Power transformers are vital equipment in power systems. Causes of transformer failure are numerous, however insulation failure accounts for more the 40%[1, 2]. Cellulose/kraft paper and oil are the two insulating materials in power transformers. The most widely used insulating oil in transformers is the mineral oil. Oil in transformer functions as both electrical insulation and cooling medium. Moisture and heat build-up are the main “culprits” in transformer oil and paper deterioration. Concerns about power transformer failure is they can lead to prolonged blackouts which can affect a large pool of power consumers. However, another current major concern about power transformers is the environmental health and risks the mineral oil pose during operation due to insulation failure. Mineral oil is non-biodegradable and toxic thus in the event of an accident such as spill or leakage takes years to degrade and harms the environment. These concerns have prompted researchers to seek biodegradable oils such as vegetable-based oils to overcome the environmental concerns of mineral oils. Literature has shown that vegetable-based oils possess the requisite qualities for use as transformer oils such as high flash point and fire point as well as high breakdown voltage and good dissipation factor (tan δ)[3–7]. In addition they possess better moisture absorption capability from transformer kraft paper [8]. However, compared to mineral oil, they have higher viscosity, pour point and poor oxidative stability.

Numerous studies in the past have been conducted that have shown various types of vegetable-based oils as candidates for transformer oil [9–19]. However, studies investigating the long term electrical and chemical performance vis-à-vis ageing and oxidative stability of these potential vegetable-based insulating oils are lacking. Furthermore, recently some researchers have reported that blending between vegetable-based oils improved their oxidative stability [20–21]. However, investigating their electrical characteristics has not been conducted. In view of the foregoing, there is a need to further carry out electrical and chemical studies on long term ageing characteristics of vegetable-based oils for use as transformer insulating oils. The objective of this paper is to investigate the dielectric properties of some vegetable-based oils as insulating oils in power transformers. Additionally, this paper also investigates the dielectric properties of blended vegetable-based oil and assessed the long-term performance and stability of the vegetable-based oil as insulating oils during accelerated ageing processes.