Ionization Tests on a Fourteen Year Old Hydrogeneretor Winding

Recently a decision was made to rewind a 1OOMW, 13.8 kV hydrogenerator which had been in service for about fourteen years. The statqr insulation on this generator was an early version of "Thermalastic", a synthetic resin vacuum impregnated mica insulation. Prior to removing the coils from the machine a series of ionization probe and tangent delta measurements were made. Selected coils, including all coils from one parallel string of the winding Were then subjected to detailed laboratory investigations. In this paper, the relation between ionization probe readings and tangent delta tip-up values is demonstrated. The effect of time of voltage application on ionization probe readings, tangent delta, and capacitance is investigated, and the importance of standardizing the time of voltage application prior to these measurements ments is emphasized. In view of the length of service encountered, the condition of the; insulation was thought to be very satisfactory.