Capacitance minimization in modular multilevel converters: A reliable and computationally efficient algorithm to identify optimal circulating currents and zero-sequence voltages

摘要

The superior harmonic performance of the modular multilevel converter (MMC) facilitates reduction in passive filtering requirements. However, in practice, any volume and weight reduction brought about by using smaller filtering components, is typically offset by the increase in stored capacitor energy inside the converter. To reduce capacitor voltage ripple, and therefore facilitate use of smaller capacitances, previous work proposes injection of harmonic components in the converter circulating currents and zero-sequence voltages. While this approach is effective, the resulting increase in converter voltage and current ratings can be significant. In this paper, harmonic reference voltages and currents are designed using an optimization technique known as linear programming. Appropriate design of the proposed linear program guarantees convexity, and complies with hard constraints on the resulting arm voltage and current ratings, while significantly reducing capacitor voltage ripple. It is also shown that hard constraints can be placed on the average arm currents, which provides effective control over semiconductor losses.