Effective Space Vector Sequences for Bootstrapped Operation of a Three-level NPC Inverter

Abstract:
This paper proposes a space vector based PWM approach to operate a bootstrapped three-level inverter (3LI) expanding upon the Multi Level Bootstrap Charge sequence introduced in a prior work. A bootstrapped 3LI offers many advantages of a 3LI at a size comparable to that of a two-level inverter (2LI) but maintaining the charge on the bootstrap capacitors while ensuring stable inverter operation becomes a challenge. SPICE simulations are carried out to look at the bootstrap capacitor voltages for different capacitor sizes and PWM sequences and arrive at a suitable choice for the capacitor size. Altering the sequence for low modulation indices is examined which reduces the total harmonic distortion (THD) of the load current while still ensuring charging of the bootstrap capacitors. Modifications to the MLBC sequence to maintain a stable neutral point voltage in the case of fluctuations are also discussed. The effectiveness of the proposed method is verified using experiments on a 5kW motor.