Optimal Control of Medium-Voltage Drives—An Overview

Carrier modulation is the most common method for inverter control of ac drive systems. Operation at switching frequencies of several kilohertz is customary to restrain the harmonic distortion of the motor currents. Lower switching frequency is preferred for medium-voltage drives, owing to the higher switching losses of the semiconductor devices. This calls for optimizing the performance of the pulsewidth modulator. Improvements are achieved by abolishing carrier modulation with its equidistant time spacing of the voltage pulses. Optimal pulse patterns can be precalculated for every steady-state operating point of the drive. The patterns are stored in a memory of the drive system, from which they are retrieved and used for inverter control. Control at transient operation is achieved by adapting the optimal steady-state pulse patterns to the respective situation. Specific trajectories of the motor current space vector are then created by online optimization. An alternative method of online optimization relies on predicting the space vector trajectories for the next possible inverter switching states. The switching state that leads to minimum switching frequency is then selected. Optimal pulsewidth modulation reduces harmonic distortion. It permits operation at very low switching frequency and reduces the switching losses. This increases the current-carrying capability of the semiconductor devices to the extent that the power rating of an inverter approximately doubles.