Reverse droop control-based smooth transfer strategy for interface converters in hybrid AC/DC distribution networks

Hybrid AC/DC distribution networks are promising candidates for future applications due to rapid advancement in power electronics technology. They use interface converters (IFCs) to link DC and AC distribution networks. However, they possess drawbacks of the AC voltage and frequency offsets when transferring from grid-tied to islanding modes. To tackle this problem, this paper proposes a simple but effective strategy based on reverse droop method. Initially, the power balance equation of the distribution system is derived, it reveals that the cause of voltage and frequency offsets is the mismatch between IFC output power and the rated load power. Then, the reverse droop control is introduced into the IFC controller. By using a voltage-active power / frequency-reactive power (U-P /f-Q) reverse droop loop, the IFC output power enables adaptive tracking of the rated load power. Therefore, the AC voltage offset and frequency offset are suppressed during the transfer process of operational modes. In addition, the universal parameter design method is discussed based on the stability limitations of the control system and the voltage quality requirements of AC critical loads. Finally, simulation and experimental results clearly validate the proposed control strategy and parameter design method.