Distributed Hierarchical Control With Separation of Concerns for Parallel-Connected UPSs

This article presents a distributed hierarchical control structure for parallel uninterruptible power supplies. A dynamic approach with separation of concerns is proposed for the secondary control, where a so-called grid forming inverter is responsible for restoring the amplitude and frequency of the output voltage, while the remaining inverters, called grid supporting, are responsible for enhancing long-term power sharing. This strategy aims to minimize the tradeoff between voltage regulation and power sharing through secondary control, which uses a low bandwidth communication link. For the primary controller, resistive droop control with virtual resistance is employed to accomplish short-term power sharing. Simple design guidelines for the secondary controllers based on simplified models developed in the article are provided. Extensive experimental results obtained from two 500-VA prototypes demonstrated the good performance of the proposed strategy. The amplitude and power sharing errors remained below 1% and 2%, respectively, for the entire load range evaluated.