: 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. |
