Theoretical and Experimental Verification of Independent Control for Parallel-Connected Multi UPS

Eduardo Kazuhide SATO  Atsuo KAWAMURA  

IEICE TRANSACTIONS on Communications   Vol.E87-B   No.12   pp.3490-3499
Publication Date: 2004/12/01
Online ISSN: 
Print ISSN: 0916-8516
Type of Manuscript: Special Section PAPER (Special Section on the 25th International Telecommunications Energy Conference (INTELEC'03))
Category: Rectifiers, Inverters and UPS
uninterruptible power supply,  parallel connection,  independent control,  stability analysis,  small-signal model,  lateral current,  

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This paper proposes an independent control for parallel-connected multiple uninterruptible power supply (UPS) systems based upon a very simple control scheme. Here, the amplitude and phase angle of the output voltage are the controllable variables. With the only measurement of the output current, the active and reactive components are calculated to define the control variables. The entire system including the equations for the circuit, control and voltage limiters is well represented by a small-signal model, in which the computation of its eigenvalues constitutes the stability proof of the system. The root locus diagram gives an overall panorama of the system performance as a function of a certain gain and it aims to aid the further understanding and the design of the control. The experimental verification is carried out using a mere proportional-integral control scheme, which is a special case of the general control equation used in the theoretical analysis. For some situations, experiments show a flow of lateral current between UPS's, which causes an unbalanced current distribution. By increasing the proportional gain of the control equation for the output voltage amplitude, the lateral current can be substantially suppressed with a consequent improvement of the load sharing. Experimental results under various conditions show excellent results in terms of synchronization, load sharing and stability for three distinct output rating UPS's connected in parallel.