Synchronised PWM strategies based on space vector approach. I. Principles of waveform generation

The authors deal with the space-vector-based generation of synchronised three-phase PWM waveforms with three-phase, half-wave and quarter-wave symmetries. The necessary and sufficient conditions for synchronisation and the waveform symmetries are brought out in terms of the inverter states. These conditions are applied to the conventional and modified forms of space vector modulation to generate synchronised PWM waveforms with all the symmetries. Further, the necessary conditions governing the numbers of switchings of the different phases in any even sector to maintain the symmetries are identified. These rules are used to develop two new clamping strategies, namely the asymmetric zero-changing strategy and the boundary sampling strategy, which exploit the flexible features of space vector methods like double-switching of a phase and clamping of two phases in a sampling period. Typical waveforms corresponding to the different strategies are presented along with their theoretical and experimental spectra