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
c)Local heating due to magnetic flux
d)Impact forces due to through fault current
e)Excessive heating due to overloading condition on inadequate cooling

These forces can cause deterioration and failure of the winding electrical insulation. Table 1 summarizes failure statistics for a broad range of transformer failure causes reported by a group of U.S. utilities over a period of years.

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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