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

Feedforward pulsewidth modulators generate volt . second differences b etween the controlling reference signal and the switched output wavefo rm during transient operation. This dynamic modulation error is partic ularly pronounced at low switching frequency, e.g., with modern high-p ower GTO inverters. The compensation of this error is a key requiremen t when designing an optimal pulsewidth modulator for these application s. 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 decomp osition of the current vector trajectory into its components enables, in addition, the identification of the instantaneous fundamental curre nt, permitting the implementation of a fast current control system. Os cillograms from a 30-kW model drive demonstrate the high quality in te rms of dynamic performance and minimum harmonic distortion obtained wi th a three-level inverter at only 200 Hz switching frequency.