Mineral insulating oils; functional requirements, specifications and production

This paper describes how requirements on transformer oil are reflected by different specified physical and chemical parameters of oil. Impulse breakdown is not included in many specifications but ASTMD3487 does have it. Breakdown behaviour with DC impulses and a heterogeneous gap is very different from the AC breakdown strength. It is designed to simulate lightning pulse striking during a thunderstorm. The result is independent of the contaminants influencing normal breakdown test. A needle and steel ball are used as electrodes, at a distance of 2.5 cm. With a negative impulse to the needle, the breakdown has been found to depend on the degree of oil refinement, with lower aromatic content giving a better/higher value. The most common refining processes are described