Page 54 - Energize August 2022

TECHNICAL

what the transformer is designed for. But the risks are higher for
those transformers exposed to frequent and high short-circuit
stresses from, for example, line-to-ground failures on power lines
due to lightning, or when moved from one place to another.
Finally, in asset management an engineer will not only
consider failure probability, but also the consequence of that
failure to gain a more complete picture of the risk involved with a
specific transformer or a larger fleet of transformers.

Other diagnostic compounds
The presence of phenols and cresols in concentrations greater
than 1 ppm indicate that solid components containing phenolic
resin (laminates, spacers, etc.,) are overheating.

Methanol (MeOH) and ethanol (EtOH)
Figure 5 - Sketch of ageing rates due to different ageing mechanisms It has been demonstrated over several years that the ageing of
impregnated paper in insulating liquid, which results in cellulose
Paper ageing and transformer risk assessment degradation, produces molecules of light alcohols, methanol
The “predicted” DP (degree of polymerisation) indicates an (MeOH) and ethanol (EtOH).
average paper condition over the whole transformer (subject In laboratory experiments, a good correlation has been
to factors such as effective circulation). New Kraft paper has established between the increase of the methanol content in
a DP in excess of 1200, and paper with a DP of 200 or less is insulating liquid and the decrease of the degree of polymerisation
considered to be unfit (subject to interpretation). The values can of the cellulose, irrespective of the type of paper, standard Kraft
be optimistic if the oil has been regenerated within the last two or thermally upgraded. Further, at the early stages of paper
years. ageing, i.e., of cellulose degradation, the methanol content is
This data should be evaluated in conjunction with routine always higher than that of furanic compounds (mainly 2-furfural),
chemical analysis and transformer history. so this behaviour suggests that methanol could be a relevant
The reduced strength of paper that results from abnormal in-oil marker to detect early paper ageing in transformers and to
ageing may be a direct or indirect cause of a failure. An assess its evolution.
understanding of the progression of cellulose ageing, and the
risks associated, enables an asset manager to make an informed Case study: insulation failure
decision regarding the condition in a real transformer, the real Overiew
situation being always more complex: temperature and water The transformer failed in service while undergoing Power-On
content varies along the winding, there are several different purification to remove moisture from the insulating oil.
qualities of paper (e.g., thermally upgraded or normal Kraft
paper), and the paper is present in many layers with temperature Transformer details
and water content gradients. Primary voltage: 11 kV
Also, cellulosic materials like pressboard and wood are used. Secondary voltage: 500 V
Simple formulas may give insight in ageing mechanisms but are VA rating: 500 kVA
never sufficient for assessing the condition of a whole winding. Vector group: Dyn11
Even so, it is still believed that one should base the analysis on Impedance: 4,1%
the weakest link, which will be the condition around the hotspot Tap changer: Off load
region in the transformer. Make: CAWSE & MALCOLM
Ageing will be most pronounced in this region within the Year manufactured: Unknown
top end of the winding, and electro-mechanical forces will be Conservator: No
strongest here. If the tensile strength in this region is insufficient Breather size: CHG2
to withstand these forces, this will be the most likely place for a Oil volume: 600 litres
failure to occur.
Furthermore, even if there was an agreed end-of-life criterion Transformer oil analysis-diagnosis
for the strength of the paper or the degree of polymerisation The dissolved gas analysis indicated a Partial discharge of
of the cellulose, there remains the question of how large the low energy density (IEC 599) with the CO 2/CO ratio of 16,5
stresses are. The risk of a failure is different for transformers indicating insulation paper degradation. (The normal ratio is
operating in different circumstances. The risk is lower for those between >3 and <11).
operating in grids where the transformer rarely experiences a The Furan analysis of 8,97 ppm indicated extensive paper
short circuit or where the short-circuit currents are lower than deterioration. (Optimistic value as the oil purification process

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