There are several temperature-detecting devices used for indication, recording or control, and, on
rare occasions, for tripping. Some devices simply measure the oil temperature, usually the top oil.
Other devices use a combination of current, by placing a small search coil around a lead, and oil
temperature to measure the total effect of load and ambient temperature. The critical temperature is
referred to as the ‘hot-spot’ temperature, and is the highest temperature that will occur somewhere
in the winding. This hot-spot is not precisely known, since it varies with the physical structure
of the transformer and the flow of cooling oil or gas. It is usually determined at the factory, and
an arbitrary temperature, e.g. 10 ◦C, is specified to be added to the total temperature as indicated
at any time. The temperature devices actuate alarms to a central dispatching office, to alert the
operators, who can either remotely unload the transformer by opening the circuit breaker, or can
dispatch an operator to the station. The hot-spot sensors are also commonly used to start and stop
cooling fans and pumps. In extreme cases, when it is not possible to remotely remove the load, or
send an operator to the station, an extreme high alarm will trip the bank.