Quantitative Analysis of Third-harmonic Neutral-point Current, its Impacts, and Mitigation in Three-level NPC Inverters

Three-level neutral-point clamped inverter (3LNPCI) is a widely popular topology in the realm of multi-level inverters. The low frequency third-harmonic neutral-point (NP) current in a 3L-NPCI and the corresponding NP-voltage oscillations on the DC-side are known characteristics of the 3L-inverter. In this paper, a quantitative analysis is presented for characterizing the DC-side third-harmonic phenomena observed in 3LNPCI. Departing from the traditional space-vector perspective, an alternate formulation based on time-domain functions and Fourier-series is adopted, and an analytical expression for the 3
^rd
harmonic NP-current is derived as a function of inverter modulation index and load power factor–this is applicable for all conventional carrier-based PWM techniques employed on 3LNPCIs. The corresponding NP-voltage variation on the DC-side and its translation onto the AC-side as low-frequency harmonic distortions in the inverter output voltage are analytically quantified. Based on this, a model-based observer is designed to mitigate the AC side distortions through accurate estimation of the disturbance variables and feed-forward cancellation. Experimental results along with a full-order switched model simulation of a three-phase 3L-NPCI system validate the theoretical analysis and mitigation technique proposed in this paper.