The Study On Control Strategy Of Cascaded Multi-level D-STATCOM

With the wide application of power electronic equipments, power quality problems of the distribution network become increasingly serious. Distribution Static Synchronous Compensator ( D-STATCOM) which can effectively solve the majority of power quality problems in the distribution network, has been more and more widely concerned. 10kV/±10M Var D-STATCOM based on cascaded multilevel converter has wide range of applications in media-voltage distribution network in China because of its capacity level and voltage. Consequently, it has great research value.Firstly, the mathematical model of the cascaded D-STATCOM is established, and the control strategy is presented. The whole controller includes two parts. One is the current closed loop controller which is used to provide reactive current to the D-STATCOM, and the other is the voltage closed loop controller which is designed to preserve the capacitor voltage in the DC side of the cascaded D-STATCOM. The current control strategy employs the common PI control method which is based on dq decoupling. And the capacitor voltage control uses the multilayer DC voltage control method which is able to control the active power distribution on general, phase and individual levels respectively. As a result, it can achieve the balance control of the capacitor voltage.Secondly, aiming at the demand for the harmonic compensation in the distribution network where the D-STATCOM is applied, the repetitive control into the traditional PI controller is introduced in the current loop. The repetitive controller and the PI controller are used in series. A double loop control is employed. The repetitive controller is used as the outside loop and its output signal is used to generate the current reference of the inner loop with the feed-forward current signal. As a result, the high tracking accuracy of the harmonic signal of the repetitive controller and the rapid response character of the PI controller are integrated in the advanced controller, contributing to the better performance of the whole control system.At last, several implement ways of the Carrier Phase Shifted SPWM are introduced in this paper. The modulation method is very important for D-STATCOM. It controls the switching device on and off and has heavily implication to the output performance of the D-STATCOM equipment. The CPS-SPWM is a very suitable way for the cascaded D-STATCOM, and it has several different implement ways. In the factual application, by choosing different modulation methods, the D-STATCOM will have different output waveforms, loss and efficiency. It is necessary for us to analysis the advantages and disadvantages of these methods and to choose the most suitable way.