Fundamental Switching Frequency Optimal Pulsewidth Modulation of Medium-Voltage Nine-Level Inverter

A low device switching frequency operation is essential in medium-voltage (MV) high-power drives to increase semiconductor device utilization as well as to achieve higher overall system efficiency. However, there exists a tradeoff between device switching frequency and harmonic distortion of machine currents. Synchronous optimal pulsewidth modulation (SOP) is an evolving technique for controlling multilevel inverters of MV ac drives at low device switching frequency without compromising the quality of machine currents. The state-of-the-art generalized SOP algorithm for five or higher level inverters leads to unequal device switching frequency, and also, transients in machine currents are possible due to the change in the number of switching instants at higher modulation index values. The goal of our study is to propose a new SOP technique that ensures equal device switching frequency and further reduces device switching frequency compared to the generalized SOP method. Also, it is expected that machine current transients at higher modulation index values will be reduced. The proposed SOP technique has been verified experimentally on a cascade nine-level inverter-fed 1.5-kW induction motor drive.