The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.

Evaluation of Power System Harmonic Effects on Transformers: Hot Spot Calculation and Loss of Life Estimation

Asaad A. Elmoudi

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Electrical and Communications Engineering for public examination and debate in Auditorium S4 at Helsinki University of Technology (Espoo, Finland) on the 20th of April, 2006, at 12 noon.

Dissertation in PDF format (ISBN 951-22-8078-7)   [2894 KB]
Dissertation is also available in print (ISBN 951-22-8077-9)


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.

Keywords: temperature, power transformers, thermal factors, power quality problems

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© 2006 Helsinki University of Technology

Last update 2011-05-26