Analysis and Mitigation of Low-Order-Harmonic Neutral-Point Voltage and Current Ripples in Five-Level NPC-Based Inverters

This paper investigates a single-phase 5-level hybrid neutral-point clamped (NPC)-based (H8 topology) inverter. The NPC-based inverters are widely deployed in industrial applications because they can achieve high efficiency and lower voltage stress across the switches. However, the significant challenge of the NPC-based inverters is the low-frequency ac capacitor-voltage ripple which contains the low-order harmonic components existing in the dc-link of the NPC-based inverters when the conventional modulation strategies are implemented. Because of the low-frequency voltage ripple, the output voltage may contain undesired low-order harmonics. In the present article, we propose a simple carrier-based modulation strategy to mitigate the low-frequency voltage oscillations and improve the output waveform purity. Moreover, the proposed technique reduces the ripple in the capacitor voltages by 97.7% from that of the conventional method. The performances of the inverter under the proposed scheme were verified in simulations and experimental studies.