Control of medium voltage drives at very low switching frequency

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Medium-voltage ac drives are widely used in industrial applications requiring adjustable frequency. This work focuses on controlling the voltage source inverter that supplies variable-frequency, switched voltage waveforms to the ac machine. The goal is to enable the inverter to operate at very low switching frequencies, down to 200 Hz, which reduces power semiconductor switching losses and allows for increased maximum load current. However, this low switching frequency leads to high harmonic distortion in stator currents, resulting in increased machine losses. To address this, synchronous optimal pulsewidth modulation is implemented for inverter control, minimizing harmonic currents under steady-state conditions. A fast controller is introduced to eliminate harmonic excursions during changes in operating points, focusing on an optimal stator flux linkage trajectory that mitigates sensitivity to machine parameter variations. Additionally, the dynamic behavior of vector-controlled medium-voltage drives is examined, as low switching frequencies exacerbate torque-flux cross-coupling. Initially, linear current controllers are designed in the frequency domain to counteract this effect. Subsequently, a nonlinear controller is developed for synchronous optimal modulation, utilizing an optimal stator flux linkage trajectory to achieve deadbeat performance and complete decoupling.