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
 ABSTRACT

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.
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