Optimal pulse width modulation for three-level inverters

The three-level neutral-point-clamped voltage source inverter is widely used in high power, medium voltage applications. This paper studies continuous and discontinuous PWM for this inverter. Detailed analysis of discontinuous modulation shows that the average switching frequency is not directly proportional to the carrier or sampling frequency, since additional switching transitions occur between different regions of discontinuity. At typical switching frequencies for high power applications (up to 2 kHz) these additional transitions contribute significantly to the inverter's total losses, so that a proper comparison of the harmonic performance can only be carried out under constant loss conditions with varying carrier frequency. This comparison is performed for a typical industrial medium voltage inverter. The paper then considers the major issues of neutral-point voltage balancing and loss distribution within the the inverter, for the identified optimal modulation schemes.