Signs of the criticality only become apparent through an internal inspection of the transformer, following a failure for example. In the presence of this criticality, grey/brown insoluble deposits (sludge) are found on the bottom of the casing, papers, windings, and oil circulation and winding cooling channels. Another sign may be the progressive increase in oil temperature with respect to the environment at same load (symptom of a reduction in thermal exchange).
Representative sampling
Should it be decided 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. In particular, it is advisable to select the paper at the top, bottom, and middle of the both primary and secondary windings for each phase, taking multiple paper samples in areas with greatest darkening or embrittlement of the papers themselves.
During external inspection of the transformer, it is necessary to take samples of insulating oil in accordance with the reference standard and the operating instructions attached to the sampling kits.
