A broad range of speed control of a permanent magnet synchronous motor driven by a modular multilevel TSBC converter

This paper provides a theoretical and experimental discussion on a modular multilevel triple-star bridge-cell (TSBC) converters. This paper also proposes a new control method for mitigating ac voltage fluctuation of the dc capacitor of each bridge cell. The proposed method is characterized by properly injecting both common-mode voltage and circulating currents to each cluster in the whole speed range. Moreover, this paper attempts to optimize the amplitude and frequency of the sinusoidal common-mode voltage with help of numerical analysis. This makes the ac voltage fluctuation the smallest. The new method can reduce both voltage fluctuation and cluster current more effectively than existing methods. Experimental waveforms using 370-V, 75-Hz, 15-kW interior-permanent magnet synchronous motor loaded at the rated torque show good start-up performance from a standstill to the rated speed, keeping both capacitor-voltage and cluster current within acceptable levels.