Cooling control of power transformers is traditionally provided by a winding temperature indicator (WTI) that is based on a measurement of the top-oil temperature and a simulation of the winding hottest spot temperature.
This method has some drawbacks and utilities are now considering the application of fiber optic temperature sensors for this critical function. Assessing the winding temperature from directly measured top oil temperature can lead to significant errors depending on the cooling mode and the shape of load profile. This situation is reflected in the current revision of the IEEE Loading Guide. The industry consensus leans toward a method based on the bottom oil temperature and a proper representation of the temperature evolution in the cooling duct.
The various hot spot temperature calculation methods are reviewed for the actual case of a 240MVA transformer where fiber optic sensors have been selected for the control of the cooling banks. It is shown that in case of rapid load change, the methods used by classic WTI can indicate a lower temperature by more than 10°C even if they are properly adjusted for the steady state conditions. It appears that with the dependability of modern fiber optic sensors, the long-term performance of transformer cooling can be better achieved with these more accurate monitoring devices.