Repetitive control scheme for circulating current suppression in modular multilevel converters

Modular multilevel converters (MMC) are intensively researched and developed in high power, high/medium voltage applications. However, the interaction between switching action and fundamental-frequency pulsation in capacitor voltage of submodules will induce 2^nd-order and other even-harmonic harmonics in circulating current. These harmonic currents will introduce extra power loss, increase current stress of power devices and even jeopardize safe operation of MMC. Traditional methods are usually designed under single harmonic frequency, resulting in increased complexity for an integrated solution. This paper presents a repetitive control scheme based on unified model of circulating currents to solve the problem. It is simultaneously effective for multi-harmonic suppression, easy to implement and can be widely applied to single-phase or three-phase MMC. Simulation and experimental results of a MMC operating at inverter mode prove the feasibility of proposed methodology.