Analysis of star and delta connected modular multilevel cascaded converter-based STATCOM for load unbalanced compensation

This paper compares the operating capabilities of STATCOMs based on Modular Multilevel Cascaded Converters (MMCC) using star and delta connections, with special attention to unbalanced load compensation. Zero sequence voltage for star connection, and zero sequence current for delta, need to be applied to overcome the phase cluster DC-voltage unbalance. Expressions are derived for both zero sequence elements as functions of the degree of load unbalance defined as the ratio of negative to positive sequence load current. They show that the zero sequence voltage in star connection reaches a very high level as the degree of load unbalance increases, making the MMCC DC-link voltage too high for correct functioning. However the delta connected MMCC can cope with the high level of load current unbalance. Experimental results are presented to validate this analysis.