A Study On The Control Strategy Of D-STATCOM For Power Grid Unbanlance Compensation

With the rapid development of modern society, requirement of good power quality become higher and higher, so the problems of reactive power shortage and power grid unbalance in the power distribution grid have been widely concerned. Custom Power, also known as DFACTS, whose aim is to improve the power quality in the power distribution grid, is a synthetic technology applying control technology, power electronics, and micro-processing technology to eliminate short-term power interruption, voltage fluctuation and flicker, compensate reactive power and reduce the harmonic distortion to improve power quality and power distribution reliability. D-STATCOM, as a representative device of Custom Power, has advantages of cost effective, powerful and excellent performance, can compensate reactive power and unbalance current simultaneously. Since the control strategy of D-STATCOM under unbalanced working condition has been the key and difficult point of research and cascade D-STATCOM has many outstanding features, the main work of this paper is to a control strategy of cascade D-STATCOM for unbalanced load compensation.Firstly this paper introduces the research background and significances, as well as the current application situation in home and abroad, the main circuit topology and the control strategy of D-STATCOM. A detailed analysis of the basic structure and working principle are given. The mathematical model of D-STATCOM under unbalanced working condition is deduced in detail by positive and negative sequence separation method and park transformation.Then this paper elaborates the differential geometry method and the sliding mode variable structure control theory systematically, which are the basis of the control strategy proposed in this paper. In order to overcome the problems of the traditional unbalance compensation control strategy, a positive and negative sequence double loop control strategy based on the input-output feedback linearization and the integral sliding mode variable structure technique is propose. Since the mathematical model of D-STATCOM under unbalanced working condition includes a positive sequence part and negative sequence part, the integral sliding mode control combined with the input-output feedback linearization method is used in them to compensate unbalanced current and reactive power simultaneously. The control strategy proposed is suitable for balanced and unbalaced working condition and has a good robustness. Finally a simulation system is built in PSCAD/EMTDC, the simulation conducts under no disturbances, only inernal disturbances, only external disturbances and both internal and external disturbances to verify the effectiveness and robustness of the control strategy proposed in this paper.