Basic description of the Transformer Monitor
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Loss of life (IEC) 4.3.3
The loss of life information is derived from the equations specified in IEC 60354 or IEC
60076-7.
The loss of life should not be confused with the operating hours described earlier. The 'Op-
erating hour' log file only counts the time during which the transformer is under voltage,
while the loss of life takes thermal ageing into account.
The relative thermal ageing of the insulation as a factor of temperature and time is deter-
mined by the Arrhenius equation:
Constants that are determined by tests on the insulators
Thermodynamic temperature in K
In the 80... 140°C temperature range, the Arrhenius equation can be replaced with the
slightly simpler Montsinger relationship.
According to scientific publications, a transformer's service life consumption doubles in the
80 to 140°C range when the temperature increases by about 6 K.
Another equation can be used to determine the relative service life consumption at tem-
perature Θ
h
relative to the normal service life consumption at temperature Θ
hN
.
The value Θ
hN
was specified for a transformer according to CEI/IEC 60354 Part 1/11.71 at
98°C. This temperature corresponds to the operation of a transformer with nominal output
at a coolant temperature of 20°C when the hot-spot over temperature is 78 K, i.e. 13 K
above the average over temperature of 65 K. These temperature conditions correspond to
the normal ageing of the insulation.
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