Condition assessment of aged and wet asphalt-mica generator windings by frequency dielectric spectroscopy measurements

This paper presents variation of dielectric properties of aged and wet asphalt mica winding samples using frequency dielectric spectroscopy measurements. The test procedure consisted of three phases: drying, wetting and electrical ageing. During drying, naturally wet samples were dried inside an oven at 70°C for 168 hrs. During wetting, dried samples were artificially wet at 28°C and RH of 75% for 198 hrs. During ageing, the samples were electrically stressed at 300% of the nominal voltage for 1007 hrs in a cyclic behavior such that each day stressing and resting were done at 12/12 hr intervals. After 1007 hrs, the samples were rested for 168 hrs and afterwards electrical ageing was repeated for further 334 hrs. In all cases, FDS measurements were conducted on the samples and loss tangent and relative permittivity parameters were used for the analysis. The results were compared with field spare winding with age of 50 years. It was found that FDS measurements can be used for separating wet and aged conditions.