Abstract
The Modular Multilevel Converter (MMC) has found considerable use in medium voltage applications like MVDC in recent times. Circulating currents that do not appear at the output and flow between the phase legs, pose serious concern causing unnecessary converter losses. Several methods of circulating current control have been proposed and they are based on simplified average models of the MMC. Such models are well suited for high voltage systems where there are large number of Sub-Modules (SMs). But, for medium voltage MMC using fewer SMs, circulating current control can be better managed by considering more accurate model. This paper proposes an improved model that includes the inductor voltage in its design. In this approach, the circulating current reference is first obtained and then it is used to choose a correct switching redundancy to control the circulating current. The proposed work uses a phase leg based PD 2N+1 modulation scheme that has inherently better harmonic performance compared to conventional arm-based modulation. Through simulation studies, it is proved here that the rms circulating current gets reduced resulting in reduced conduction loss. The proposed work is applicable for MVDC applications which involve fewer number of SMs.
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