Control Architecture for Parallel Inverter in Uninterruptible Power Systems Chi Zhang, Student Member, IEEE, Josep M. Guerrero, Fellow, IEEE, Juan C. Vasquez, Senior Member, IEEE, Ernane A.A. Coelho, Member, IEEE

 

Abstract— In this paper, a control strategy for the parallel operation of three-phase inverters forming an online uninterruptible power system (UPS) is presented. The UPS system consists of a cluster of paralleled inverters with LC filters directly connected to an AC critical bus and an AC/DC forming a DC bus. The proposed control scheme comprises two layers: (i) a local layer that contains a “reactive power-to-phase droop” in order to synchronize the phase angle of each inverter and a virtual resistance loop that guarantees equal power sharing among inverters; and (ii) a central controller that guarantees synchronization with an external real/fictitious utility, and critical bus voltage amplitude restoration. Improved transient and steady-state frequency, active, reactive and harmonic power sharing, and global phase-locked loop resynchronization capability are achieved. Detailed system topology and control architecture are presented in this paper. Further, a mathematical model was derived in order to analyze critical parameters effects on system stability. The proposed control approach has been validated by means of experimental results obtained for several case-study scenarios. Index Terms— UPS system; parallel inverters; voltage restoration; droop control; virtual impedance

LINK:https://vbn.aau.dk/files/219215517/Zhang_Chi_trans_manu_doublecol.pdf