A Novel Control Strategy for Parallel Operation of Three-phase UPS Inverters

A Novel Control Strategy for Parallel Operation of
Three-phase UPS Inverters

By Sang-Hoon Lee
Under the supervision of Prof. Dong-Seok Hyun, Ph.D.
Dept. of Electrical Engineering
The Graduate School of Hanyang University

Parallel operation of uninterruptible power supply (UPS) system has been used to
increase power capacity of the system or to secure reliable supply of power to critical loads.
During parallel operation, load sharing control to maintain the current balance is critical
for reliable operation, since load sharing is very sensitive to differences in components of
each module such as amplitude /phase difference, line impedance, and output LC filters. To
solve these problems various control algorithms have been researched. However, these
methods cannot be applied to UPS systems with different ratings. For this case, master and
slave control algorithms for parallel operation is adequate. If the ratings of UPS systems are
different, the value of passive LC filters will be different, and it will affect current sharing.
This paper presents the general problems associated with parallel operation of UPS systems,
and control strategy for parallel operation with different ratings. The validity of the proposed
control strategy is investigated through simulation and experiment with two UPS systems.
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