A Harmonic Circulation Current Reduction Method for Parallel
Operation of UPS with a Three-Phase PWM Inverter
Kyung-Hwan Kim†, Wook-Dong Kim* and Dong-Suk Hyun**
†*EHWA Technologies and Information, Seoul, Korea
**Dept. of Electrical Eng., Hanyang University, Seoul, Korea
ABSTRACT
In a parallel operation of UPS, there are two types of circulating currents between UPS. One is the low order circulating
current with a fundamental frequency caused by the amplitude and phase differences of UPS output voltages, and the other
is the harmonic circulating current with PWM switching frequency caused by non-synchronized PWM waveforms among
UPS. The elimination of the low order circulating current is essential for optimal load sharing in parallel operations of UPS,
which can be accomplished by the phase and magnitude control at each UPS. The harmonic circulating current may cause
troubles and deteriorate in performance of the controller for optimal load sharing in parallel operation of UPS. This paper
presents a PWM synchronizing method to eliminate the harmonic circulation current in parallel operation of UPS. The
effectiveness of the proposed scheme has been investigated and verified through experiments by a 50kVA UPS.
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