Capacitance minimization in modular multilevel converters: Using model predictive control to inject optimal circulating currents and zero-sequence voltage

The superior harmonic performance of the modular multilevel converter facilitates reduction in passive filtering requirements. However, in practice, any volume and weight reduction brought about by using smaller filtering components, is offset by the increase in stored capacitor energy inside the converter. This paper proposes injection of high-frequency circulating currents and zero-sequence voltages, that significantly reduce capacitor voltage ripple. This can ultimately facilitate use of smaller capacitances. Optimal reference voltages and currents are designed off-line, with a model predictive control scheme used to track the references on-line while also compensating for any disturbances introduced by the practical system.