Estimation of Electric Transformer Service Life from Oil Degradation Kinetics

Petroleum-based transformer oil performs the dual roles of insulator and electrolyte depending on charges in the oil. The availability of charges for conduction influences the level of voltage breakdown in the mineral oil and depends on the intensity of physicochemical processes such as redox action, hydrolysis, photodegradation, sludge/particle development, and gas formation arising from thermal, mechanical, chemical, and electrical stresses in the oil. Previous investigators have focused mostly on single oil aging indicators to build models for estimation of the aging rates of transformer oils, within a time frame that is too short relative to the operational service life of transformers. In this research, the interdependence of oil voltage breakdown and changes in oil physicochemical characteristics was investigated experimentally using T24 and T14 oils. Tests were performed with a DTS-100D oil tester for voltage breakdown and a HKV 4000 series high kinematic viscosity bath. The calculated activation energies are $26.2{\mathrm{KJm}}^{-3}$ for T24 and $25.4{\mathrm{KJm}}^{-3}$ for T24 transformer oils, respectively. Neglecting catastrophic failure from environmental hazards, the estimated service lives of T14 and T24 transformers that are herein analyzed are 40 and 43 years, respectively. Their degradation constants were experimentally determined to be $7.38\times {10}^{-12}$ and $6.98\times {10}^{-12}{\mathrm{L}}^{-1}{\mathrm{S}}^{-1}$ , respectively.