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. The paper presents a new injection technique to extend the operating ranges of MMCC STATCOMs when used for negative sequence and reactive current compensation. A non-sinusoidal 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.