Abstract
Maintaining the mechanical integrity of a power transformer during large fault currents requires a clamping system that exerts sufficient compressive forces on the windings. This paper presents a thermomechanical model for the clamping force in a power transformer. The model predicts changes in clamping forces due to operating conditions, such as load and ambient temperatures, to indicate when the transformer is most vulnerable to mechanical failures. The model was tested and evaluated on one year of operational data collected from a 40 MVA ONAN transformer equipped with extensive temperature monitoring and novel clamping force sensors. By using only transformer design data, fundamental material parameters and results from the heat run test, the model was able to replicate the general trends observed in the measured clamping force. The model was able to predict the daily mean clamping force throughout the year with a mean absolute error of 6.6 kN, which corresponds to 2.9% of the initial clamping force at the start of the year. For daily variations, i.e., the difference between maximum and minimum clamping force for each day, the model was able to predict variations with a mean error of 2.1 kN. However, the model overestimates the changes in clamping force, as it is too sensitive to changes in loading and ambient temperature.