Advanced compensation mode for cascade multilevel static synchronous compensator under unbalanced voltage

An improved control scheme for delta-connected cascade multilevel static synchronous compensator (STATCOM) operating under unbalanced conditions is proposed. The individual phase instantaneous current control is suitable for delta-connected cascade multilevel STATCOM which can be equivalent to three single-phase inverters. To compensate the system imbalance, the reactive control mode and the voltage control mode are studied, and the discrete current loop controller has been designed. Accordingly, the cascade multilevel STATCOM can maintain normal operation under unbalanced voltage by using the individual phase instantaneous current control strategy while the unbalanced system can be effectively compensated by the two modified compensation modes. The proposed control scheme has been verified by simulation and experimental results based on a three-phase 36-chain cascade multilevel STATCOM laboratory prototype are shown.