Control of Medium-Voltage Drives at Very Low Switching Frequency

Medium-voltage ac drives are employed in numerous industrial setups that demand adjustable frequency. The present work focuses on the control of the voltage source inverter, which feeds the ac machine of the drive system with variable-frequency, switched voltage waveforms. The objective is to allow the inverter operate at very low switching frequency down to 200 Hz. The switching losses of the power semiconductors are then reduced which permits increasing the maximum load current of the inverter. Setting the switching frequency to very low values entails high harmonic distortion of the stator currents. The machine losses increase, as a consequence. To overcome this problem, synchronous optimal pulsewidth modulation is employed for inverter control; it minimizes the harmonic current at steady-state conditions. A fast controller is introduced: it eliminates harmonic excursions that occur when the operating point changes. Rather than the stator current, the method is based on the evaluation of an optimal stator flux linkage trajectory, which introduces insensitivity against variations of the machine parameters. A further issue of concern in the present work is the dynamic behavior of vector-controlled medium-voltage drives: low switching frequency values intensify the cross-coupling between torque and flux in vectorcontrolled systems. In a first approach, linear current controllers are designed in the frequency domain to compensate this undesired effect. A nonlinear controller is subsequently introduced, especially for operation at synchronous optimal modulation: it makes use of an optimal trajectory of the stator flux linkage vector to achieve deadbeat performance and complete decoupling.