International Journal of Electrical and Computer Engineering (IJECE) Vol. 7, No. 2, April 2017, pp. 650~658 ISSN: 2088-8708, DOI: 10.11591/ijece.v7i2.pp650-658  650 Journal homepage: http://iaesjournal.com/online/index.php/IJECE Dielectric Strength Improvement of Natural Ester Insulation Oil via Mixed Antioxidants: Taguchi Approach Sharin Ab Ghani 1 , Zulkarnain Ahmad Noorden 2 , Nor Asiah Muhamad 3 , Hidayat Zainuddin 4 , Muhammad Ilman Hakimi Chua Abdullah 5 , Imran Sutan Chairul 6 1,2 Institute of High Voltage and High Current (IVAT), Universiti Teknologi Malaysia, 81310 Johor Bahru, Malaysia 3,6 School of Electrical and Electronic Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia 4 Faculty of Electrical Engineering, Universiti Teknikal Malaysia Melaka, 76100 Durian Tunggal, Melaka, Malaysia 5 Faculty of Engineering Technology, Universiti Teknikal Malaysia Melaka, 76100 Durian Tunggal, Melaka, Malaysia Article Info ABSTRACT Article history: Received Jun 9, 2016 Revised Nov 20, 2016 Accepted Dec 11, 2016 Recently, natural ester insulation (NEI) oils are found to be the best candidates to replace mineral-based insulation oils for oil-immersed transformer applications. However, NEI oilsare prone to oxidation due to their poor oxidative stability which can be improved by adding antioxidants into the oils. Latest studies have also shown that the use of selected antioxidants improves the AC breakdown voltage (BdV) of NEI oils. However, the experiments in previous studies were designed using the conventional one-factor-at-a-time (OFAT) method, which requires a large number of samples to be tested in order to determine the optimum response. Thus, a Taguchi-based designed experiment is introduced in this study in replacement of the OFAT method. It is found that this method is capable of determining the optimum concentrations of propyl gallate (PG) and citric acid (CA) which will maximize the AC BdV and improve the oxidative stability of the NEI oil. An AC breakdown voltage test is conducted in accordance with the ASTM D1816 standard using Megger OTS60PB portable oil tester, in which the electrode gap distance is kept fixed at 1 mm. The results indicate that the addition of PG and CA antioxidants increases theAC BdV of the rapeseed-based NEI oil. It is found that the optimum concentrations of PG and CA antioxidant is 0.05 and 0.25 wt.%, respectively. Lastly, the model developed in this study is analysed using analysis of variance (ANOVA). Validation test is also conducted on the optimized NEI oil to determine its dielectric strength and oxidative stability. Keyword: Antioxidants Breakdown voltage Design of experiments Natural ester insulation oil Optimization Oxidative induction time Taguchi Copyright © 2017 Institute of Advanced Engineering and Science. All rights reserved. Corresponding Author: Sharin Ab Ghani, Faculty of Electrical Engineering, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya 76100 Durian Tunggal, Melaka, Malaysia. Email: sharinag@utem.edu.my 1. INTRODUCTION Transformers are one of the key components in power network and their main function is to convert voltage and transfer energy. The reliability of power transformers is largely determined by the condition of their insulation [1]. In liquid-filled transformers, the insulation liquid has two important functions: (1) it provides electrical insulation and (2) it serves as a coolant by absorbing heat when there is temperature increase in the transformer windings and core. For more than a hundred years, liquid-immersed transformers are typically filled with mineral insulation oils due to their wide availability, good properties and low cost. However, due to environmental issues resulting from the use of mineral insulation oils, it becomes more