Transformer winding temperature determination

by JN Bérubé, J Aubin and W McDermid, Neoptix

 

Published in: Transformers and Substations Handbook: 2014

Chapter 2: Design and installation of a substation - page 46

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The loading capability of power transformers is limited mainly by winding temperature. It has been the practice to assess this temperature from a measurement of oil temperature at the top of the tank, with an added value calculated from load current and winding characteristics. With more frequent occurrences of overloading, it has been found that this simplified approach is not suitable for several types of overload and transformer design. In an attempt to close this gap, IEEE and IEC loading guides are being revised with more sophisticated models aiming at a better representation of oil temperature inside the winding, and consideration of variations in winding resistance, oil viscosity and oil inertia. Still, direct measurement of winding temperature with fibre optic sensors provides a definitive advantage over a value calculated from uncertain parameters provided by the manufacturer and uncertain equations characterising the cooling pattern.

 

Fibre optic temperature sensors

For nearly 30 years, fibre optic temperature sensors have been available for measurement in high voltage transformers. The first units were fragile and needed delicate handling during manufacture. In the past 10 years, though, significant developments have taken place to improve their ruggedness and facilitate connection through the tank wall. The fibre optic probe on the authors’ company’s T/Guard system consists of a 200-micron glass fibre sheathed with a permeable protection Teflon tube. This probe is designed to endure manufacturing conditions, including kerosene desorption, and long-term immersion in transformer oil. The temperature-sensing element is based on the proven GaAs technology. An original algorithm is used to extract temperature information, providing accurate and reproducible measurements, even when probes are interchanged.

 

Take note

  • Fibre optic winding sensors have improved to the point where direct measurement of winding temperature is the preferred measuring method.
  • Fibre optic winding sensors have reached maturity for application in power transformers.
  • On-line monitoring of winding temperature can provide a dynamic evaluation of insulation degradation.