Dynamic multi-agent dc-bus reconfiguration in modular motor drives with a stacked polyphase bridge converter

A stacked polyphase bridge converter consists of multiple standard, three-phase two-level voltage source inverters connected in series to the same dc voltage source. The authors present a decentralised and dynamic dc-bus reconfiguration strategy for this type of converter. The innovation of the proposed strategy lies in its ability to isolate or reactivate one of the inverters in the series connection online, even in motoring mode, enabling increased fault tolerance and an extended speed range. Both the required hardware alterations and the multi-agent control strategy are addressed. Its major benefits compared to the state-of-the-art are that only two additional active, unidirectional semiconductor switches are required per inverter, and that the control is based on local computations, local measurements and neighbour-to-neighbour communication only. Hence, the scalability and reliability of the hardware are extended towards the control. Simulations and experimental results on a 4 kW modular axial-flux PMSM prove the feasibility of the concept, and validate that the dc-bus can be reconfigured in 100 ms, without torque interruption.