Received January 22, 2022, accepted February 4, 2022, date of publication February 7, 2022, date of current version February 18, 2022. Digital Object Identifier 10.1109/ACCESS.2022.3149802 Fuzzy Based Condition Monitoring Tool for Real-Time Analysis of Synthetic Ester Fluid as Transformer Insulant MRIDULA , SHUFALI ASHRAF WANI, A. J. AMALANATHAN, AND R. SARATHI , (Senior Member, IEEE) Department of Electrical Engineering, IIT Madras, Chennai 600036, India Corresponding author: R. Sarathi (rsarathi@iitm.ac.in) This work was supported by the Science and Engineering Research Board (SERB), New Delhi, India, under Grant CRG/2020/003525/EEC (for advanced transformer insulation). ABSTRACT The main aim of the present study is to explore the use of synthetic ester fluid as a transformer insulant. For this, the performance of thermally aged synthetic ester is investigated using corona inception voltage (CIV) measurement, fluorescent analysis, and dielectric spectroscopic studies. Then a fuzzy model is introduced to correlate the outcome of these studies to predict the key performance index and lifetime estimation of insulation structures. CIV tests for the thermally aged ester fluid, under AC voltage show 53.2% and 34% reduction in inception values due to ageing time and ageing temperature respectively. Furthermore, the effect of magnetic field on the performance of thermally aged ester is also studied and a drastic reduction of 54.3% and 39.4% respectively in CIV is observed. Statistical studies also indicate accelerated degradation of the insulation performance under the influence of the magnetic field, showing high scatter in CIV data indicated by decreasing beta values. Other studies such as fluorescent spectroscopy depict progressive shifts in fluorescence maxima and a decrease in emission intensity with ageing. Dielectric spectroscopy results show a manifold increase in dissipation factor due to thermal ageing. A fuzzy monitoring tool uses spectral data as input and at the output indicates the level of deterioration. The Rule-base of the model is framed using dielectric study. INDEX TERMS Ester, corona, magnetic field, frequency, hysteresis, Weibull, fluorescence, dielectric loss, fuzzy model. I. INTRODUCTION Transformers form an essential part of the power system network. The information about the longevity of trans- formers is determined basically by the health of their insulation. Mineral oil is the most commonly used insulating liquid used in transformers. However, moisture intolerance, non-biodegradability and corrosive nature due to sulphur formation provoke the search of finding better alternatives of existing mineral oil [1]–[5]. The new age insulating liq- uids viz., synthetic ester fluids are gaining popularity due to their high biodegradability, higher flash point, fire point and moisture tolerance. The increased moisture tolerance allows more water, resulting from cellulose to be absorbed by ester fluids. This results in slower ageing of solid insulation The associate editor coordinating the review of this manuscript and approving it for publication was Giovanni Pau . thereby enhancing the life of transformers [6], [7]. However, degradation of transformer insulation due to inevitable ther- mal ageing causes impairment of these properties and results in impending faults. Such faults if left unattended may lead to catastrophic failure of insulation structure and hence con- tinuous assessment of insulation condition becomes essen- tial. Since the condition of the oil is a direct indicator of transformer condition, therefore condition monitoring of transformers is majorly associated with the timely assessment of the condition of oil. A widely used traditional technique for transformer condi- tion monitoring is Dissolved Gas Analysis (DGA). However, it helps in incipient fault recognition and cannot assess the ageing condition of oil. Chemical tests for moisture and acidity calculation of oil are also helpful in predicting the health of aged oil but these are time-consuming [8]. One of the major problems in transformers is the discharge initiated VOLUME 10, 2022 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ 18055