A new programmed PWM technique to selectively eliminate several lower order harmonics at the output of a neutral point clamped (NPC) inverter topology is investigated. The switching function approach is utilized to derive relevant analytical expressions for input/output variables. A thorough evaluation of the NPC inverter topology based on the switching function approach is described. Further, optimal power control strategies are proposed for an NPC inverter employing programmed PWM patterns. For a constant frequency variable-voltage NPC inverter power supply, the proposed strategy is to maintain a minimum specified total harmonic distortion (THD) employing a low-output impedance filter, which is a performance index that guarantees high-quality output voltage and simultaneous immunity to unbalanced and nonlinear loads. In the case of an NPC inverter powering an ac motor drive, the proposed strategy is to maintain a minimum specified harmonic current factor (HCF), which is a performance index that guarantees a low percentage of the undesirable harmonic current. Finally, the power control strategies proposed in this paper are achieved without substantial increase in inverter switching frequency and are therefore suitable for high-power applications employing gate turn-off thyristor (GTO)-type devices. Selected results have been verified experimentally.
