Research On Control Strategy Of Cascaded H-bridge Multilevel STATCOM

With the development of science and technology,the demand for power supply is dramatically increasing.Because of the loss caused by power factor,the loss of power transmission is becoming more and more serious.Flexible AC Transmission Technology(FACTS)has been paid more and more attention in the field of power transmission and supply.Flexible AC transmission technology is a new technology which can effectively improve the quality of power transmission and distribution.As one of the core technologies of the flexible AC transmission system,STATCOM is one of the hotspots in this field.Cascaded H bridge STATCOM is composed of multiple H bridge modules.It possesses high voltage level,large equipment capacity,high switching frequency,easy expansion and high reliability.For the research on control strategy of the cascade H-bridge multilevel STATCOM,this paper mainly includes the following parts:Firstly,this paper establishes the mathematical model of cascaded H-bridge topology according to the working principle of cascaded H-bridge multilevel STATCOM.On this basis,this paper selects the suitable modulation strategy by comparing several common modulation strategies,considering the harmonic distortion rate of the output waveform,whether it is conducive to the expansion of modular structure,and whether it is convenient to achieve accurate current control and so on.The conclusion has been verified in the MATLAB/Simulink platform.Secondly,this paper designs the double close-loop control based on the compensation current and the total DC voltage for the cascade H bridge multilevel STATCOM.The inner loop adopts the combined control strategy of PI and repetitive control,and the voltage feed forward link is introduced to realize the accurate control of the output current.The DC voltage loop adopts traditional PI control to ensure the stability of three-phase DC voltage.The proposed control strategy are verified in the MATLAB/Simulink environment.Besides,the selection of capacitance parameters for cascaded H bridge multilevel STATCOM is analyzed and designed.And the causes for the unbalanced three-phase DC voltage imbalance are theoretically analyzed.It is concluded that the unbalanced three-phase DC voltage imbalance is mainly caused by zero-sequence voltage caused by neutral point drift and negative-sequence current caused by compensation of unbalance load.On basis of this,a clustered voltage balancing strategy based on negative-sequence current injection is proposed,and the experimental verification of the interphase voltage balancing strategy is carried out under the MATLAB/Simulink platform.