Zero-sequence manipulation to maintain correct operation of NPC-PFC rectifier upon neutral line disconnection and reconnection

In this paper, the reason for operational instability occurring upon neutral line disconnection and reconnection of active three-phase three-level neutral-point-clamped power factor correction rectifiers is revealed and simple solution allowing to maintain correct operation is proposed. It is shown that the neutral line disconnection/reconnection inverts the sign of the partial dc-link voltages balancing loop; thus, reversing the sign of zero-sequence component created by the balancing loop controller would eliminate the mentioned drawback by keeping the balancing loop negative feedback. The proposed finding is experimentally verified on a commercial 25-kVA NPC-PFC front end.