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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