Life span estimation of oil-impregnated paper high voltage current transformers based on long duration tests under simultaneous thermal and electrical stresses

Abstract

A large number of power system apparatus around the world are over 30 years old in operation. Replacement of power system assets involves high investments. On the other hand, the failure of such equipment causes a high economic impact. The lifespan estimation of high voltage apparatus is an important information to determine the best moment for the replacement. Although the current transformer is an essential component in the electrical power system and its failure may result in serious consequences, there are no accelerated aging tests for this equipment prescribed by technical standards. Life estimation for this equipment is generally made based on field experience, something that cannot be done for new models and/or new manufacturers. In this work, experimental results of life tests performed on reduced models, representing high voltage current transformers with oil-impregnated paper insulation, are presented and discussed. The analysis is based on the application of a multi-stress model, which combines the Montsinger rule and Inverse Power Law. The proposed approach considers the existence of threshold values for electrical and thermal stresses, below them, electrical and thermal degradation are negligible. An order of magnitude for the electrical stress threshold value (E0) and for the electrical aging exponent (n) is obtained from the application of the test results to the model used in this investigation. The scope of the investigation includes several diagnostic tests which are performed after interruptions of the long-duration test. The diagnostic tests include some classical high voltage power frequency measurements, tests on oil samples and finally, dielectric frequency response (DFR), one of the most recent insulation diagnostic techniques. The application of the thermoelectrical multi-stress model to the results indicates that the estimated lifespan for the test samples is about 50 years, that the exponent related to electrical stress n is in the range of 1.3-3 and that the electrical stress threshold E0 is in the range of 1.17 to 1.21 p.u. The results of diagnostic tests carried out as part of this investigation revealed the relevant variation of several parameters along the aging test and, combined with the findings obtained in the autopsy of the failed test samples indicated a dielectric failure caused by the combined action of electrical and thermal stresses. The variations of these parameters are of great value for evaluating the degradation conditions of the insulation over its service life. The cross-analysis of several different diagnostic tests is recommended for decision-making regarding the replacement of equipment in service.