THE TRAJECTORY TRACKING APPROACH - A NEW METHOD FOR MINIMUM DISTORTION PWM IN DYNAMIC HIGH-POWER DRIVES

Feedforward pulsewidth modulators generate volt . second differences between the controlling reference signal and the switched output waveform during transient operation. This dynamic modulation error is particularly pronounced at low switching frequency, e.g., with modern high-power GTO inverters. The compensation of this error is a key requirement when designing an optimal pulsewidth modulator for these applications. The trajectory tracking approach uses the steady-state trajectories of the current vector, computed from the optimal switching sequences, as templates. A tracking controller counteracts if the actual current vector deviates from this given path at transient operation. A decomposition of the current vector trajectory into its components enables, in addition, the identification of the instantaneous fundamental current, permitting the implementation of a fast current control system. Oscillograms from a 30-kW model drive demonstrate the high quality in terms of dynamic performance and minimum harmonic distortion obtained with a three-level inverter at only 200 Hz switching frequency.