Global interest in environmentally friendly ships has surged as a result of greenhouse gas reduction policies and the demand for carbon neutrality. Despite growing demand for electric propulsion systems, there is a lack of research and development on crucial components. Efficiency and stability are primarily influenced by the performance of inverters, which are essential for driving propulsion motors. Existing inverter control techniques can be of two types: continuous-PWM (pulse width modulation) methods for harmonic performance enhancement and discontinuous-PWM methods for efficiency improvement by reducing losses. However, there are limitations in that each PWM method exhibits substantial variations in inverter performance based on its operating conditions. To address these challenges, this study proposes the hybrid pulse-width-modulation (HPWM) method for optimal inverter operation. By analyzing the inverter’s operating conditions, the proposed HPWM method adopts various pulse-width-modulation (PWM) strategies based on a modulation index to achieve harmonic improvement and loss reduction. Our focus is on comparing and analyzing diverse PWM techniques under varying modulation indices and frequency conditions to attain the optimal operating conditions. Experimental validation of the proposed method was conducted using a 2.2 kW dynamometer. In comparison with existing PWM methods, the proposed method demonstrated superior performance.

This study proposes a novel fault diagnosis algorithm based on Equivalent Series Resistance (ESR) estimation to enhance the accuracy of capacitor life diagnosis techniques for the DC link in marine electric propulsion systems. Accurate ESR estimation is critical for maintaining the reliability and efficiency of DC-Link capacitors, which play a key role in stabilizing voltage, reducing harmonics, and ensuring the smooth operation of electric propulsion systems. By preventing capacitor failures, this algorithm contributes to reducing the risk of catastrophic damage to entire systems. The ESR value is determined by extracting AC voltage and current data within the frequency range of 10 kHz to 30 kHz using a band-pass filter. To improve reliability, the algorithm compensates for input errors based on the modulation index and switching pattern, with error data stored in a lookup table. By addressing limitations in existing ESR estimation techniques, the proposed method reduces estimation errors across the entire range and enhances fault diagnosis accuracy. Experimental results validate the algorithm’s improved accuracy, reliability, and stability, demonstrating its effectiveness in preventing damage to power conversion devices.

This study proposes a novel finite control set model predictive control strategy (FCS-MPC) to suppress common-mode voltage (CMV) in electric propulsion systems while maintaining current quality. The key innovation is a virtual multi-vector synthesis method that eliminates zero-voltage vectors by adaptively generating small and medium vectors based on the modulation index and voltage sector. Unlike conventional CMV-reduction methods that compromise current quality or rely on fixed switching states, the proposed method enhances voltage resolution without relying on zero vectors. Simulation results demonstrate that, compared to conventional MPC, the proposed method reduces the CMV from approximately 100 V to 33 V—a 66.7% reduction. In terms of current quality, it achieves a 22.0% reduction in total harmonic distortion (THD) compared to the conventional reduced-CMV MPC method under low modulation conditions, while avoiding its excessive switching frequency. Experimental validation confirms both stable waveform generation and robust CMV suppression, while the proposed MPC reduces DSP execution time from 30.88 μs to 22.71 μs, thereby increasing computational availability to 77.3% and enabling real-time implementation on low-cost hardware. These results confirm the practicality of the proposed controller for real-time marine propulsion systems requiring both electromagnetic compatibility and high current quality.

Bearing damage caused by electric discharge machining (EDM) is a concern in electric ship propulsion drives, particularly during low-speed operations such as maneuvering and slow steaming. In a modular multilevel converter (MMC) operated with nearest-level modulation (NLM), rounding of the three-phase submodule insertion numbers produces a residual imbalance that appears as common-mode voltage (CMV) and charges the bearing film capacitance. The peak bearing voltage rises from 6.4 V at 60 Hz to 20.0 V at 10 Hz, while the thinning lubricant film lowers the dielectric breakdown threshold. Always-on CMV reduction approaches apply a corrected switching candidate in every control period, including intervals where the bearing voltage stays well below the threshold. This paper proposes a selective NLM correction driven by predicted bearing voltage risk: a reduced-order RC model predicts the bearing voltage the conventional NLM candidate would produce, and a hysteretic controller applies a zero-CMV candidate only when that prediction approaches the insulation threshold. Using a worst-case discharge criterion and thresholds of 5.9–29 V derived from elastohydrodynamic film thickness estimates, simulations at 10 Hz show that the method eliminates EDM events over the full evaluated threshold range. It achieves the same zero-EDM outcome as always-on correction while reducing the correction mode activation ratio from 100% to at most 30.8%, and remains inactive where conventional NLM is already safe.