Control of NPC Three Phase Three Level Inverter Using a Switching Logic-based Control Algorithm With Constant Switching Frequency

This paper proposes a control strategy for Neutral point clamped(NPC) three phase three level inverter which employs a hysteresis based frequency limiting switching algorithm to decide the switching instants. The low pass filtered inverter output voltage is used as the feedback signal. Depending on the slope of the feedback voltage, the control algorithm varies slightly for different regions of reference signal to provide constant switching frequency. The reference signal modification is another key feature that improves the quality of the output waveforms. This method ensures a fast transient response comparable with conventional hysteresis control method along with well defined harmonic spectrum and improved THD. Simulation results are included to verify the proposed concept.