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This paper presents a novel dc capacitor voltage balance control method for cascade multilevel static synchronous compensator (STATCOM) and a general analytical method for balance control strategy. Considering that the imbalance of dc capacitor voltage is caused by the inconsistency of active power absorbed and consumed by chain, a balance control strategy based on active voltage vector superposition is proposed, in which an active voltage component is superposed to chain's output voltage to change its absorbed active power. A general analytical method based on vector analysis is also presented, by which the performance of balance control strategy can be analyzed, including stability and regulation capacity. To find out the most appropriate balance control strategy, a comparison still based on vector analysis among the proposed and other two commonly used methods is provided, from which it can be known that the proposed balance control strategy has the advantage of good stability and strong regulation capacity, and simulations are performed to prove it. The effectiveness of proposed control scheme has been verified by experimental results based on a three-phase 36-chain cascade multilevel STATCOM laboratory prototype.
Liu et al. (Wed,) studied this question.