Analysis of unbalanced clustered voltage and control strategy of clustered voltage balancing for cascaded H-bridge STATCOM

To explore the clustered voltage balancing mechanism of the cascaded H-bridge static synchronous compensator(STATCOM),this paper analyzes the causes of unbalanced clustered voltage.The negative-sequence current caused by the compensation of unbalanced reactive power or detection and control errors and the zero-sequence voltage caused by voltage drift of the STATCOM neutral point contribute to unbalanced clustered voltage.On this basis,this paper proposes a control strategy to inject negative-sequence current and zero-sequence voltage simultaneously.The injection of negative-sequence current may cause current asymmetry in the grid,and the zerosequence injection has a relatively limited balancing ability in the clustered voltages.The proposed control strategy can not only generate a faster balancing response than the traditional zero-sequence voltage injection method,but also lower the extent of current asymmetry compared with the traditional negative-sequence current injection method.Then,the negative-sequence current and zero-sequence voltage injection are further transformed into the dq frame to establish a unified frame.The effectiveness of the proposed control strategy is verified by the simulation and experimental results.