Fast and fault-tolerant model predictive control of MMCs under selective harmonic elimination

Selective harmonic elimination (SHE) is a suitable pulse-width modulation (PWM) technique for some applications of modular multilevel converters (MMC) with lower number of semiconductor submodules (SM). However, SHE can make it difficult to control circulating currents in the MMC. Moreover, the MMC is vulnerable to SM switch failures that reduce its output power magnitude and quality. This paper proposes a fault-tolerant model predictive control strategy for MMCs operating with SHE (FT SHE-MPC). The effects of switch failure on phase voltage waveforms are studied first. Then, a method is proposed to design the FT SHE waveform to preserve its harmonic performance. Next, the proposed FT SHE-MPC is designed to increase the capacitor voltages during an SM failure to maintain the fundamental component of the faulty phase voltage and control the circulating currents, maintaining balance among line voltages and the output power of the MMC. Additionally, the proposed method offers as much as 60% faster FT transition time and as much as 36% lower capacitor voltage ripple. The advantages of the proposed method are verified through offline and real-time simulation studies on a three-phase nine-level MMC in PSCAD/EMTDC software and RTDS.