Signs (visual inspection)

The direct visual signs of this criticality are only highlighted by internal transformer inspection. In the case of failure (or end of life) of twin machines, for example, it is good practice to perform a paper diagnostics (through sampling, analysis and interpretation) in order to determine an experimental reference for the transformer concerned. In the presence of “degradation of insulating papers” the following can be observed:

  • signs of paper embrittlement (loss of mechanical properties),
  • insoluble deposits (e. g. sludge or copper sulfide) on the insulating papers
  • obstructions of the oil circulation ducts used to cool the windings and the papers themselves.

Through the effect of intimate contact with insulating papers, insulating oil becomes a valuable vector for indirectly diagnosing the state of the papers themselves. Through oil analysis, it is thus possible to identify and quantify paper degradation products to evaluate the thermal life consumed.

Paper degradation results in a loss of mass, and clamps can loosen with a progressive increase in vibrations. Over time, the resulting mechanical stress is added to electrical stress and thermal stress, significantly increasing the probability of failure.

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.

It is important that appropriate sampling protocols be used and that sampling kit be used to ensure preservation of the sample until the phase of preliminary analysis. Likewise, it is important that the kit offers suitable data collection tools (transformer plate data, progressive numbering of each sample, sampling point with three-dimensional mapping of conductors, etc. ).

During the normal life cycle of equipment it is necessary to take representative samples of the insulating oil in accordance with the reference standard and the operating instructions attached to the sampling kits (read more).

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