Thermal analyses of power transformers according to standards

During the past years, the increase in the number of nonlinear loads has been affecting the power quality negatively. The harmonic currents resulting from such loads yield to an increase in power losses of transformers and hence an increase in the temperature of these transformers. The main criteria that defines the life and loading limits of a transformer is its thermal performance. The most important parameter of the thermal performance of a transformer is the hot spot temperature. The hot spot temperature is the temperature of conductor in a region where it generally is near the top of winding. This parameter is accepted to represent the temperature limit of a transformer's loading.The best way to have information about hot spot temperature is to place sensors in the exact top temperature zone and to transfer the measured values out of the tank of the transformer. But, this method is not suitable for the transformers that are currently in operation. It is necessary to place the sensors during design and implementation of the transformer.In most of the power transformers of Turkish Electricity Transmission Company (TEİAŞ), there is no mechanism to measure the winding temperature. In this study, the estimating methods of top oil and hot spot temperature are explained based on the IEEE standards. The top oil temperature of two different types of power transformers of TEİAŞ are measured and compared with those of theoretical. Also, the hot spot temperature is estimated based on the IEEE standards. As a summary, the effect of harmonics on losses, top oil temperature rise, hot spot temperature and the useful operating life of transformers are analyzed in this thesis.Key Words :Power Transformer, Hot Spot, Top Oil Temperature, Harmonics