Impact of capacitor balancing strategies on converter ratings for star-connected cascaded H-bridge STATCOM

This paper presents a comparison between two DC-voltage balancing strategies for cascaded star-connected multilevel STATCOM: zero-sequence voltage, and negative-sequence current control. Different levels of unbalance and grid strengths are considered. The impact of the control strategies on the convert ratings in terms of voltage and current requirements are compared and the operating range of the converter using each control strategy is theoretically derived. The results show that for a given system ratings, the operating range of the converter using negative-sequence current control is wider than the zero-sequence voltage control when the STATCOM is connected to weak grids. On the contrary, in case of connection to strong grids the zero-sequence voltage control can provide wider operating range. PSCAD simulation results are provided to prove the theoretical results.