A simple modulation strategy to control the neutral point voltage deviation in three-level NPC inverters

The importance of the half wave symmetry for DC link neutral balancing of three-level NPC inverter has been described. This concept has been applied to regular sampled carrier based PWM. To achieve half wave symmetry, the carrier in-phase PWM requires an odd carrier/reference frequency ratio while the carrier phase-opposite requires an even ratio. There are no even harmonics in the output line voltage while carrier phase-opposite PWM presents more side-band harmonics especially for continuous modulation. For discontinuous modulation, there is no such difference. To verify the proposed concept, more simulation results are provided and discussed. Self balancing of DC link voltage can be achieved for most situations except heavy DC link capacitance discrepancy.