Abstract
M.Ing.
The study looks into the impact of harmonic distortion on power-plant equipment in general, and then
focuses on the impact it has on power transformers operating near the thermal limit. The feasibility of
the study is firstly evaluated and then the theory on harmonics and transformer losses is analysed. The
study had been narrowed down to power transformers due to the high numbers of failures nationally and
internationally attributed to unknown causes. A transformer model is then developed through theoretical
considerations. Finally, a case study is done on the capability of a fully loaded transformer under
harmonics conditions evaluated through transformer capability calculations and the proposed
transformer model. Thereafter the transformer model developed is verified with measured results.
The main impact of harmonic current distortion on power transformers is an increase in the rated power
losses that results in a temperature rise inside the power transformer. The heat build-up can lead to
degradation of insulation, which can shorten the transformer’s life and lead to eventual breakdown. The
harmonic current distortion impacts transformer losses – namely, ohmic losses, the winding eddy
current losses and other stray losses. All of these harmonic effects on transformer losses are verified
theoretically, mathematically and practically.
The harmonic impact on the transformer capability is then evaluated through a numerical example of a
transformer feeding a harmonic load. The transformer capability is determined via two methods –
namely, harmonic capability calculations in the standard “IEEE Recommended Practice for Establishing
Transformer Capability when Supplying Nonsinusoidal Load Currents”, [11] and a proposed
transformer model derived from theoretical and mathematical analysis. The results show that an
increase in the winding eddy current losses can decrease the maximum permissible nonsinusoidal load
current substantially. If the load current of the transformer is derated accordingly it translates into a loss
of the output power capacity of the power transformer. The standard recommended capability
calculations for winding eddy current losses are conservative and not satisfactorily accurate. This results
in a large loss of power capacity. The proposed transformer model includes a parameter that estimates
the winding eddy current loss in the transformer that results in a smaller loss in power capacity.
Furthermore, it was shown that the harmonic current distortion levels could exceed the permissible
levels although the harmonic voltage distortion levels are within acceptable levels. The proposed
transformer equivalent model is thereafter practically verified with experimental results of papers
published by M.A.S. Masoum, E.F. Fuchs and D.J. Roesler, [19], [20] and [29].