Synchronous optimal pulsewidth modulation and stator flux trajectory control for medium voltage drives

High-power PWM inverters for medium voltage drives operate at low switching frequency to maintain the switching losses of the power semiconductor devices at an acceptable level. For these applications, synchronous optimal pulsewidth modulation methods permit a good trade-off between switching losses and harmonic distortion of the machine currents. The optimization of the pulse patterns is done off-line on the assumption of steady-state conditions. Dynamic modulation errors, and high overcurrents as a consequence, are encountered when the operating conditions change during operation. To overcome this problem, the stator flux linkage vector, including its harmonics, is subjected to closed loop control. A stator flux trajectory controller is employed for this purpose. Experimental results obtained from an industrial 1-MVA, 4.16 kV three-level inverter ac drive are presented.