PWM control of a 5-level single-phase current-source inverter with controlled intermediate DC-link current

Multilevel converters offer benefits of reduced harmonics and increased power ratings compared to their two level counterparts, but to date most of the research in this area has been directed towards Voltage Source Inverters rather than Current Source Inverters. Partly, this is because DC link capacitors are a more effective energy storage medium than link inductors, with less losses and easier operating requirements. However, for higher power applications, and with improvements in inductors using superconducting magnetic energy storage systems (SMES), current source inverters can have particular advantages in some circumstances. Hence it is useful to explore how a multilevel CSI system might be managed. This paper presents a 6-switch single-phase 5-level current source inverter, and compares its topology to a previously reported 8-switch system. For both converter topologies, it is identified that control of the intermediate 1/2-level DC link current is a major challenge, and a closed loop strategy is proposed to control this current under varying load conditions. Simulation and experimental results are presented to confirm the operation of the new 6-switch topology under PWM control, and to verify that the 1/2-level link current can be maintained at half of the total DC current with very little current ripple under differing load conditions.