Dynamic loads caused by the tower shadow, wind shear and power system disturbances affect large-scale WTGS more and more prominently. In order to improve the reliability of a WTGS, study on active control strategies and characteristics of dynamic loads of a WTGS becomes very necessary and also attractive. WTGS with doubly-fed induction generator(DFIG) is viewed as the research object of this thesis. The research documented in the thesis addresses electromechanical co-simulation model and active control strategies of loads on blades, tower, and drive train of a WTGS. The main studies and achievements are as follows.① Considering a traditional first-order transfer-function model of an electric pitch drive system(EPDS) being incapable of reflecting dynamic load characteristics of a WTGS, an improved simplified transfer-function model of an EPDS is proposed, in order to enhance the numerical simulation efficiency for WTGS load calculation and to optimize pitch angle control strategies. Firstly, a detailed dynamic model of an EPDS is conducted considering the voltage equations and the control strategy of an induction motor. Then, the improved simplified transfer-function model for an EPDS is derived from its detailed transfer-function model by using methods of transfer function approximation, sensitivity analysis and block diagram reduction. Finally, in order to validate the improved simplified transfer-function model, its frequency-domain performances and its impacts on the stability of the pitch angle control system are analyzed. And dynamic performances of an EDPS are analyzed by using the improved model and compared with that using a testing system for EPDS, and that using a simplified first-order transfer-function model.② Since the influences of electrical system dynamics on WTGS dynamic load characteristics is rarely considered in the existing WTGS loads simulation model, an electromechanical co-simulation model for a DFIG WTGS is proposed in the thesis by using several different simulation platforms. The goal of the proposed electromechanical co-simulation model is to modeling the coupling characteristics between mechanical structure and electrical system of a WTGS. Firstly, the wind model, the aerodynamic model, the structural dynamics model are present for elastic mechanical structure of a WTGS. And the DFIG and its control strategies models are present for electrical system of a WTGS. Based on four simulation softwares/program packages, Turb Sim, Aero Dyn, FAST, and Matlab/Simulink, the electromechanical co-simulation model for a DFIG WTGS is present by integrating the above subsystem model and the improved simplified transfer-function model of an EPDS. And based on the proposed electromechanical co-simulation model, simulation comparisons of dynamic load characteristics a WTGS are performed under two different EPDS models, and under two different generator system models. The loads simulation results of the co-simulation model are also compared to loads measurements of a practical WTGS.③ To mitigate harmonic loads of a WTGS caused by non-uniform wind speed across the rotation plane, this paper proposes a proportional resonant(PR) based individual pitch control(IPC) strategy. Firstly, based on the relation of the rotational reference frame of blades and the fixed reference frame of hub, the impact of blade harmonic loads on the rotor hub are analyzed. Then, to improve the robust performances of the proposed IPC strategy, its PR controller parameters are designed by averaging the period time-varying linear model of the blade system. And to reduce the phase error of the control system caused by dynamics of the flexible blades and pitch drive systems, the phase compensator of the IPC strategy is designed. Finally, the stability of the proposed IPC strategy is analyzed in frequency domain when system parameters are time-varying. And, its performances to mitigate harmonic loads of a WTGS are simulated and compared with the traditional collective pitch strategy.④ To mitigate dynamic loads of a WTGS caused by rotor unbalance, this paper proposes an decoupled Coleman transformation based pitch control strategy. Firstly, the loads characteristics are analyzed and simulated for a WTGS with unbalanced rotor. Then, considering the dynamics of the Coleman transformation, the Coleman transformation model of the blade and pitch drive system is deduced, and the coupling between the tilt and yaw variable of the Coleman transformation model is analyzed in frequency-domain. Considering the variables’ coupling of the Coleman transformation model, the decoupled controller of the proposed pitch control strategy is designed by using the method of feed-forward composition. And based on the decoupled Coleman transformation model of the control system, the main controller of the pitch control strategy is design by using single-input-single-output theory. Finally, the stability of the proposed pitch control strategy is analyzed in frequency domain. And load mitigation performance of the proposed pitch control strategy is tested by simulations of a WTGS with unbalance rotor.⑤ To reduce fatigue loads of WTGS drive train caused by torsional vibration, this thesis propose a model predictive control(MPC) based damping control strategy for WTGS drive train system. Firstly, the characteristics of the drive train torsional vibration are analyzed based on different transfer-function models of the drive train system. Then, taking into consideration of varying parameters of a WTGS drive train system, the predictive model, the error adjuster, and the objective function are designed for the MPC based damping control strategy. Finally, zeros and poles of the MPC based damping control strategy are analyzed under varying parameters of drive train. And its ability to mitigate fatigue loads of a drive train system is simulated and compared with the traditional BPF based damping control strategy for WTGS drive train.