Dc-link voltage balancing of six-phase wind energy systems with series-connected machineside converters and NPC grid-side converter

This paper presents an investigation of the dc-link voltage balancing control for an asymmetrical six-phase wind energy conversion system, which utilises series-connected two-level machine-side converters and a three-level grid-side neutral point clamped (NPC) converter. In such a topology, the dc-link voltages drift apart if there are asymmetries at the machine-side, so dc-link voltage balancing control is required. The work investigates the capability of the NPC converter to balance the dc-link voltages and examines additional control, provided by the machine-side converters, which can extend the control range. Simulation results confirm that a satisfactory control can be achieved when both converter sets contribute to the dc-link voltage balancing.