Losses and temperature rise within power transformers subjected to distorted currents

The currently high energy efficiency demands and control requirements impose an extensive usage of power electronic devices and circuits in both domestic and industrial applications. These intrinsic nonlinear loads generate prominent level distortion of the electric current waveforms, which lead to additional losses and thermal stress within the power supply distribution system. Thus, due to the supplementary harmonic losses, the power transformer, experience an increase of its hot-spot temperature that may cause the machine insulation failure and ultimately the transformer lifetime expectancy reduction. In order to overcome these undesirable consequences, the reduction of the apparent power rating of the transformer (derating) is commonly adopted. This paper quantitatively investigates the nonsinusoidal steady state of the transformer by evaluating the losses distribution within the machine, its hot-spot temperature rise and the corresponding transformer loss of life. The computation procedure is exemplified on a 250 kVA liquid filled three-phase power distribution transformer that supplies both linear and nonlinear industrial loads.