A Novel Electrolytic Capacitor-Less MMC With Magnetic Power Decoupling Method

Abstract

This article presents a novel flux-cancellation-based modular multilevel converter (MMC) with power decoupling capability. Conventional MMC suffers from low-frequency (fundamental and second-order) ripple power in the submodule (SM) capacitors. The low-frequency ripple power makes the SM capacitors bulky, responsible for a significant portion of the volume and weight of the whole system. The ripple power on the SM capacitor of the three-phase module is phase-shifted by 2 pi/3 rad from each other. The proposed flux cancellation is based on directing the low-frequency ripple power to a common core of a three-winding transformer to cancel out. A three-port bidirectional isolated dual half-bridge is introduced for flux cancellation. For this scheme, the main problem is the presence of leakage inductances of the transformer, and another one is three-port isolated bidirectional converter modeling as it has only input ports. The average model is proposed for the flux cancellation circuit. It provides a practically simple model for the complex circuit. Second, a virtual-capacitance-based power decoupling controller is proposed to overcome the leakage inductance problem by the benefits of simple current estimation and of no need for the current sensor. The proposed power decoupling method reduces the system size significantly and enables the MMC to operate with a nonelectrolytic SM capacitance of few microfarads. The operating principle and control design of the proposed MMC are presented. The proposed MMC is validated by simulation and experimentation.