IEEE Std 1538a-2015 pdf download – IEEE Guide for Determination of Maximum Winding-Temperature Rise in Liquid-Filled Immersed Transformers.
4.1 Direct measurement by fiber-optic detectors 2 4.1.1 Introduction Fiber-optic temperature detectors may be used to measure temperatures in distribution and power transformers. The main application of fiber-optic probes has been the measurement of local temperatures in the windings, particularly at the hottest-spot temperature in the windings. The probes have usually remained in the windings, and in some cases they have been used for the measurement of temperatures under operating conditions. There are fiber-optic probes on the market with embedded sensors at certain intervals along their length. Such probes could measure the local temperatures along the whole length of the conductor. However, for use in power transformers, such probes are not very practical. One reason is that the probe has to be embedded in a slot in a single strand. This creates complications, for example, at the transposition of parallel strands. Another reason is that the current distribution is not completely balanced between parallel conductors, in spite of a proper transposition scheme. Furthermore, at the design stage it is not obvious which one of the parallel conductors carries the highest current. This makes selection of the proper conductor for probe installation uncertain, because probes would have to be installed in all of the parallel conductors, which would be both expensive and time consuming. Consequently, point sensors are more likely to be used in most transformer applications. When using point sensors then, choosing the best installation locations is crucial to determining the hottest- spot temperature with reasonable accuracy. The term “reasonable accuracy” is used here to mean that the measured maximum temperature is close enough to the real hottest-spot temperature, such that it can be used as a reference for safe operation of the transformer and for the estimation of consumed transformer life.