High-performance Integrated Modulation Scheme for Modular Multilevel Converters

Abstract

In this paper, a virtual bus-clamping technique (VBC) is introduced for a modular multilevel converter (MMC) in which submodules (SMs) in each arm are virtually clamped to either positive or negative dc-bus voltage. The effectiveness of the proposed technique is verified through the simulations in PLECS platform for five-level MMC, and its performance is compared with the carrier-based space-vector pulse width modulation (SVPWM) under various operating conditions. The VBC technique helps in the reduction of voltage harmonic distortion, capacitor voltage ripple, circulating current (CC), and converter switching losses at high modulation indices. However, the conventional SVPWM technique displays a better performance compared to VBC in terms of efficiency, SM capacitor voltage ripple, CC and common-mode voltage (CMV) commutations at the lower modulation indices. Therefore, an integrated modulation scheme with superior performance characteristics over a wide operating range of modulation for MMC is proposed. This scheme employs the SVPWM method in the low modulation range and VBC method in the high modulation range including field weakening region of the motor. The combined modulation scheme performs satisfactorily throughout the MMC operating range and the seamless transition from SVPWM to VBC and vice versa is established.