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) Forces due to the flow of
through-fault currents
e) Excessive heating due to overloading
or inadequate cooling
These forces can cause deterioration
and failure of the electrical insulation of the transformer windings.
Statistics for the causes of transformer failures experienced in U.S.
utilities are not readily available.
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.
Some examples of actions taken to
detect undesirable operating conditions are as follows:
1) Temperature monitors for winding or
oil temperature are typically used to initiate an alarm requiring
investigation by maintenance staffs. At this stage, the operators may
start to reduce the load on the transformer to avoid reaching a
condition where tripping the transformer would be required.
2) 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 partial discharge and thermal degradation of the
cellulose insulation.
The gas detection relays may be used to
trip or to generate an alarm depending on the utility practice.
Generally, gas analysis is performed on samples of the oil that are
collected periodically. A continuous gas analyzer is available to
allow online detection of insulation system degradation.
3) Sudden-pressure relays under oil
respond to the pressure waves in the transformer oil caused by the
evolution of gas associated with arcing.
4) Sudden-pressure relays in the gas
space respond to sudden changes in the gas pressure due to evolving
gases from an arc under oil.
5) Oil-level detectors sense the oil
level in the tank and are used to generate an alarm indicating minor
reductions in oil level and trip for severe reductions.
6) Online devices monitor bushings of
the transformers, CTs installed in those bushings, and surge
arresters installed on the transformers and generate an alarm
indicating that repair is needed urgently so that major damage is
avoided.
Details of the modern techniques for
monitoring these components are given by Coffeen et al.
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