Evaluation of power system harmonic effects on transformers : hot spot calculation and loss of life estimation

The significance of harmonics in power systems has increased substantially due to the use of solid state controlled loads and other high frequency producing devices. An important consideration when evaluating the impact of harmonics is their effect on power system components and loads. Transformers are major components in power systems. The increased losses due to harmonic distortion can cause excessive winding loss and hence abnormal temperature rise. Existing standards give a procedure to determine the capability of an existing transformer subject to non-sinusoidal load currents based on conservative assumptions. The eddy current loss generated by the electromagnetic field is assumed to vary with the square of the rms current and the square of the frequency (harmonic order h). Actually, due to skin effect, the electromagnetic flux may not totally penetrate the strands in the winding at high frequencies. In addition, the temperature rise due to harmonics is estimated based on constant harmonic load currents and the average daily or monthly temperatures to which a transformer would be subjected while in service. It is the purpose of this research effort to quantify the increased winding losses due to harmonics and the corresponding temperature rise in transformers. This is accomplished using a 2-D FEM model adapted for winding loss calculation. A corrected harmonic loss factor that considers conductor skin effect is proposed and verified by measurements. Thermal dynamic models are investigated and modified to consider a time varying distorted load cycle. The increased temperature is used with an industry accepted insulation loss of life formula to evaluate a transformer's capability.reviewe