IEEE 1538a:2015 pdf free download – IEEE Guide for Determination of Maximum Winding-Temperature Rise in Liquid-Filled -Immersed Transformers
4.1.1 lntroduction
Fiber-optic temperature detectors may be used to measure temperatures in distribution and powertransformers. The main application of fiber-optic probes has been the measurement of local temperatures inthe windings，particularly at the hottest-spot temperature in the windings.The probes have usuallyremained in the windings, and in some cases they have been used for the measurement of temperaturesunder 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, foruse in power transformers,such probes are not very practical.One reason is that the probe has to beembedded in a slot in a single strand. This creates complications,for examplc, at the transposition ofparallel strands.Another reason is that the current distribution is not completely balanced between parallelconductors,in spite of a proper transposition scheme.Furthermore, at the design stage it is not obviouswhich one of the parallel conductors carrics the highest current. This makes selection of the properconductor for probe installation uncertain, because probes would have to be installed in all of the parallelconductors，which would be both expensive and time consuming.Consequently, point sensors are morelikely 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 themeasured maximum temperature is close enough to the real hottest-spot temperature,such that it can beused as a reference for safe operation of the transformer and for the estimation of consumed transformerlife. The probable location of the hottest-spot is usually determined by analysis of the eddy-lossdistribution, the oil-flow distribution, and the heat-transfer characteristics of the winding.Unfortunately,large thermal scatter often overrides the theoretical predictions. Thus, a certain minimum number of probesare needed.The following subclauses recommend numbers and locations of probes in core-typetransformers. It is then assumed that the windings are of approximately equal length, are designed withaxial or zig-zag oil circulation, that the load current is sinusoidal, and that the transformer is ON-, OF-, orOD-cooled.
NOTE—These recommendations for the number of probes are only one perspective and may vary according to specificmanufacturers guidelines.
Thermal tests may be conducted at rated load in accordance with IEEE Std C57.12.90TM.Thermal data,including the hottest fiber-optic probe recordings，may be used to verify the hottest-spot temperatureperformance of the transformer. Such data are also useful to verify thermal models to be used to calculatethe hottest-spot temperature of other designs. Data from tests at loads above and below rating assists furtherin the development of thermal models to predict the performance of the transformer at different loads andalso to predict the performance of other designs.
4.1.3Location of sensors on the limb selected
lt is normally not of interest to install sensors in regulating windings, stabilizing windings, or in circulatingoil. An exception might be a need to check that the regulating winding capacity is properly sized.Otherwindings in special configurations should also be considered as candidates for location of fiber-opticprobes if significant loading is expected for those windings.