A New Four-Level Inverter-Fed Motor Drive for Marine Propulsion Systems: Topology, Control, and Analysis

Multilevel inverters (MLIs) with medium-voltage (MV) capacity have emerged as a promising solution for dealing with rising power demand and increasing electrification in marine propulsion systems. This article introduces a new four-level inverter (NFLI) with a reduced component count and superior harmonic performance for MV applications. The proposed NFLI requires multiple floating capacitors (FCs) and their voltage should be regulated at one-third of the net dc-bus voltage. A generalized voltage balancing method based on the simple comparison logic is proposed to regulate the FCs voltage, which can be easily integrated with any type of multi-carrier pulse width modulation (PWM) scheme without any modifications. However, a higher capacitor voltage ripple is observed with conventional multi-carrier PWM schemes, especially at low-frequency operation, which significantly affects the motor operation and switching device's voltage stress. To overcome this issue, a modified multi-carrier PWM scheme is introduced along with the proposed voltage balancing method. Consequently, the capacitor voltage ripple can be minimized at low-frequency operation. Through the simulation and experimental studies on a laboratory prototype, the NFLI's performance utilizing the proposed voltage balancing and PWM scheme is validated under transient and steady-state conditions including low-frequency operation.