Temperature effect on insulation resistance of generator stator bar affected by water ingress

For water-cooled stator bar of power generator, leaks of water at the copper strand brazing location will reduce dramatically its insulation resistance. Therefore, the insulation resistance measurement is an important and simple test to perform in the periodic maintenance program. The temperature has an important effect on the insulation resistance. To correctly trend the insulation resistance of the winding, a correction factor has to be applied. This paper describes experimental data performed on both dry and wet epoxy-mica insulation from bars removed from an operating generator. The results are compared with different correction models. They show an important difference of the temperature coefficient between the dry and wet bar.