Diagnostic of insulation condition of oil impregnated paper insulation systems with return voltage measurements

The insulation system of power transformers consists of oil and cellulose, whose dielectric properties are strongly influenced by moisture, temperature and aging. Their condition can therefore, be evaluated using dielectric response measurements. This paper describes the return voltage measurement (RVM) diagnostic method based on scanning the polarisation spectra of the oil-paper insulation in the time range of 0.1s to 1200s and excitation voltage 500V (DC). The effects of moisture, temperature and aging were studied. The accelerated aging process of oil-paper samples were investigated under air environment up to a temperature of 115°C. Typical transformer proportions of copper, aluminium, zinc and iron were added to oil-paper insulation during the aging process. The oil-paper insulation samples were aged at different aging times at 250, 500, 1000 and 2000 hours. The investigations of temperature influence on the return voltage characteristics were studied to determine the state of the insulation under operating condition at different operating temperature from 24 to 65°C. The RVM spectra were evaluated in terms of peak magnitude variation and time shift of peak occurrence in relation to the temperature and moisture condition as well as aging.