The Main Circuit And Control Strategy Of Large Capacity Cascade STATCOM

Cascade STATCOM has many advantages such as fast response, small size, low cost, easy to redundancy. Cascade STATCOM is a typical representative of the third generation of reactive power compensation device, Its application can improve the power system voltage stability limits significantly, improve low voltage ride through capability of wind farms, improve the power system power quality, and reduce the loss of the electric energy effectively. Cascade STATCOM has a very broad prospect in the smart grid as the technology of the emerging development in the21st century. Therefore, carry out in-depth and extensive research for Cascade STATCOM is necessary. Based on actual project, the main circuit and control strategy of Cascade STATCOM are depth and extensive studied in this paper.First of all, based on the instantaneous reactive power theory, the paper introduces working principle of the Cascade STATCOM and establishes a synchronous rotating coordinate system transformation matrix. Besides, it establishes and analyzes the mathematical model of cascade STATCOM under synchronous rotating coordinate system. Combines with the actual Cascade STATCOM product R&D, selects the main circuit topology and device parameters of Cascade STATCOM.Secondly, the Cascade STATCOM control strategy is researched and designed. Modulation wave inverting unipolar frequency doubling of the carrier phase shift technology is used as Component-level control strategy, H-bridge carrier phase followed by a difference that180°/N, the carrier frequency is selected as500Hz. Direct current decoupling control strategy and three-phase DC capacitor voltage balance control strategy are used as device-level control, They ensure the response of Cascade STATCOM dynamic reactive power and the stability of DC capacitor voltage. Constant bus voltage control strategy and the constant power control strategy are used as System-level control strategy.Once more, in the power system voltage unbalance conditions, the problems of ignore the negative-sequence and negative sequence voltage feedforward decoupling control strategy are analyzed. Mathematical model of Cascade STATCOM is established, on this basis, an improved positive and negative sequence pairs of inner current control strategy is proposed.It not only solves the problems of Cascade STATCOM over current and Converter chain voltage unbalanced in the power system voltage unbalance conditions, and the performance device is optimized in the power system voltage balance conditions.Finally, methods and conclusions are proven effective through PSCAD simulation.