Depletion rate of oxidation inhibitor in hydroprocessed mineral insulating oil and naphthenic oils

Throughout the world majority of power transformers in use utilize mineral insulating oil and cellulose insulation system for management of dielectric stresses. This system has proven to be extremely reliable and cost effective. The mineral insulating oil provides approximately 80% of the dielectric strength of a transformer. Mineral insulating oils undergo oxidative degradation (Ageing) in the presence of oxygen, moisture and heat to give rise to a number of oxidation products which impair the long term stability and cooling efficiency of the oil as well. Oxidation of transformer oil cannot be eliminated but can be delayed by incorporating inhibitors which will offer enhanced oxidation stability. By regular monitoring and maintaining the level of concentration of the inhibitors, the healthy life of the transformer oil can be extended to a considerable period of time. In this paper we present details of investigations carried out to obtain the information on the depletion rate of oxidation inhibitor in hydroprocessed mineral insulating oils and naphthenic oils. The extended thermal oxidation tests were carried out in the laboratory by using oxidation stability tests at 120^oC as per IEC 61125 Method C. Also the compatibility experiments were carried out in the laboratory by mixing different proportions of hydroprocessed mineral insulating oils with naphthenic oils and conduction oxidation stability tests. And ageing experiments were performed with Kraft paper and conductors in sealed tube at 150 ^oC for 1000 Hrs. The result show that there is very slow rate of oxidation inhibitor depletion in hydroprocessed mineral insulating oils apart from their good compatibility with naphthenic oils and no generation of gases under the same experimental conditions and temperatures.