R. Nasrollahi - Department of Electrical Engineering, Central Tehran Branch, Islamic Azad University, Tehran, Iran
H. Feshki Farahani - Department of Electrical and Computer Engineering, Ashtian Branch, Islamic Azad University, Ashtian, Iran
M. Asadi - Faculty of Electrical and Computer Engineering, Arak University of Technology, Arak, Iran
M. Farhadi-Kangarlu - Faculty of Electrical and Computer Engineering, Urmia University, Urmia, Iran
P. Amiri - Faculty of Electrical Engineering, Shahid Rajaee Teacher Training University, Tehran, Iran

kground and Objectives: Due to the increased sensitive loads, improving power quality in distribution grids by custom power tools is one of the important fields of electrical engineering. This paper proposes a new kind of three-phase three-wire dynamic voltage restorer (without including storage sources or DC link) and also its control method.Methods: The proposed structure includes an AC/AC converter, low-pass filters at the input and output sides, and three-phase injection transformers. The control system is based on the combination of feedback and feedforward control that its advantages are high speed, good response quality, and very simple implementation. To overcome the harmonics raised from AC/AC converter switching on the main line, a SOGI-PLL has been used. Also, SOGI-PLL operates independently on each phase so that the asymmetric voltage variations can be identified.Results: The proposed control method is capable to compensate the power quality problems such as voltage sag, swell, and harmonics in balanced and unbalanced conditions. The detailed modelling and design of the proposed controller are verified through computer simulations and experimental results under different operating conditions. Simulation and experimental results show that the proposed control strategy can compensate the power quality events as close as possible to the desired values under different operation modes.Conclusion: In this paper, a three-phase three-wire dynamic voltage restorer (DVR) was assessed using direct AC/AC converters without a supply source and DC link. A control system based on combined feedback and feedforward control (CFBFFC) and SOGI-PLL has been proposed for the DVR. The simulation results on a three-phase ۲۰kV system as well as the experimental results obtained from a single-phase ۲۲۰V system verified the performance of the DVR and the control system. It was shown that this structure can compensate for ۰.۵pu voltage sag, above ۱pu voltage swell, and all kinds of harmonic faults.

Dynamic Voltage Restorer, Power Quality, AC/AC Converter, Combined Feedback and Feedforward Control