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The ageing characteristic of cellulosic insulation within a transformer is permanent and irreversible. Mineral oil however, can be changed, cleaned and/or regenerated restoring the oil quality.
Loss of mechanical or electrical strength when paper is neglected can NEVER be regained. The life of the transformer is the life of the cellulosic paper. In short, transformers do not die of old age, but are killed by neglect.
A loss of 75% of the initial tensile strength of the cellulosic insulation is now the accepted “End of Life” criteria.
This represents a Degree of Polymerisation (DP) of approximately 200.
Newer transformers have been developed against a different set of design criteria than units pre 1955. With computer aided design and a range of newer insulating products, electrical stresses are higher per unit thickness of insulation, there is far less oil per kVA and cooling passages, vents and radiators are generally smaller.
With less oil /kg of insulation there is less capacity for hot oil to remove moisture from the paper and a reduced ability to dilute concentrated soluble contaminants.
For these reasons, it is even more important to provide good preventative and ongoing performance based maintenance.
Transformers are the single most expensive item of high voltage plant in a substation.
The life of a transformer is the life of its paper insulation. It is essential therefore, to control the various factors that lead to paper degradation.
Transformer main insulation is the:
PAPER which provides
and
OIL which provides
There are three critical factors in a transformer that must be controlled:
Moisture kills transformers.
Small, even microscopic amounts of moisture accelerate deterioration of cellulose insulation and a cause significant loss of life.
The strength of cellulose will rapidly degrade with increasing moisture even in the absence of oxidation. Moisture will divide between cellulose and oil in a ratio, at equilibrium, in which the moisture content of the cellulose is hundreds of times greater than that of the oil.
The thermal life of Class 105 insulation is halved for each increase of 8 degrees C or conversely doubled for each decrease of 8 degrees C.
Transformers should not exceed a top oil operating temperature of 60°C.
Every 4 – 6 degrees C above 60°C will halve
the remaining paper life.
