A multilevel chain-link topology for low voltage, variable frequency applications

The increasing demand of Electric Vehicle from the automotive industry is leading research towards higher power density and more efficient electric drives. Usually, 2-Level bidirectional AC/DC converters are utilized and the use of wide band gap devices is often considered in order to increase the power conversion efficiency. On the other hand, similar results can be obtained by a more complex power conversion system, based on standard silicon devices, such as a multilevel inverter. Amongst all the possible multilevel converter topologies, Modular Multilevel Converters (MMC) is the less considered in Electric Vehicle applications. The fundamental drawbacks of MMC which limits its applicability in low voltage, variable frequency applications is that each sub-module handles a power ripple with components at fundamental frequency and at twice the fundamental frequency which impacts the sub-module capacitance sizing, especially at low frequency. This paper proposes a modification to the MMC topology in order to expand its applicability to low voltage, high speed drives. The basic observation is that by adding a power port linking upper and lower arm sub-modules, instantaneous power can be exchanged and the voltage ripple components can be completely removed from the sub-module capacitors without impacting other parts of the converter. The paper introduces the design and analytical modelling of this novel chain-link topology and validate the theoretical claims through simulation and experimental tests.