The electrical windings and the magnetic
core in a transformer are subject to a number of different forces during
operation, for example
a)Expansion and contraction due to thermal
cycling
b)Vibration
c)Local heating due to magnetic flux
d)Impact forces due to through fault
current
e)Excessive heating to to overloading or
inadequate cooling
This guide deals primarily with the
application of electrical relays to detect the fault current that results from
an insulation failure. The current a relay can expect to see as a result of
various types of winding insulation failures.
The detection systems that monitor other
transformer parameters can be used to indicate an incipient electrical fault.
Prompt response to these indicators may help avoid a serious fault. For example
a)Temperature monitors for winding or oil
temperature are typically used to initiate an alarm requiring investigation by
maintenance staff.
b)Gas detection relays can detect the
evolution of gases within the transformer oil. Analysis of the gas composition
indicates the mechanism that caused the formation of the gas; e.g., acetylene
can be caused by electrical arcing, other gases are caused by corona and
thermal degradation of the cellulose insulation.
The gas detection relays may be used to
trip or alarm depending on utility practice. Generally, gas analysis is
performed on samples of the oil, which are collected periodically.
Alternatively, a continuous gas analyzer is available to allow on-line
detection of insulation system degradation.
c)Sudden-pressure relays respond to the
pressure waves in the transformer oil caused by the gas evolution associated
with arcing.
d)Oil level detectors sense the oil level
in the tank and are used to alarm for minor reductions in oil level and trip
for severe reductions.
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