This paper presents the comparison of two methods to estimate the insulation temperature increase and lifetime reduction in power transformers due to the presence of stationary power quality disturbances. The internal insulation can suffer an accelerated aging by temporary increases of rms values of currents caused by waveform distortion (harmonic current). The recurrent presence of increase in rms values during long time periods will produce a cumulative effect that degrade the dielectric material. Models for insulation aging are employed. A discussion about the effects on insulation lifetime is presented.