Generalized discontinuous DC-link balancing modulation strategy for three-level inverters

This paper presents a new generalized discontinuous space-vector modulation approach applicable to neutral point clamped (NPC) inverters. By the proposed modulation scheme the DC-link voltage can be actively controlled to maintain a stable neutral point even if unbalanced loading of the DC-link capacitors occur due to e.g. nonlinear loads containing even harmonics. The maximum unbalance for which the modulation scheme are able to compensate is theoretically investigated and found to be a function of both load angle, modulation index and output power level. The proposed modulation method is compared to conventional modulation schemes with regard to both switching losses and wave form quality. For the same number of switchings, the proposed modulation scheme offers up to 25% lower switching losses than conventional modulation schemes while maintaining the same output wave form quality. The functionality of the modulation scheme is validated by simulation results.