Modified MMC and its capacitor voltage ripple suppression method employed in mediumvoltage wind generator system

Abstract

Modular multilevel converter (MMC) is widely used in medium-and high-voltage applications, but there are largecapacitor voltage ripples on the sub-modules (SMs) when applied to low-speed applications, such as wind generator system. The method based on the injection of common-mode voltage and high-frequency current is the most effective solution atpresent, but it will bring new problems. To solve these problems, a modified MMC is proposed in this study. Firstly, coupledinductors and LC resonance circuit are adopted as the power transfer channel between the upper arm and lower arm of theMMC, to eliminate the low-frequency voltage ripple. The theoretical analysis and design guideline of the coupled inductors andLC resonance circuit are discussed in detail before the explanation of SM voltage fluctuation suppression method. Secondly, animproved voltage fluctuation suppression method, named limited compensation method, is presented to replace theconventional full compensation method to decrease the magnitude of injected high-frequency current, thereby reducing thecurrent ratings of power devices, size of inductors, and the cost of system. In the end, MATLAB/SIMULINK simulations and a 2kW down-scaled prototype are performed to validate the effectiveness of the proposed solution and its control scheme.