Condition Assessment of Natural Ester-Mineral Oil Mixture Due to Transformer Retrofilling via Sensing Dielectric Properties

Hesham S Karaman; Diaa-Eldin A Mansour; Matti Lehtonen; Mohamed M F Darwish

doi:10.3390/s23146440

Condition Assessment of Natural Ester-Mineral Oil Mixture Due to Transformer Retrofilling via Sensing Dielectric Properties

Sensors (Basel). 2023 Jul 16;23(14):6440. doi: 10.3390/s23146440.

Authors

Hesham S Karaman¹, Diaa-Eldin A Mansour^{2

3}, Matti Lehtonen⁴, Mohamed M F Darwish¹

Affiliations

¹ Department of Electrical Engineering, Faculty of Engineering at Shoubra, Benha University, Cairo 11629, Egypt.
² Department of Electrical Power Engineering, Egypt-Japan University of Science and Technology (E-JUST), Alexandria 21934, Egypt.
³ Department of Electrical Power and Machines Engineering, Faculty of Engineering, Tanta University, Tanta 31511, Egypt.
⁴ Department of Electrical Engineering and Automation, School of Electrical Engineering, Aalto University, 02150 Espoo, Finland.

Abstract

Mineral oil (MO) is the most popular insulating liquid that is used as an insulating and cooling medium in electrical power transformers. Indeed, for green energy and environmental protection requirements, many researchers introduced other oil types to study the various characteristics of alternative insulating oils using advanced diagnostic tools. In this regard, natural ester oil (NEO) can be considered an attractive substitute for MO. Although NEO has a high viscosity and high dielectric loss, it presents fire safety and environmental advantages over mineral oil. Therefore, the retrofilling of aged MO with fresh NEO is highly recommended for power transformers from an environmental viewpoint. In this study, two accelerated aging processes were applied to MO for 6 and 12 days to simulate MO in service for 6 and 12 years. Moreover, these aged oils were mixed with 80% and 90% fresh NEO. The dielectric strength, relative permittivity, and dissipation factor were sensed using a LCR meter and oil tester devices for all prepared samples to support the condition assessment performance of the oil mixtures. In addition, the electric field distribution was analyzed for a power transformer using the oil mixtures. Furthermore, the dynamic viscosity was measured for all insulating oil samples at different temperatures. From the obtained results, the sample obtained by mixing 90% natural ester oil with 10% mineral oil aged for 6 days is considered superior and achieves an improvement in dielectric strength and relative permittivity by approximately 43% and 48%, respectively, compared to fresh mineral oil. However, the dissipation factor was increased by approximately 20% but was at an acceptable limit. On the other hand, for the same oil sample, due to the higher molecular weight of the NEO, the viscosities of all mixtures were at a higher level than the mineral oil.

Keywords: accelerated aging; condition assessment; diagnostic process; dielectric properties measurement; dynamic viscosity; mineral oil; mixture oil; natural ester oil; power transformer.

Grants and funding

This paper is created upon work supported by Science, Technology & Innovation Funding Authority (STIFA), Egypt, under a grant (31292).