Dielectric absorption characteristics of generator stator insulation

It has been the practice in Manitoba Hydro over a 30-year period to apply a constant direct-voltage to generator stator windings for a period of at least 10 minutes prior to the application of a ramp voltage according to IEEE Std 95. The constant voltage is between 15% and 20% of the maximum applied during the subsequent ramp test. The primary purpose of the 10-minute period at constant voltage is to measure the insulation resistance and polarization index from which to determine whether it is safe to proceed with the ramp test. However the continuous record of current as a function of time provides the data from which the absorption coefficients K and n can be calculated according to the relationship I_a=KCVt^-n. The coefficient K is known to be highly temperature dependent. However if the influence of temperature can be removed, it has been suggested that changes in K with time may be indicative of aging of the insulating materials. Absorption data from generator windings in Manitoba Hydro obtained over a 30-year period are examined to determine the characteristics of shellac, asphalt, polyester and epoxy bonded mica as well as evidence of aging of these materials