Hybrid modulation for neutral point voltage ripple reduction in DC-fed three-level motor drive Systems

This paper presents a hybrid modulation method to limit the neutral point (NP) voltage ripple and reduce the requirement for dc link capacitors during the startup process for three level motor drive systems. The neutral point voltage calculation model is presented to study the large NP voltage ripple for three level modulation during the startup process. Moreover, the possible overvoltage issue on the semiconductors considering the different switching speed of the devices for two level modulation in three level NPC converters is analyzed in detail. To avoid such overvoltage on the semiconductors, zero neutral point voltage ripple modulation (ZNPVR) method is proposed to limit the NP voltage ripple when the output fundamental frequency is low. The implementation method of the proposed method is discussed in detail. When the output fundamental frequency is high, the modulation can smoothly transit to normal NTSV modulation for lower current ripple and higher efficiency. Both simulation and experimental results are shown to verify the analysis based on a 3 kW three level VSI dc-fed motor drive system.