is possible to provide the earthing transformer
with a secondary winding for continuous auxiliary
transformers are usually oil immersed and may
be installed outdoor. In cases where a separate
reactor is connected between the transformer
neutral and earth, the reactor and the transformer
can be incorporated in the same tank.
earthing transformer is going to be used together
with an arc-suppression reactor, the rated current
(and its duration) of the earthing transformer
will be determined by the data for the arc-suppression
If the earthing
transformer is used for directly earthing or
through a current limiting reactor, the neutral
current through the transformer will be high
but the duration is limited to a few seconds.
Earthing transformer must be designed to withstand
the thermal and mechanical effects of the rated
neutral current. The characteristic of earthing
transformer should be selected to match the property
of the system.
Most of the
time in service the loading of earthing transformers
is very low. It is the randomly occurring short – duration
currents that cause any heating of significance.
Ageing of the cellulose materials is then not
a matter of concern. Regarding acceptable temperatures
there are two aspects to consider. The temperature
during currents of say 10 seconds duration must
not cause softening of the winding conductor
material, say 250o C for copper and 200 o C for
with duration in the order of hours or more,
temperatures that cause excessive gas development
in the oil should be avoided. Temperatures for
items in direct contact with the oil should then
not exceed 140 o C
earth fault protection relay may not be effective
at low currents. To prevent damage to earthing
transformer by such current, oil thermometer
with alarm/trip contacts is recommended. Alternatively,
the value of maximum continuous earth fault current
may be declared and sensitive protection provided.
transformers that carry continuous load because
of a secondary winding that supplies local consumers,
the temperature limits in IEC 60076-2 (1993-04)
Power transformers Part 2; Temperature rise apply.