DC Link Voltage Unbalance Control in Three-Phase UPQCs based on NPC Topologies

The DC link voltage unbalance phenomena in three-phase UPQCs based on NPC (Neutral Point Clamped) converters is studied. The work identifies the causes for the phenomena and proposes a control strategy to mitigate it. The strategy is conceived for carrier-based modulating techniques. Specifically, the middle point of the carriers is used to produce a zero current sequence that is used to balance the DC link voltages. An interesting outcome is the mathematical justification for using either the series or the parallel converter to control the unbalance as a function of the operating point of the system. As a result, the UPQC can keep the load voltage and PCC power factor while the DC link voltage unbalance is minimized even during transient conditions at both the PCC voltage level and load level. Simulated preliminary results are used to probe the theoretical considerations.