Unified Control of Three-Level PFC Rectifiers in 3-Wire and 4-Wire Configurations

This article presents a comprehensive analysis of neutral wire disconnection event in active three-phase three-level neutral-point-clamped (NPC) power factor correction (PFC) rectifier operating as front-end of uninterruptible power supply (UPS). It is revealed that upon four-wire to three-wire configuration transition, partial DC link voltages control loops become positive-feedback, yielding operational stability loss. Consequently, it is proposed to reverse partial DC link voltage loop gain polarities (e.g., by swapping the feedback signals) upon the above-mentioned event detection. As a result, partial DC link voltage loops retain negative feedback and stable operation of the system is preserved. The proposed approach was verified experimentally on an off-the-shelf 25 kVA commercial UPS system.