Improved Carrier-based Modulation for Four-level Neutral Point Clamped Inverters

Abstract

This paper proposes to implement an improved carrier modulation technique to reduce the distortion of voltage and load currents for the four-level NPC inverter. By changing the offset function compared to conventional carrier-based modulation methods, the proposed method adds a sub-offset component of the control voltage in the two-level inverter, causes the duty-time to hold the redanduncy state vectors to be evenly divided in a switching sequence, which improves the harmonic characteristics. Furthermore, this paper presents a strategy for changing the offset based on the idea of reducing the common mode voltage amplitude in order to expand the working area of the inverter to reach the modulation index m = 1 compared to the SinPWM (SPWM) method. From there, three improved carrier modulation techniques are proposed: Improved SPWM technique, Improved average offset technique and Improved Extended SPWM technique. The proposed techniques to improve harmonic distortion (THD), expand the operation region of inverter, shown by simulation results for 4-level NPC inverter.