Modeling of the Dielectric Response of a Stator Winding Insulation from a DC Ramp Test

The aging due to mechanical, thermal, electrical and environmental stresses of a stator insulation system inherently involves alterations of the material properties which 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 even to improve the useful life, condition-based maintenance and diagnostic tests are periodically conducted on stator insulation system. Among the tests performed to assess the condition of winding insulation, various methods are commonly used like partial discharge measurements, hi-pot, step voltage and ramped voltage tests to name just a few. The I-V curve measured during a ramped direct voltage test, as the one obtained in a polarization/depolarization test, includes the contribution of various current components, the capacitive, the absorption, the surface leakage and the bulk conduction currents. These contributions, especially the absorption and the conduction (surface or volume) currents, are not as easy to separate in a ramped voltage test as in a polarization/depolarization test (step voltage test). A modeling of the I-V curve obtained with the ramped voltage test, allowing an easy separation of the leakage current from the absorption and the capacitive currents, is presented in this paper. This modeling is compared with a field measurement on complete stator winding and with a lab measurement on spare coil. The calculated values of the absorption and leakage currents obtained during a ramped direct voltage test are compared with the same components deduced from a polarization/depolarization test.