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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.

The high-switching frequency operation is highly undesirable for multilevel inverters (MLIs) in electric transportation systems. In this article, a high-performance sequential model predictive control (SMPC) with reduced switching frequency is proposed for MLIs. The proposed SMPC is applied to a four-level inverter (FLI), in which the inverter ac current models are formulated by using the forward Euler integration method, whereas the inverter floating capacitor (FC) voltage models are formulated by using Heun's integration method. The use of Heun's method reduces the computational error of the FC's voltage, leading to a reduction in the number of switching transitions, thereby minimizing their tracking error. This process further leads to a reduction in their ripples with the available switching states. Also, it produces a lower current and voltage harmonic distortion while operating at a low switching frequency in comparison with conventional model predictive control (MPC) methods. The steady-state and transient performances of the proposed SMPC are demonstrated through the experimental studies on a dSPACE/DS1103 controlled laboratory prototype. The FLI's performance with the proposed SMPC is further analyzed in terms of average switching frequency, FC voltage ripple, total harmonic distortion, and computational burden. Finally, a comparative analysis of the proposed and conventional MPC methods is presented.