Experience with ISO-Paraffinic Insulating Oil for Power Transformers

Abstract

For many years Manitoba Hydro had used naphthenic oil refined by solvent extraction and hydrogen treatment in its power transformers. The oil purchased in 1960 through 1979 contained no more than 0.08% oxidation inhibitor. By 1990 naphthenic oil was specified as having at least 0.15% and not more than 0.4% oxidation inhibitor. However, during the 1990s insulating oil meeting this description experienced premature ageing under conditions of service. This resulted in the need to reclaim using Fuller's Earth or replace with new the oil in a number of large generator step-up transformers after less than 10 years of service. The unsatisfactory experience caused us to evaluate more reliable sources of insulating oil. Since 1999 Manitoba Hydro has used a synthesized iso-paraffinic insulating oil in new generator step-up, HVDC converter, and transmission transformers. This choice was made as a result of the improved oxidation stability and heat transfer capabilities of this fluid. In addition, the selected iso-paraffinic oil has low kinematic viscosity at -40°C which is a vital requirement in our climate. As many of these transformers installed since 1999 have been in service for at least 10 years, this paper tracks the results of insulating oil quality tests on typical units based on measurements of colour, interfacial tension, acidity, and dissipation factor. Other diagnostic tests involving the oil have included dielectric strength, water content, dissolved gas-in-oil and furan-in-oil. Manitoba Hydro participated in thermal ageing tests sponsored by CEATI on a number of small transformers filled with two types of naphthenic oil as well as the iso-paraffinic oil. These tests confirmed the superior performance of the iso-paraffinic oil compared to that of a commonly used naphthenic oil.