Analysis of proportional-resonant damping factors in the parallel operation of UPSs

Accurate load-sharing and circulating current mitigation is a particular problem in parallel-connected inverters without intercommunication. This paper analyzes how using multiple-resonant controllers in the voltage regulation loop impacts the parallel operation of uninterruptible power supplies (UPSs) with droop control. It is shown how adding damping factors of all modes of the resonant controller reduces the circulating current between the UPSs, also improving the power-sharing closed-loop stability and performance. Moreover, the proper choice of these damping coefficients can replicate the effects of adding virtual resistance loops to the droop controller structure. An analysis of such coefficients on the stability margins of the voltage regulation system is carried out and the methodology is validated by experimental results considering the parallelism of two 3.5 kVA UPSs with parametric differences in their $LC$ filters.