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The aim of this paper is to investigate the ability of cascaded H-bridge STATic COMpensator in star and delta configuration to exchange negative-sequence current with the connecting grid. Zero-sequence voltage for the star and zero-sequence current for the delta configuration are utilized to guarantee dc-capacitor voltage balancing. A general solution for the required zero-sequence voltage/current for any kind of unbalanced condition is provided. The obtained solutions show that a singularity for both configurations exists under specific conditions, leading to infinite zero-sequence requirement. In case of star configuration, this singularity occurs when the amplitude of the positive- and negative-sequence components of the current output of the converter are equal. Analogously, the singularity for the delta configuration occurs when the amplitude of the positive- and negative-sequence components of the voltage at the converter terminals are equal. This singularity impacts the ratings of the system and imposes a limitation in the utilization of the compensator for unbalance compensation purposes, both in industrial and utility applications. Experimental results are presented to validate the theoretical analysis.
Behrouzian et al. (Thu,) studied this question.