Optimal pulsewidth modulation of cascade nine-level (9L) inverter for medium voltage high power industrial AC drives

Synchronous optimal pulsewidth modulation (SOP) is an emerging modulation technique for medium voltage industrial AC drives. SOP modulates multilevel converters with low device switching frequency while maintaining the quality of machine stator currents. The goal of our study is to implement SOP technique for cascaded nine-level (9L) inverter with average device switching frequency limited to rated fundamental frequency. SOP technique performs offline calculations to determine optimal switching angles for each steady-state operating point. The sequential steps in SOP are as follows: obtain number of switching angles for each steady-state operating point, perform offline optimization to determine switching angles and assign switching angles to each power semiconductor device based on the optimal switching angles. The switching angles are assigned to each power semiconductor device based on the criteria to minimize the device switching frequency as well as minimize the unbalance in dc-link capacitor voltages. The experimental results from low power prototype of cascaded 9L inverter feeding the 1.5-kW induction motor drive illustrated that harmonic distortion of stator currents is kept minimal, while average device switching frequency has been limited to rated fundamental frequency.