Improved NL-PWM Method Based on Sawtooth Carrier Modulation for Common-Mode Voltage Reduction in MMC

Nearest level PWM (NL-PWM) is commonly used in low-level modular multilevel converter (MMC) due to its good harmonic characteristic. However, common-mode voltage (CMV) results from the asymmetry of the modulated three-phase voltage, leading to bearing failures, deterioration of the stator insulation, and electromagnetic interference (EMI). In this paper, an improved NL-PWM method based on sawtooth carrier modulation for complete CMV elimination is proposed. The proposal can improve the output current quality and also keep the maximum modulation ratio as 1 compared with the conventional NL-PWM. Finally, the validation of the proposed method is demonstrated by simulations.