{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,3,9]],"date-time":"2026-03-09T02:28:29Z","timestamp":1773023309820,"version":"3.50.1"},"reference-count":53,"publisher":"MDPI AG","issue":"15","license":[{"start":{"date-parts":[[2022,7,27]],"date-time":"2022-07-27T00:00:00Z","timestamp":1658880000000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Energies"],"abstract":"<jats:p>Mineral oil has long been used as an adequate coolant and dielectric medium in power transformer design. However, it is flammable and environmentally risky as it may be leaked or spilled. Therefore, ester fluids, which have been increasingly used in the last two decades, look promising as an ideal dielectric option. This research aims to better understand how using ester fluid insulation in power transformers impacts their physical and electrical dimensions, including their load-losses, impedance, masses, and equipment dimensions. Three case studies were carried out in a Portuguese electrical equipment manufacturer\u2019s facility, with varying electrical parameters and physical properties of the mineral oil and ester-filled power transformers. The main results enhanced the known good electrical behavior of ester fluids, namely creating a lower electric field around winding wedges, yet the use of ester fluids led to higher load-losses, larger masses, additional radiators and, consequently, higher manufacturing costs. Nevertheless, the contribution of ester-filled power transformers to the improved environmental safety (reducing spillage and fire risks), among other advantages, makes ester fluids a truly eco-friendly option for power transformer design.<\/jats:p>","DOI":"10.3390\/en15155418","type":"journal-article","created":{"date-parts":[[2022,7,27]],"date-time":"2022-07-27T21:11:05Z","timestamp":1658956265000},"page":"5418","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":12,"title":["Eco-Friendly Ester Fluid for Power Transformers versus Mineral Oil: Design Considerations"],"prefix":"10.3390","volume":"15","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-9904-7527","authenticated-orcid":false,"given":"Teresa","family":"Nogueira","sequence":"first","affiliation":[{"name":"School of Engineering, Polytechnic Institute of Porto (P. Porto), 4249-015 Porto, Portugal"},{"name":"Center for Innovation in Engineering and Industrial Technology (CIETI), P. Porto, 4249-015 Porto, Portugal"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-7106-5771","authenticated-orcid":false,"given":"Jos\u00e9","family":"Carvalho","sequence":"additional","affiliation":[{"name":"School of Engineering, Polytechnic Institute of Porto (P. Porto), 4249-015 Porto, Portugal"},{"name":"Center for Innovation in Engineering and Industrial Technology (CIETI), P. Porto, 4249-015 Porto, Portugal"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-2835-3169","authenticated-orcid":false,"given":"Jos\u00e9","family":"Magano","sequence":"additional","affiliation":[{"name":"Research Center in Business and Economics (CICEE), Universidade Aut\u00f3noma de Lisboa, 1150-293 Lisboa, Portugal"},{"name":"Higher Institute of Business and Tourism Sciences (ISCET), 4050-180 Porto, Portugal"}]}],"member":"1968","published-online":{"date-parts":[[2022,7,27]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"01053","DOI":"10.1051\/e3sconf\/20171401053","article-title":"Comparison of mineral oil and esters as cooling liquids in high voltage transformer in aspect of environment protection","volume":"14","author":"Dombek","year":"2017","journal-title":"E3S Web Conf."},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"e01159","DOI":"10.1016\/j.heliyon.2019.e01159","article-title":"Studies of different types of insulating oils and their mixtures as an alternative to mineral oil for cooling power transformers","volume":"5","author":"Rouabeh","year":"2019","journal-title":"Heliyon"},{"key":"ref_3","doi-asserted-by":"crossref","unstructured":"Pukel, G.J., Schwarz, R., Baumann, F., Muhr, H.M., Eberhardt, R., Wieser, B., and Chu, D. (2013). Power transformers with environmentally friendly and low flammability ester liquids. E I Elektrotechnik Und Inf., 1\u20136.","DOI":"10.1007\/s00502-012-0110-4"},{"key":"ref_4","first-page":"14","article-title":"Ester Fluids for Power Transformers at >100kV","volume":"1","author":"Midel","year":"2014","journal-title":"Transform. Mag."},{"key":"ref_5","doi-asserted-by":"crossref","unstructured":"\u0160\u00e1rpataky, M., Kurimsk\u00fd, J., and Raj\u0148\u00e1k, M. (2021). Dielectric Fluids for Power Transformers with Special Emphasis on Biodegradable Nanofluids. Nanomaterials, 11.","DOI":"10.3390\/nano11112885"},{"key":"ref_6","doi-asserted-by":"crossref","unstructured":"Rozga, P., Beroual, A., Przybylek, P., Jaroszewski, M., and Strzelecki, K. (2020). A Review on Synthetic Ester Liquids for Transformer Applications. Energies, 13.","DOI":"10.3390\/en13236429"},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"012026","DOI":"10.1088\/1742-6596\/2213\/1\/012026","article-title":"Ester-based Dielectric Fluid for Power Transformers: Design and Test Experience under the GreenEst Project","volume":"2213","author":"Carvalhosa","year":"2022","journal-title":"J. Phys. Conf. Ser."},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"102","DOI":"10.1016\/j.trpro.2019.07.017","article-title":"Different Ways to Improve Natural Ester Oils","volume":"40","author":"Totzauer","year":"2019","journal-title":"Transp. Res. Procedia"},{"key":"ref_9","doi-asserted-by":"crossref","unstructured":"Silva, C.S., Magano, J., Matos, A., and Nogueira, T. (2021). Sustainable Quality Management Systems in the Current Paradigm: The Role of Leadership. Sustainability, 13.","DOI":"10.3390\/su13042056"},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"6","DOI":"10.1109\/MEI.2009.5313705","article-title":"Experimental investigations on insulating liquids for power transformers: Mineral, ester, and silicone oils","volume":"25","author":"Perrier","year":"2009","journal-title":"IEEE Electr. Insul. Mag."},{"key":"ref_11","doi-asserted-by":"crossref","unstructured":"Boris, H., Gockenbach, E., and Dolata, B. (July, January 30). Ester fluids as alternative for mineral based transformer oil. Proceedings of the 2008 IEEE International Conference on Dielectric Liquids, Chasseneuil, France.","DOI":"10.1109\/ICDL.2008.4622541"},{"key":"ref_12","unstructured":"Borsi, H., and Gockenbach, E. (July, January 26). Properties of ester liquid midel 7131 as an alternative liquid to mineral oil for transformers. Proceedings of the IEEE International Conference on Dielectric Liquids, Coimbra, Portugal."},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"58","DOI":"10.1016\/j.epsr.2013.01.007","article-title":"Comparative evaluation of alternative fluids for power transformers","volume":"98","author":"Ortiz","year":"2013","journal-title":"Electr. Power Syst. Res."},{"key":"ref_14","unstructured":"Bhuyan, K., and Chatterjee, S. (2012). Non Linear Voltage Distribution in Windings of Power Transformer. Int. J. Eng. Res. Technol., 1."},{"key":"ref_15","doi-asserted-by":"crossref","unstructured":"Bhuyan, K., and Chatterjee, S. (2009, January 18\u201320). Surge Modelling of Transformer Using Matlab-Simulink. Proceedings of the 2009 Annual IEEE India Conference, Ahmedabad, India.","DOI":"10.1109\/INDCON.2009.5409414"},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"18","DOI":"10.1109\/MEI.2002.1014964","article-title":"Challenge of mixed insulating liquids for use in high-voltage transformers.1. Investigation of mixed liquids","volume":"18","author":"Fofana","year":"2002","journal-title":"IEEE Electr. Insul. Mag."},{"key":"ref_17","doi-asserted-by":"crossref","unstructured":"Liu, R., and Tornkvist, C. (2009, January 18\u201321). Ester fluids as alternative for mineral oil: The difference in streamer velocity and LI breakdown voltage. Proceedings of the 2009 IEEE Conference on Electrical Insulation and Dielectric Phenomena, Virginia Beach, VA, USA.","DOI":"10.1109\/CEIDP.2009.5377896"},{"key":"ref_18","doi-asserted-by":"crossref","unstructured":"Dang, V.-H., Beroual, A., and Perrier, C. (2011, January 26\u201330). Comparative study of statistical breakdown in mineral, synthetic and natural ester oils under AC voltage. Proceedings of the IEEE International Conference on Dielectric Liquids, Trondheim, Norway.","DOI":"10.1109\/ICDL.2011.6015408"},{"key":"ref_19","doi-asserted-by":"crossref","unstructured":"Wang, X., Tang, C., Huang, B., Hao, J., and Chen, G. (2018). Review of Research Progress on the Electrical Properties and Modification of Mineral Insulating Oils Used in Power Transformers. Energies, 11.","DOI":"10.3390\/en11030487"},{"key":"ref_20","doi-asserted-by":"crossref","first-page":"319","DOI":"10.1016\/S0378-3812(02)00025-0","article-title":"Review of thermodynamic properties of refrigerants + lubricant oils","volume":"199","author":"Marsh","year":"2002","journal-title":"Fluid Phase Equilib."},{"key":"ref_21","doi-asserted-by":"crossref","first-page":"6","DOI":"10.1109\/57.981322","article-title":"Vegetable oils for liquid-filled transformers","volume":"18","author":"Oommen","year":"2002","journal-title":"IEEE Electr. Insul. Mag."},{"key":"ref_22","doi-asserted-by":"crossref","first-page":"1626","DOI":"10.1109\/TDEI.2011.6032833","article-title":"A comparative study of physicochemical, dielectric and thermal properties of pressboard insulation impregnated with natural ester and mineral oil","volume":"18","author":"Liao","year":"2011","journal-title":"IEEE Trans. Dielectr. Electr. Insul."},{"key":"ref_23","doi-asserted-by":"crossref","first-page":"2021","DOI":"10.1109\/TDEI.2016.7556474","article-title":"Jatropha curcas methyl ester oil obtaining as vegetable insulating oil","volume":"23","author":"Sitorus","year":"2016","journal-title":"IEEE Trans. Dielectr. Electr. Insul."},{"key":"ref_24","doi-asserted-by":"crossref","first-page":"34","DOI":"10.1109\/MEI.2019.8636104","article-title":"Palm fatty acid ester as biodegradable dielectric fluid in transformers: A review","volume":"35","author":"Tokunaga","year":"2019","journal-title":"IEEE Electr. Insul. Mag."},{"key":"ref_25","unstructured":"(2022, May 12). Cargill Dielectric Fluids: Superior High-Performance Formulations for Power and Distribution Transformers. Available online: https:\/\/www.cargill.com\/bioindustrial\/dielectric-fluids."},{"key":"ref_26","first-page":"1","article-title":"Experiences in Service with New Insulating Liquids","volume":"436","author":"Martin","year":"2010","journal-title":"CIGR\u00c8"},{"key":"ref_27","unstructured":"(2022, May 20). Jim Fitch How to Test Flash Point. Available online: https:\/\/www.machinerylubrication.com\/Read\/19\/flash-point-test."},{"key":"ref_28","doi-asserted-by":"crossref","unstructured":"M\u00fcnster, T., Werle, P., H\u00e4mel, K., and Preusel, J. (2021). Thermally Accelerated Aging of Insulation Paper for Transformers with Different Insulating Liquids. Energies, 14.","DOI":"10.3390\/en14113036"},{"key":"ref_29","doi-asserted-by":"crossref","unstructured":"Garc\u00eda, B., Ortiz, A., Renedo, C., Garc\u00eda, D.F., and Montero, A. (2021). Use Performance and Management of Biodegradable Fluids as Transformer Insulation. Energies, 14.","DOI":"10.3390\/en14196357"},{"key":"ref_30","doi-asserted-by":"crossref","first-page":"110783","DOI":"10.1016\/j.rser.2021.110783","article-title":"A critical review of plant-based insulating fluids for transformer: 30-year development","volume":"141","author":"Shen","year":"2021","journal-title":"Renew. Sustain. Energy Rev."},{"key":"ref_31","doi-asserted-by":"crossref","first-page":"1546","DOI":"10.1109\/TDEI.2020.008765","article-title":"A review on pre-breakdown phenomena in ester fluids: Prepared by the international study group of IEEE DEIS liquid dielectrics technical committee","volume":"27","author":"Rao","year":"2020","journal-title":"IEEE Trans. Dielectr. Electr. Insul."},{"key":"ref_32","doi-asserted-by":"crossref","first-page":"124023","DOI":"10.1109\/ACCESS.2021.3109867","article-title":"Study of the Impregnation of Power-Transformer Cellulosic Materials with Dielectric Ester Oils","volume":"9","author":"Sanz","year":"2021","journal-title":"IEEE Access"},{"key":"ref_33","doi-asserted-by":"crossref","first-page":"129","DOI":"10.1016\/j.applthermaleng.2016.05.020","article-title":"Thermal degradation assessment of Kraft paper in power transformers insulated with natural esters","volume":"104","author":"Delgado","year":"2016","journal-title":"Appl. Therm. Eng."},{"key":"ref_34","doi-asserted-by":"crossref","unstructured":"Jariyanurat, K., Maneerot, S., Pattanadech, N., and Potivejkul, S. (2016, January 25\u201328). Electrical characteristics of natural ester impregnated pressboards under different periods of impregnation. Proceedings of the 2016 International Conference on Condition Monitoring and Diagnosis (CMD), Xi\u2019an, China.","DOI":"10.1109\/CMD.2016.7757835"},{"key":"ref_35","doi-asserted-by":"crossref","first-page":"374","DOI":"10.1109\/TDEI.2008.4483455","article-title":"A Comparison of the Impregnation of Cellulose Insulation by Ester and Mineral oil","volume":"15","author":"Dai","year":"2008","journal-title":"IEEE Trans. Dielectr. Electr. Insul."},{"key":"ref_36","doi-asserted-by":"crossref","unstructured":"Prevost, T.A. (2009, January 26\u201330). Dielectric properties of natural esters and their influence on transformer insulation system design and performance\u2014An update. Proceedings of the 2009 IEEE Power & Energy Society General Meeting, Calgary, AB, Canada.","DOI":"10.1109\/PES.2009.5275167"},{"key":"ref_37","doi-asserted-by":"crossref","unstructured":"Havran, P., Cimbala, R., Kurimsk\u00fd, J., Doln\u00edk, B., Kolcunov\u00e1, I., Medve\u010f, D., Kir\u00e1ly, J., Kohan, V., and \u0160\u00e1rpataky, \u013d. (2022). Dielectric Properties of Electrical Insulating Liquids for High Voltage Electric Devices in a Time-Varying Electric Field. Energies, 15.","DOI":"10.3390\/en15010391"},{"key":"ref_38","doi-asserted-by":"crossref","unstructured":"Haegele, S., Vahidi, F., Tenbohlen, S., Rapp, K., and Sbravati, A. (2017, January 25\u201329). Investigation of interfacial surface creep breakdown at oil-pressboard interfaces in natural ester liquid and mineral oil. Proceedings of the 2017 IEEE 19th International Conference on Dielectric Liquids (ICDL), Manchester, UK.","DOI":"10.1109\/ICDL.2017.8124605"},{"key":"ref_39","doi-asserted-by":"crossref","unstructured":"Sbravati, A., Rapp, K., Schmitt, P., and Krause, C. (2017, January 25\u201329). Transformer insulation structure for dielectric liquids with higher permittivity. Proceedings of the 2017 IEEE 19th International Conference on Dielectric Liquids (ICDL), Manchester, UK.","DOI":"10.1109\/ICDL.2017.8124705"},{"key":"ref_40","doi-asserted-by":"crossref","first-page":"1894","DOI":"10.1109\/TPWRD.2014.2305449","article-title":"Assessing the Use of Natural Esters for Transformer Field Drying","volume":"29","author":"Villarroel","year":"2014","journal-title":"IEEE Trans. Power Deliv."},{"key":"ref_41","doi-asserted-by":"crossref","first-page":"174651","DOI":"10.1109\/ACCESS.2019.2957171","article-title":"Thermal Modelling of a Power Transformer Disc Type Winding Immersed in Mineral and Ester-Based Oils Using Network Models and CFD","volume":"7","author":"Santisteban","year":"2019","journal-title":"IEEE Access"},{"key":"ref_42","doi-asserted-by":"crossref","first-page":"1511","DOI":"10.1109\/TDEI.2009.5361569","article-title":"Comparative PD pulse burst characteristics of transformer type natural and synthetic ester fluids and mineral oils","volume":"16","author":"Pompili","year":"2009","journal-title":"IEEE Trans. Dielectr. Electr. Insul."},{"key":"ref_43","doi-asserted-by":"crossref","first-page":"8","DOI":"10.1109\/MEI.2018.8445430","article-title":"Increased loadability of transformers using natural ester and cellulosic materials as high temperature insulation systems","volume":"34","author":"Vasconcellos","year":"2018","journal-title":"IEEE Electr. Insul. Mag."},{"key":"ref_44","doi-asserted-by":"crossref","first-page":"506","DOI":"10.1109\/TDEI.2011.5739456","article-title":"Electrical characteristics of natural and synthetic insulating fluids","volume":"18","author":"Arazoe","year":"2011","journal-title":"IEEE Trans. Dielectr. Electr. Insul."},{"key":"ref_45","unstructured":"(2000). Power Transformers\u2014Part 3: Insulation Levels, Dielectric Tests and External Clearances in Air (Standard No. IEC-60076-3)."},{"key":"ref_46","doi-asserted-by":"crossref","unstructured":"Trkulja, B., Drandi\u0107, A., Milardi\u0107, V., and \u017diger, I. (2021). Evaluation of Methodology for Lightning Impulse Voltage Distribution over High-Voltage Windings of Inductive Voltage Transformers. Energies, 14.","DOI":"10.3390\/en14165144"},{"key":"ref_47","doi-asserted-by":"crossref","first-page":"297","DOI":"10.1016\/j.proeng.2017.09.717","article-title":"Power transformer winding model for lightning impulse testing","volume":"202","author":"Trkulja","year":"2017","journal-title":"Procedia Eng."},{"key":"ref_48","unstructured":"Makuc, D., Ikanovi\u0107, J., and Lenasi, K. (2003). Electric Field Analysis of the Insulation Structure of Power Transformer. Computer Engineering in Applied Electromagnetism, Springer."},{"key":"ref_49","unstructured":"(2022, April 05). Omgfreestudy Difference between Core Type and Shell Type Transformer Characteristics. Available online: https:\/\/omgfreestudy.com\/difference-between-core-type-and-shell-type\/."},{"key":"ref_50","doi-asserted-by":"crossref","first-page":"312","DOI":"10.1016\/j.proeng.2017.09.719","article-title":"Transformer electric field calculation using BEM and FEM","volume":"202","author":"Trkulja","year":"2017","journal-title":"Procedia Eng."},{"key":"ref_51","unstructured":"Kulkarni, S., and Khaparde, S. (2012). Transformer Engineering Design, Technology, and Diagnostics, CRC Press."},{"key":"ref_52","doi-asserted-by":"crossref","unstructured":"Ziomek, W., Vijayan, K., Boyd, D., Kuby, K., and Franchek, M. (2011, January 5\u20138). High voltage power transformer insulation design. Proceedings of the 2011 Electrical Insulation Conference (EIC), Annapolis, MD, USA.","DOI":"10.1109\/EIC.2011.5996148"},{"key":"ref_53","unstructured":"(2014). Commission Regulation (EU) No 548\/2014 Regulations. Off. J. Eur. Union, L, 152\/1."}],"container-title":["Energies"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/1996-1073\/15\/15\/5418\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,10]],"date-time":"2025-10-10T23:57:15Z","timestamp":1760140635000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/1996-1073\/15\/15\/5418"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2022,7,27]]},"references-count":53,"journal-issue":{"issue":"15","published-online":{"date-parts":[[2022,8]]}},"alternative-id":["en15155418"],"URL":"https:\/\/doi.org\/10.3390\/en15155418","relation":{},"ISSN":["1996-1073"],"issn-type":[{"value":"1996-1073","type":"electronic"}],"subject":[],"published":{"date-parts":[[2022,7,27]]}}}