Neutral-Point-Less (NPL) Multilevel Inverter Topology with Single DC-link Capacitor: H-type Inverter

High-voltage and high-power multilevel inverters (MLIs) have gained attention as the transportation electrification trend is rapidly expanding towards high-capacity mass transit systems such as electric aircraft, trains, and ships. Conventional MLIs such as neutral point clamped (NPC) and T-type inverters provide high-voltage and high-power operation capabilities but require stacked dc-link capacitors with neutral point connection for zero voltage vector. This neutral point connection to the stacked dc-link capacitor generates a neutral current oscillating at three times the fundamental frequency, causing capacitor voltage imbalance and overvoltage stress on capacitors and switching devices. This paper proposes a neutral-point-less (NPL) MLI topology with a small and single dc-link capacitor, the H-type inverter, and its operating principle is introduced. The performance of the H-type inverter is investigated through simulation and compared to the conventional three-level T-type inverter. The simulation results show that the H-type inverter has 75% reduced dc-link capacitor current and voltage ripple, as well as less current waveform distortion, which will lead to smoother power output and a higher volumetric power density by substantially reducing the required capacitor size. Simulation of the inverter shows that it operates with an efficiency of 99.23% while operating with a switching frequency of 10 kHz. The prototype of the H-type inverter operates as proposed at low power.