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Voltage balancing control of a symmetrical nine-phase machine with series-connected DC links

This paper explores the possibility of using a nine-phase machine, with three isolated neutral points, in order to produce a high dc-link voltage by cascading three lower voltage three-phase voltage source inverters. Each inverter drives one winding set of the machine. Consequently, used inverter switches and capacitors have three times smaller voltage rating when compared to the single high-voltage dc link, a desirable feature in high-power and high-voltage drives, such as wind turbines or ship propulsion. It is shown that any imbalance in machine phases or the inverters produces unequal power sharing between winding sets, and consequently unequal dc-link voltages. Therefore, after initial analysis, a dc-link voltage balancing controller has been developed and implemented together with current/power sharing control. Feasibility of the proposed system has been verified by simulation with different types of imbalance. Both motoring and generating operation are considered.