Design of Advanced Voltage Controller for Three-Phase UPS inverters by KYUNG-HWAN KIM

Design of Advanced Voltage Controller for
Three-Phase UPS inverters
by
KYUNG-HWAN KIM
Under the supervision of
Prof. DONG-SEOK HYUN, Ph.D.
Dept. of Electrical Engineering
The Graduate School of Hanyang University

Inverter system, which uses LC component as the output filter, is essential for
a UPS (Uninterruptible Power supply), UPQC (Universal Custom Power Conditioner) and PCS (Power Conditioning System) for the photovoltaic power system.

This paper presents DSP (Digital Signal Processor) controlled voltage
controller for a 3-phase UPS inverter, which is able to compensate the voltage
distortions due to unbalanced and nonlinear loads. The paper discussed the problem
of conventional control schemes for the compensation of voltage distortion when
they are applied to UPS inverter, and proposes advanced synchronous reference
frame control scheme which is able to overcome the problem. In order to solve the
problem of low damping ratio of LC filter, the inverter current feed-forward
compensator including the selection of the feed-forward gain is proposed. And the
digital filter for a compensation of the voltage distortion due to unbalanced and
nonlinear loads is also proposed, which makes PI controllers in the synchronous
reference frame are able to operate with DC values even under nonlinear and
unbalanced load, likewise ensures PI controllers are able to provide zero steady
state error.
In order to improve the practical usability of the proposed scheme in the paper,
the realization of the PLL and the parallel operation, which are essential for UPS,
using the proposed controller is also presented. Especially, this paper shows that
the feed-forward compensation of the inverter current and the PWM
synchronization method proposed in the paper ensures a high precise load-sharing
performance in parallel operation of inverter system.
The effectiveness of the proposed scheme has been investigated and verified in
digital simulation. This thesis also investigates the digital implementation of the
proposed systems and all of the related theoretical concepts and control systems
have been verified in experimental prototype of the UPS systems.