Research On Control Strategy Of Cascade Multilevel STATCOM

With the development of the power system, more and more attention is paid to the effects of reactive power to power grid. At rated voltage, the balance of reactive power has become an important symbol for measuring power quality. Immediate reactive power compensation is of greatly realistic importance. The compensatory techniques of traditional reactive power compensators have fall behind and be not good enough to meet the compensatory requirement for power system. STATCOM (Static Synchronous Compensator) is a new-style dynamic reactive power compensator, since it has superiority in fast response, high precision, good performance for filtering harmonics and restraining voltage fluctuation. At the present time, STATCOM has become one of the research focuses in FACTS (Flexible Alternative Current Transmission Systems).In this paper, the compensatory principle of STATCOM is analysized. Three topologies of main circuit for large-capacity STATCOM are compared. Based on the analysis of their advantages and disadvantages, Cascade multilevel STATCOM is chosen as the main research object. Two detection methods of instantaneous reactive power theory, namely p-q and ip-iq detection, are studied. The thesis selects the latter and applys in the current detection of STATCOM.In order to improve the control property of STATCOM, the paper establishes a control system of cascade multilevel STATCOM based on state feedback decoupling. For the modulation of inverter in cascade multilevel STATCOM, carrier phase shifting SPWM method is applied. For the whole STATCOM, it has a problem of mutual coupling between active current and reactive current in ac (alternating current) side. Two phase dq variables are decoupled by adopting state feedback. The simulation results show that, in an ideal situation, the equipment has quick dynamic response and good follow-up property when the reference signal of reactive current is changed suddenly.Owing to the strongly coupled and nonlinear characteristics of electric power system and dynamic change of loads, it is proposed to apply ADRC (Active Disturbance Rejection Control) to the controller of system that include cascade multilevel STATCOM. The dynamic mathematical model is established. In order to eliminate the non-linearity of the model, the differential geometry method based on coordinate transformation and state feedback is introduced. Furthermore, considering the uncertain factors in the system, the ESO (Extended State Observer) in ADRC is applied to evaluate and compensate them. Finally, the nonlinear controller is designed. The simulation results show that, compared with the traditional PID controller, the nonlinear controller in this thesis can achieve better voltage regulation.The research results in the thesis have strengthened the theoretical foundation for practical application of cascade multilevel STATCOM. With the development of power electronics technology and control measures, cascade multilevel STATCOM will have a wider application prospect.