Operational Characteristic Analysis And Control Stratrgy Of Pmsg Under Unbalanced And Distorted Operating Conditions

As a clean and renewable energy, wind power has gained more and more attention under the background of fossil energy shortage and more serious environmental pollution.Wind energy is very rich in China, but the region which is suitable for large-scale wind energy development is always far from the main power grid. So wind generators are always connected to a"weak grid",which has voltage drop,voltage unbalance,harmonic distortion and other forms of operation risks. The operation of wind generators and power system faces many challenges. Therefore, it is important to analyze the operational characteristics of wind generators under unbalanced and distorted operating conditions, improve the control strategy of wind generators and analyze the influence of wind generators on the near power system, which is of great significance to the efficient and stable operation of wind generators. In this paper, the operational characteristics and control strategy of direct-driven permanent magnet synchronous generators (DDPMSG) under unbalanced and distorted operating conditions are studied.Firstly, the mathematical model and control system of DDPMSG are introduced. The structure and principle of DDPMSG are analyzed. A detailed simulation model is established on PSCAD. The theoretical analyses of operational characteristics of DDPMSG under unbalanced and distorted operating conditions are presented respectively, whose correctness is verified by case studies.Secondly, in order to improve the performance of DDPMSG under unbalanced and distorted operating conditions, a novel control strategy combining repetitive (REP) control and quasi-proportional resonant (QPR) control is proposed for DDPMSG grid-side converter. In the proposed control strategy, the current commands are dependent on the objectives of converter control when the grid voltage is unbalanced. The hybrid control strategy has the advantages that it can eliminate the steady-state tracking error on the fundamental frequency current while suppress the total harmonic distortion significantly in the two-phase stationary reference frame. In this paper, the block diagram of control system is presented, and the design methods of each block are studied. Meanwhile, the effect of the control strategy is investigated. At last, case studies are presented and the simulation results show that the proposed control strategy can effectively suppress active power pulsation and reduce the total harmonic distortion under unbalanced and distorted operating conditions.Finally, a steady-state asymmetric calculation method for power system with DDPMSG after the injection of negative sequence current is used. In the positive and negative sequence networks, DDPMSG with the control strategy under unbalanced condition could be equivalent to the controlled current source. Then voltage and current equations based on positive and negative sequence networks of power system are listed.The sequence components of parameters of power system could be gained by solving the simultaneous equations combining the voltage and current equations and the current commands equations of the control strategy. The influence of DDPMSG on the negative sequence currents of the system near DDPMSG is analyzed,which is also compared with the synchronous generator. Finally, two case studies based on two-machine system and 13-node distribution network are presented, and comparative analyses between theoretical calculations and simulation results verify the correctness of the calculation method and conclusions.