Circulating Current Optimization Control of MMC

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Abstract
Modular Multilevel converter (MMC) is a preferred topology for higher power applications. The instantaneous voltage mismatch between the phase legs and dc bus in MMC leads to predominantly second-harmonic circulating current (CC). The sizes of submodule (SM) capacitor and arm inductor, power losses and current rating of semiconductor switches in MMC are influenced by the CC. This paper proposes a circulating current optimization control (CCOC) scheme for MMC, which reduces the arm current peak value without increasing the SMs capacitors size for the same maximum voltage ripples. It enables the MMC to handle more power with the same rated switches. This controller works on the principle of controlling the magnitude and phase angle of CC to follow their respective optimized references. A detailed design and implementation method of CCOC is presented for all possible operating conditions of MMC in terms of modulation indices and load power factors. A comparative evaluation performed between CCOC and other major CC control schemes highlights its main features. The efficacy of the proposed scheme is validated using simulations and also using experimental studies on a seven-level three-phase grid-connected MMC prototype. The results demonstrate the effectiveness of the CCOC scheme and verify the proposed principles.

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