Average Neutral Point Current Control Strategy in Neutral-Point-Clamped Converters

A control strategy for regulating the average neutral point (NP) current in neutral-point-clamped (NPC) converters is proposed in this paper. Previously presented strategies in the literature control the instantaneous NP current. However, the instantaneous maximum NP current profile is time-varying, which results in difficulties to regulate the average NP current linearly. The proposed strategy in this paper overcomes this drawback by directly controlling the average NP current. Furthermore, it can compensate nonzero average NP currents produced by unbalanced dc loads connected to the converter. In this strategy, the required nonzero average NP current reference is compensated by switching one or two phases between the negative and positive dc-rails for a calculated time interval. The proposed strategy also provides the range for stable operation of the NPC converter by formulating the maximum average current which can be injected into the NP. The effectiveness of the proposed control strategy is validated by performing simulation tests in MATLAB/Simulink environment under variable dc-loading conditions.