Effect of Soluble Copper Ions on Insulating Oil’s Electrical Properties: An Experimental Study

Corrosion and electrical erosion of metals within transformers lead to the presence of metal ions in the oil. Unlike metal particles, these soluble ions diffuse readily in the oil and are challenging to filter out, posing a significant threat to the transformer's reliable operation. This paper employs experimental evaluations to elucidate the effects of copper ions on the dielectric properties, breakdown performance, and partial discharge characteristics of insulating oil, coupled with a theoretical analysis. Dielectric spectroscopy test reveals that soluble copper ions alter the dielectric properties of the oil, specifically causing a reduction in its relative permittivity due to ion-induced endogenous electric fields (about 60% decline in the low frequencies). Additionally, the ions result in increased dielectric losses at low frequencies and notably enhanced conductivity at high frequencies, potentially due to electron tunneling and thermally activated polarization. AC Breakdown tests indicate that the breakdown voltage of the oil decreases as copper ion concentration increases. The relatively lower ionization potential and charge saturation effects are identified as contributing factors to the decreased breakdown voltage. Partial discharge experiments confirm that copper ions lead to reduced inception voltages (with a maximum decline of 17.5% in the test), and the frequency of discharges between needle-plate gaps in the insulating oil increases with ion concentration. The findings offer critical insights into the influence of metal ions on the electrical properties and discharge behavior of insulating oils.