Progress in DC Testing of Generator Stator Windings: Theoretical Considerations and Laboratory Tests

The aging due to mechanical, thermal, electrical, and environmental stresses of a stator insulation system inherently involves alterations of the material properties that are detrimental to its service operation. When these properties are deteriorated to the point where the material can no longer operate safely under normal stress conditions, it implies that it has reached the end of its useful life. To prevent such forced outages and improve the useful life, condition-based maintenance and diagnostic tests are periodically conducted on stator insulation systems. Among the tests performed to assess the condition of winding insulation, various methods are commonly used, such as partial discharge measurements, hipot, step voltage, and ramped voltage tests (RTs), to name just a few. DC testing, where the current is continuously monitored, whether during a step voltage test, such as the polarization/depolarization test, or during a linearly increasing voltage test, such as the RT, is commonly used in the field in order to assess the quality of stator winding insulating systems of large synchronous rotating machines. Both of these tests are related to the dielectric response of the insulation system. This paper presents theoretical considerations on the dielectric response of the various types of machine winding insulation systems encounter in the field as well as laboratory results on individual bars and coils.