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A two-stage carrier-based PWM (CB-PWM) strategy is proposed for a three-phase three-level neutral-point-clamped inverter to maintain its constant neutral point potential (NPP). The CB-PWM is a prevalent modulation method used to control inverters. However, the onemodulation-signal CB-PWM cannot mitigate the oscillation of the NPP. The two-modulation-signal CB-PWM, which can mitigate the oscillation of the NPP, requires many additional restrains with implicit principles. Besides, the capability to regulate the drift NPP in the second method is moderate. The proposed strategy in this Letter is to divide the modulation into two stages. The primary stage utilises the one-modulation-signal method. The secondary stage utilises the two-modulation-signal as the auxiliary to the former one. The modified strategy fixes the dc voltage imbalance and mitigates the oscillation of the NPP under any modulation index range and heavy load conditions. The results of simulations and experiments validate the proposed strategy as feasible and reliable.

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