ENHANCED CONTROL OF FOUR-LEVEL NNPC INVERTER FOR MEDIUM-VOLTAGE DRIVE APPLICATIONS

Multilevel inverters have been extensively employed in motor drive systems owing to their capability to produce high-quality output voltage waveforms, reduce dv/dt stress, and improve overall system efficiency. Among these, four-level topologies are particularly attractive for medium-voltage applications because of their superior waveform quality, higher voltage rating, and better suitability for high-power drives. However, conventional four-level inverters often suffer from capacitor voltage imbalance, which can significantly degrade output quality, induce harmonic distortion, and compromise drive reliability. The Four-level Nested Neutral-Point-Clamped (4L-NNPC) inverter has emerged as a promising solution, offering improved flying-capacitor (FC) voltage balancing under rated conditions. Nevertheless, maintaining stable FC voltages remains challenging under dynamic operating conditions, where irregular capacitor charging can lead to voltage drift, increased ripple, and higher common-mode voltage, thereby reducing the overall performance of the drive. Therefore, this paper proposes a high-performance control strategy to improve total harmonic distortion (THD) and reduce common-mode voltage (CMV) in a field-oriented control (FOC) medium-voltage induction motor drive fed by a 4LNNPC inverter. A voltage-balancing control scheme is integrated to regulate FC voltages across a wide range of operating conditions, ensuring reliable and efficient motor operation. Simulation results demonstrate the effectiveness of the proposed strategies compared to conventional methods, confirming enhanced voltage stability, lower THD, and reduced CMV, which collectively contribute to superior drive performance and robustness.