Two level inverters generate Common Mode Voltage (CMV) in the motor windings, causing motor failures due to bearing currents. Furthermore, a leakage current will flow throughout the stator windings and motor frame creating common mode electromagnetic interference. Taking these problems into consideration and the fact that the multilevel inverters offer better Total Harmonic Distortion (THD) as well as the possibility to reduce the CMV by modulation strategy, they it became a point of interest. Compared to higher level topologies, a good ratio between complexity and performance is offered by the three-level topology. As this type of inverter synthesises the output voltage from series connected DC link capacitors, problems such as neutral point balancing might appear, causing voltage stress on the semiconductors.
The aim of this project is to develop modulation strategies that address both DC link voltage balancing and CMV reduction.
Four modulation strategies that address these problems were developed and validated through simulation. Furthermore, in order to experimentally validate them, a Neutral Point Clamped (NPC) inverter was designed and built. Analysis, regarding CMV and Electromagnetic Interference (EMI) together with neutral point balance assessment, was performed on each developed modulation which concluded that all proposed strategies offer better CMV and EMI spectrum compared with classical NTV and ZCM.