Modulated Model Predictive Control with Common Mode Voltage Injection for MMCC-STATCOM Based Unbalanced Load Compensation

This paper presents a novel modulated model- predictive control (MMPC) scheme for Modular Multilevel Cascaded Converter-based STATCOMs (MMCC-STATCOM) to compensate unbalanced load current and regulate reactive power flow. By adding a common mode voltage (CMV) to the phase-voltages of the star-connected MMCC current model, the method allows natural injection of a non-sinusoidal voltage to the neutral point of the converter, hence achieving inter-phase cluster voltage balance. Moreover the imposed CMV is shown to extend the operating ranges of MMCC STATCOMs when used for negative sequence current compensation. The proposed MMPC method incorporates a modified branch and bound (B&B) algorithm to optimize the per-phase switch duty ratios. It is shown to be computationally more efficient compared to model-predictive control schemes using optimal voltage level method combined with voltage sorting schemes. Experimental results with different weighting factors confirm the effectiveness of this control scheme, and compared favorably with the conventional scheme of injecting only a sinusoidal zero sequence voltage.