Analysis of the Chemical and Electrical Characteristic of Vegetable Insulating Oils Exposed to Accelerated Aging

초록

Electrical insulation is one of the most important components in a high voltage apparatus. Most oil-filled transformers use insulating and cooling fluids derived from petroleum crude oil, however these mineral oils have disadvantages in regards to environmental pollution and can explode, causing fires. Vegetable insulating oils have a high flash point and are more environmentally friendly when compared to conventional mineral oils. The accelerated aging processes used for these experiments were performed using two methods. First, sealed accelerated aging vessels containing insulating paper (Kraft and Diamond-Pattern) and insulating oils (both mineral and vegetable oil) were aged at 140℃ for 500, 1000, 1500, and 2000 hours. The second method case used real transformer vegetable and mineral insulating oil samples which underwent accelerated aging for 42, 93, 143, 190, 240, 300 thermal cycles ranging from 30℃ to 120℃. The mineral and vegetable insulating oils were tested in regards to breakdown voltage, water content, total acidity, viscosity, volume resistivity, insulating paper tensile strength, and insulating paper and oil permittivity. The breakdown voltage of the vegetable insulating oil was found to be higher than that found for the mineral oil; the accelerated aging progress decreased the breakdown voltage. The vegetable oil had a higher water saturation level than that found for the mineral oil; the vegetable oil possessed superior water characteristics (water saturation) and breakdown voltage. The viscosity of the insulating liquid is important in the impregnation process. Due to the higher viscosity of vegetable oil, special care has to be taken, especially when designing the cooling system for a large transformer.