Long term electrical and thermal behavior of transformer oil/paper insulation under simulated service conditions

A bench-top test method was designed to establish the long-term electrical and thermal behavior under simulated service conditions in order to allow the efficient screening of candidate oils at a reduced final cost when compared to model-type tests. The method is based on the use of small test models aged at stresses slightly above normal operating values with temperature and voltage as the accelerated parameters. The test models are similar in construction to cable models apart from their square conductors and overlapping paper tapes. The test program is such that the temperature is cycled between 105°C and 50°C over a period of 5000 h. The power factors of the test models and partial discharges were monitored at regular intervals and a series of 60 Hz breakdown tests were conducted on a number of samples at 1000, 3000, and 5000 h. A description is given of the various steps of the program, and results of tests done on two different transformer oils are provided. Data of electrical and chemical measurements are discussed in relation to the properties of the oils tested