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The potential of having a more efficient and cost-effective source of power in the market has driven the incentive to inject distributed generation through smart grids. Distributed generation has been proposed to be a source of sustainable energy. However, the increased use of power electronic devices linking the distributed generation to the existing grid can produce a considerable amount of harmonics. Moreover, the use of power factor correction capacitors at the distributed generation interfacing the grid can amplify the harmonic distortion level due to system resonance. Thus, the injected frequency components to the grid can play a significant role in introducing enhanced stresses on the electrical insulation system on vital power system components; like transformers. In this paper, the effect of the harmonics' injection, as a consequence of the distributed generation interface to the grid, on the degradation of the transformer insulation system is studied. Experiments on degrading transformer paper insulation samples with harmonic-polluted voltages are performed. Dissipation factor increment ratio is used as a measure of the aging effect on insulation. It has been found that the dissipation factor increment ratio of the transformer paper insulation is significantly higher, when stressed with distorted voltage waveforms compared to the power frequency voltage stresses.