Zero-sequence voltage injection control scheme of modular multilevel converter supplying passive networks under unbalanced load conditions

Modular multilevel converters have a promising application in passive network supply, such as offshore platforms and remote loads. In these applications, load imbalance is inevitable and the established unbalanced control strategies for active network applications, which are mainly based on negative-sequence voltage canceling, are no longer feasible. Novel control strategy for unbalanced load conditions is required to perform under the constraint of constant line-to-line voltages and uncontrollable load currents. And the major control object is to eliminate both the overcurrent and the thermal overload problems of arm currents due to the load imbalance. The former may cause the converter to switch offline, while the latter may exceed the I2t capability of power semiconductors. This paper proposes a control strategy to balance the arm currents. On the basis of the equivalent relationship of dc-link active power and ac-link average active power, the dc component of the arm current is determined by the ac-link average active power in the corresponding phase. By adding a common zero-sequence voltage to the reference voltages, the average active power in each converter phase can then be regulated, and the arm current can be controlled. By balancing the peak value of the arm currents, the safe operation area of the converter can be increased under unbalanced operation conditions. Simulation results and experimental results verify the validity of the control scheme.