Temperature measurement in thermal cycling of stator bars - the influence of the sensor installation technique

Thermal cycle testing is required sometimes as a type test for stator bar production for hydro generators. Based on the outcome of the test, a production is qualified by the owner or operator of the generating station. Because of this, it is important that the measurements performed during this test are as precise and non-intrusive as possible. During a previous investigation on thermal cycle testing, it was noted that the temperature measurement technique needs clear and definite specification to increase the repeatability within the author's employer company laboratories and amongst different laboratories.To find a better suited measurement technique, several tests were performed with different thermocouple types and several installation methods and at different locations. The comparisons revealed that there can be a wide variation between temperature readings on the exact same bar depending on which installation method, interface, position and thermocouple size are used. The testing also showed that the stability of the reading is dependent on the installation method and the material used to fix the thermocouple. Furthermore, depending on the installation method, already the thermocouple size can cause variations of more than 10°C. Also the size of the thermocouple itself was found to have a major impact on the temperature reading.