Unbalanced and Reactive Load Compensation Using MMCC-Based SATCOMs With Third-Harmonic Injection

Modular multilevel-cascaded converter-based STATCOMs are scalable to higher voltages without requiring a step-up transformer with multiple windings and can be realized using a low switching frequency, giving lower harmonic content and, hence, a reduced filtering requirement. This paper presents a new injection technique to extend the operating ranges of modular multilevel-cascaded converters STATCOMs when used for negative sequence and reactive current compensation. A nonsinusoidal voltage or current containing a fundamental and its third-harmonic component is injected to achieve phase cluster voltage balance. This technique reduces the maximum dc-link voltage for the star configuration, and the maximum converter phase circulating current for the delta case, compared to applying only sinusoidal zero-sequence components for mitigating the same degree of load unbalance. The analysis is confirmed experimentally, showing that the third-harmonic injection can allow a significant improvement of STATCOM capability for simultaneous compensation of unbalanced load and reactive current.