A Control Method of HVDC-Modular Multilevel Converter Based on Arm Current Under the Unbalanced Voltage Condition

This paper proposes an enhanced control method for a high-voltage direct-current (HVDC) modular multilevel converter (MMC). To control an MMC-HVDC system properly, the ac current, circulating current, and submodule (SM) capacitor voltage are considered. The ac-side current is a fundamental frequency component, and the circulating current is a double-line frequency component. Existing control methods control the ac current and circulating current by separating each component. However, the existing methods have a disadvantage in that the ac-side current must be separated into the positive and negative sequences for control under an unbalanced voltage condition. The circulating current consists of not only negative-sequence components but also positive- and zero-sequence components under an unbalanced voltage condition. Therefore, an additional control method is necessary to consider the positive- and zero-sequence components of the circulating current. The proposed control method has the advantage of controlling not only the ac-side current of the MMC but also the circulating current without separating each of the current components to control each arm current of the MMC. In addition, it can stably control the positive- and zero-sequence components of the circulating current under the unbalanced voltage condition.