A New Framework For Handling Some Critical Issues on Neutral Point Clamped Three-Level Inverter With Unbalanced DC-Links

In this paper, a new framework for handling the common-mode voltage (CMV), neutral point (NP) voltage oscillation and distorted space vector issues are proposed. It is seen that, if the maximum, medium and minimum voltage outputs among the three phases of the inverter can be determined, the reduction of the CMV, the cancellation of the NP voltage oscillation and the compensation of the distorted space vector for the neutral point clamped three-level inverter (NPC TLI) can then be easily implemented. Especially, by injecting a specially designed zero-sequence in the three-phase reference voltage, the volt-second balance can also be maintained in the case that the dc-links are unbalanced. The main contribution of this work is twofold: (i) A relatively simpler framework that is capable of reducing the CMV, eliminating the NP voltage oscillation and compensating the distorted space vector, simultaneously, is proposed; (ii) The total harmonic distortion (THD) of the output current will reach such a desirable level that is far less than the one processed by the existing methods. The relevant simulations for NPC TLI are conducted to confirm the advantages and effectiveness of the proposed algorithms.