Gain Tuning for Stability in the Low Line Impedance Region for Autonomously Controlled Parallel-Connected UPS System

This paper proposes a gain tuning technique to achieve stability in the low line impedance region for multiple autonomously controlled UPSs connected in parallel. In this independent control, with the information of the output current, controllers can determine the output voltage variables (phase angle and amplitude variation). Firstly, an investigation on the stability is carried out through computer simulations. A root locus analysis based on a small-signal model shows that a specific pair of eigenvalues is sensitive to the variation of the gains of the control equation for the phase angle. With this premise, a gain tuning technique determines a stable value of the control gains according to the locus of the eigenvalues. Experimental results validate the independent control with this method for gain tuning and exhibit a good performance in terms of stability and sharing of active and reactive currents