Enhancing short-term overcurrent capability of MMC for energy storage system by active circulating current injection

Certain applications of modular multilevel converter (MMC), such as MMC based super capacitor energy storage system, require MMC to have improved short-term overcurrent capability. This paper proposes an active circulating current injection control method to enhance short-term overcurrent capability of MMC. The positive peak current and negative peak current in the arm of an MMC exhibit asymmetry during active power conversion. Based on this characteristic, the asymmetry can be equalized by the injected circulating current to suppress the peak current of the MMC arm. Thus, potential damage to switching devices caused by overcurrent can be avoided. This paper analyzes the ability of the proposed method to reduce the peak current of the MMC arm, and verifies the effectiveness of the proposed method through the simulation model of the MMC based super capacitor energy storage system.