The mineral or synthetic oil is mostly being used in conjunction with paper as the dielectric medium in most of the high voltage equipment. However, impact on environment, lack of petroleum oil reserves and disposal problems with used oils, have prompted researchers to direct their focus onto biodegradable and renewable insulating materials. The new insulating liquid materials development is guided by multiple factors such as environmental requirements and other safety and economic considerations. Therefore transformers manufacturer have to face new specifications related to these new requirements. The Vegetable-oil based transformer fluids increasingly replacing mineral oil-based products in the market place. They are successful because they perform better than mineral oil products and they provide definite environmental and safety gains. This paper reviews the current status of vegetable oils use as transformer oil, including their production, processing, and characterization. The vegetable oils most used as transformer oils are presented and their main advantages described in comparison with mineral oil. The various experimental work carried out in different countries is described, giving an overview of the current research carried out on the vegetable oils. In addition scope and challenges being faced in this area of research are clearly described.

Use of vegetable oils as transformer oils – a review

As a non-renewable resource, mineral oil has been re-evaluated for its value and suitability as the coolant and insulator in transformers because of its environmental unfriendliness, fire hazards, and operating efficiency dissatisfaction. As a result, alternative dielectric liquids meeting the demands for more reliable, safer, and cleaner energy has been a critical need. Plant-based insulating fluids, non-toxic to the environment and ultimately biodegradable, have been flowing into the mainstream for achieving cost efficiencies, performance advantages, and enhancing safety. With more than 20 years of field experience, plant-based oils are now employed in more than 600,000 transformers worldwide, and have been established an enviable performance track record. Far from being just a mineral oil replacement, but more significantly, plant-based oils have filled a gap in some specific application scenarios where mineral oils fail to satisfy the fire-safety and environmental standards. This article reviews the research status of plant-based insulating fluids for transformer, including both challenges and outlook circa 2020. Plant-based oils have many inherent advantages over mineral oil, but also have numerous and unique challenges.

A critical review of plant-based insulating fluids for transformer: 30-year development

Transformer mineral oil has been replaced by alternative oils such as synthetic oil and natural esters due to their biodegradability and environmentally friendly nature. This paper presents performance of coconut oil as such an alternative. Generally, as the other alternative oils, coconut oil has high conductivity due to the presence of free fatty acids. In this study, three different types of coconut oil samples consisting of virgin, copra and RBD (refined, bleached and deodorized) were initially tested by frequency dielectric spectroscopy (FDS) measurements to see how the conductivity was improved by dehydration and neutralization. The FDS results were fitted by inverse power dependence and Cole-Cole models to estimate the conductivity and response functions. Afterwards, a set of new coconut oil samples extracted from copra were thermally aged at 120°C under sealed conditions and compared with that of mineral oil. The performances of oil samples were evaluated using the test results of breakdown voltage, acidity, interfacial tension and FDS measurements under different aging periods such as 2, 5 and 7 weeks. Another 4 sets of new coconut and mineral oil samples were subjected to simulated thermal faults and electrical faults which include aging for 12 hours at 160°C, one hour at 200°C, exposing to partial discharges for four hours and subjecting to 20 low energy breakdowns. The performance comparisons were done by FDS measurements and dissolved gas analysis. In parallel, field-aged coconut oil samples collected from a sealed distribution transformer with 11 years of service were also tested by FDS measurements. In general, coconut oil shows its suitability as an alternative to the mineral oils for transformers, despite limitations found in some of their physical properties. It was found that the FDS results were in good agreement with chemical test results and with the estimated conductivity values.

Performance of coconut oil as an alternative transformer liquid insulation

This paper presents a comparative evaluation of the aging characteristics between a high oleic natural ester dielectric liquid (HONE) and mineral oil based on an accelerated single-temperature thermal aging experiment at 150°C for a period of 3000 hours. The important physical (e.g., viscosity, moisture content, acidity) and dielectric (e.g., power factor, alternating current (AC) dielectric breakdown) properties of both the oils were measured and analyzed at different aging intervals. As expected, results show differences in aging behaviors between mineral oil and the natural ester liquid. The relative moisture content in HONE had stable and lower values during the aging duration as compared to the case of mineral oil, where the relative moisture content increased with the aging duration. Further, unlike the observations in mineral oil, the power factor (PF) and acidity of HONE increased drastically with increasing aging times. The most relevant observation made from this study was that in spite of this significant increase in the PF and acidity of HONE with aging, its AC dielectric breakdown voltage was quite stable, which is not observed in the case of mineral oil. This indicates that for natural ester liquids, higher values of PF (or dielectric dissipation factor) and/or acidity due to aging may not necessarily give a true picture on the dielectric performance of the ester. It is therefore important to note here that the threshold limits of some of the diagnostic parameters which are already established for mineral oil do not apply for ester liquids.

Comparative aging characteristics between a high oleic natural ester dielectric liquid and mineral oil

Failure analysis of transformer liquid — solid insulation system under selective environmental conditions using Weibull statistics method

The usage of vegetable oil for power transformer insulation has increased, compared to mineral oil, for its better environmental characteristics. However, although the temperature rise depending on load difference and the relationship between hottest spot temperature and life time depending on long-term operation for mineral oil immersed transformer has been studied, there is being established for vegetable oil immersed transformer. Therefore, the MV power transformers either filled with conventional mineral oil or with vegetable oil are thermally aged and the transformer oils are sampled regularly. With these samples, the chemical, mechanical and electrical characteristics of the two oils are investigated by the aging time. According to our experiment, the degradation rate of vegetable oil can be compared to that of mineral oil.

Accelerated aging effects of mineral and vegetable transformer oils on medium voltage power transformers

Electrical insulation is one of the most important part in a high voltage apparatus. Recently, researchers are interested in the environmental friendly vegetable oil from environmental viewpoint. Accelerated aging transformer insulating material in vegetable oil was compared to that of mineral oil. Accelerated aging oil samples produced in the oven at for 500, 1000, 1500, 2000hours. And Real transformer insulation oils samples of vegetable oil and mineral oil were aged by thermal cycles repeating from to . Samples were analyzed at 42, 63, 93, 143, 190, 240, 300 cycles. The mineral and vegetable insulating oils were investigated for breakdown voltage, water content, total acid number, viscosity, volume resistivity, insulating paper and oil permittivity, and dissolved gas analyses. The breakdown voltage of the vegetable insulating oil is higher than that found for the mineral oil; the accelerated aging progress decreased the breakdown voltage. The vegetable oil had a higher water saturation than the mineral oil; the vegetable oil has the superior water characteristics and breakdown voltage. And high viscosity of vegetable oil, care has to be taken, especially when designing the cooling system for a large transformer.

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Analysis on the Chemical and Electrical Characteristic of Vegetable oil by Accelerated Aging

Aging of transformer insulating material in natural ester insulating oil is compared to that in conventional transformer oil. Aging of insulating paper and insulating oil have been studied by performing accelerated thermal aging test. Sealed aging test vessels containing cooper, laminated core, Kraft paper and insulating oil(natural oil or mineral oil) were aged at for 500, 1000, 1500 and 2000 hours. Insulating oils after aging are investigated with total acid number, breakdown voltage and viscosity. Also, degradation of insulating paper after aging is determined using breakdown voltage and mechanical strength. Accelerated aging studies demonstrate a slower aging rate for natural ester insulating oil compared to the rate for conventional mineral oil.

Analyses on Electrical-Mechanical Characteristics of Environment-friendly Transformer Insulating Material by Accelerated Thermal Aging

In this paper, the FT-IR, conductivity and flow electrification phenomena of Vegetable oils were discussed. Vegetable oil has high flash point and eco-friendly properties compared to conventional mineral oil. Accelerated aging transformer oil samples produced in the oven at 140[] for 500 hours, 1000 hours, 1500 hours, 2000 hours. Serial device to measure the flow electrification phenomena was designed and fabricated. Pico ammeter and the Labview data measurements were utilized. The effects of Temperature and antistatic agent on fluid flow electrification were investigated. Both Mineral and vegetable oil showed similar trends for temperature change. The polarity reversal occurred in mineral oil with antistatic agent above 10 ~ 30[ppm] and in vegetable oil with antistatic agent above 5[ppm].

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Jung-Il Jeong

Papers

Accelerated aging effects of mineral and vegetable transformer oils on medium voltage power transformers

Analysis on the Chemical and Electrical Characteristic of Vegetable oil by Accelerated Aging

Analyses on Electrical-Mechanical Characteristics of Environment-friendly Transformer Insulating Material by Accelerated Thermal Aging

Influence of Temperature and Benzotriazole on the Flow Electrification by aging Mineral, Vegetable Oil