Transformer Temperature Measurement Methods
There are two main transformer temperature measurement methods: oil surface temperature measurement and winding temperature measurement.
Oil Surface Temperature Measurement
Oil surface temperature measurement also has two methods: pressure-type measurement and platinum resistance temperature measurement.
In the pressure-type probe, a liquid is sealed inside. As the temperature changes, the liquid expands to different degrees. At the end of the liquid conduit is a sensing device that measures the pressure value. This pressure is then converted into a temperature signal and transmitted. Common signal types include 0–5 V voltage signal, 4–20 mA current signal, Pt100 platinum resistance signal, and RS485 signal. This temperature measurement method is widely used.
Platinum resistance temperature measurement uses a platinum resistance probe directly. Currently, few manufacturers produce this type of thermometer, and the cost is high, so it is not widely applied in practice.
Winding Temperature Measurement
Winding temperature measurement also has two methods: transformer conversion method and fiber optic temperature measurement.
The transformer conversion method is based on pressure temperature measurement. It first measures the oil surface temperature, then superimposes a copper-oil temperature difference on the oil surface temperature based on the relationship between the bushing CT secondary current and temperature conversion, thereby simulating the winding temperature. This measurement method is not accurate - it is only a theoretical calculated value. However, it is the most commonly used method.
Fiber optic temperature measurement is a newer technology that requires temperature sensing components to be pre-installed in the transformer. However, due to insulation concerns, this method has inherent limitations, especially in UHV transformers, where application is quite challenging.
Infrared Thermometer for Inspection
Recently, manufacturers have also been required to provide portable infrared thermometers. However, this can only be used for inspection purposes. It measures the temperature of external structural components and cannot be considered a temperature measurement method for the main transformer itself.