EFFECT OF INSULATING OIL LEVEL ON DISSIPATION FACTORS OF OIL-PAPER INSULATION USED IN DISTRIBUTION TRANSFORMERS

The rate of failure of distribution transformers which are connected in the National Grid in Tanzania is high. Earlier studies found out that one of the major causes of transformer failures is internal winding short circuit and that this type of failure may be accelerated by low insulating oil level in the transformer tank. In the distribution transformer, oil-paper insulation system is used to insulate the transformer winding. In addition, insulating oil is used to cool the transformer winding. This paper proposes the use of dissipation factors measured over wide frequency range to predict oil level in distribution transformer. The effect of insulating oil level on the dissipation factor curves over wide frequency range, i.e. from 1 mHz up to 1 kHz, has been assessed using a model implemented in Finite Element Method software (COMSOL Multiphysics). The simulation results were validated through comparison with experimental results from the literature. The results show that there is significant change in dissipation factors in the low frequency region when the insulating oil drops below the position of paper layers.