Signs (visual inspection)
Signs of the problem only become apparent when the inside of the transformer is inspected, following a breakdown for example. Where the problem is present, grey deposits are typically observed on the copper conductors (copper sulfide) or silver contacts (silver sulfide). On insulating papers the copper sulfide contamination shows as grey spots and streaks.
Representative sampling
When deciding to carry out an internal inspection of the transformer, following a failure or in order to carry out a thorough inspection, it is strongly recommended to take samples of the insulating papers in accordance with relevant protocols and procedures. It is particularly recommended that both primary and secondary papers are taken from the upper, lower and middle sections of individual windings at each phase, taking more samples from areas where the papers are darker or more brittle.
When inspecting the transformer externally, samples of the insulating oil must be taken in accordance with the reference standard and operating instructions enclosed with sampling kits.
