The conditions in which transformer oil is used in transformer, switches and other high voltage equipment are rather demanding on the quality and purity of the oil. In the process oil is heated by currents, contaminated by particles of solid insulation and chemical reactions with the internal surfaces of the equipment, solves water and gas from the outside. Each of these separately and all together significantly accelerate aging of transformer oil and can make it harmful for the equipment.
A sample of transformer oil is taken from the lower part of transformer oil tank after rinsing of the drain opening by oil. The sampling vessel must be clean and very dry.
Local and international standards define certain parameters which the oil should comply with.
Oil analysis and testing is performed directly before filling of oil into power equipment.
New transformer oil is tested before filling into transformers. Purity and quality guaranteed by the manufacturer of oil does not prevent water, air and solid particles form entering the oil during handing and storage. Even new transformer oil must be tested and purified to extend its service life and ensure reliable equipment operation.
Before a transformer is energized, the oil in the transformer must be briefly tested for minimum dielectric strength, solids, acidity and flashpoint.
If different brands of oil are used in the transformer, the blend must be tested for stability, which should not be inferior to stability of one of the oils with the lowest stability value.
Used oil must be tested according to regulations on operation of industrial equipment.
Before testing the vessel containing the oil is inverted and turned slowly around several times to eliminate air bubbles from the oil. A ceramic oil test vessel with electrodes is filled three times. Oil is poured on the walls of the vessel in a thin stream so as to prevent formation of air bubbles. The level of oil in the vessel must be at least 15 mm higher than the top of the electrode. After each rinsing, the oil is completely drained.
Transformer oil is allowed to settle in the vessel for 15-20 minutes for air bubble removal. Then voltage is gradually increase at the rate of 1 – 2 kV per second. After a breakthrough is achieved (spark between the electrodes), voltage is dropped to zero. Six runs are done, and the average of the five subsequent tests is taken, the first breakthrough is not considered.
After each breakthrough carbon particles are removed from the space between the electrodes by clean glass or metal rods. The liquid then settles for 10 minutes.
Transformer oil sampling process may vary from facility to facility, depending on the equipment used and purity requirements.
At any rate, a timely analysis may well prevent significant costs and losses.