Influence of copper particles on breakdown voltage and frequency domain spectroscopy characteristics of vegetable insulating oil

In order to obtain the influence of metallic particles on the insulating performance of vegetable insulating oil, copper particles were chosen to contaminate the vegetable insulating oil. The AC breakdown voltage of rapeseed insulating oil with different copper particle concentrations was measured. The dielectric properties i.e. relative permittivity, dielectric loss tangent and volume resistivity of rapeseed insulating oil versus frequency with various copper volumetric concentrations were also measured by dielectric spectrometer. The characteristics of the breakdown voltage and the three dielectric parameters versus frequency were analyzed. Measurement results show that the AC breakdown voltage of vegetable insulating oil decreases with the increase of copper particle concentration, and it decreases faster with increasing copper particle concentration when the size of cooper particles is larger. The relative permittivity and dielectric loss tangent increase with increasing copper volumetric concentration, and the volume resistivity decreases. The difference is more obvious at low frequencies. Copper particles in the vegetable insulating oil damage its insulating properties, therefore it is necessary to pay attention to the metallic particle concentration in vegetable insulating oil when applied in transformers.