A generalized approach for the study of the non linear behaviour of stator winding insulation

Measurements performed on stator winding insulation indicate variations in capacitance and tan δ values with voltage even in the absence of partial discharge activity. One of the most obvious reasons for this variation is the presence of the non-linear field stress grading system employed at the slot ends. Other reasons include interfacial and space charge polarization phenomena, due to a variety of reasons including contamination of the windings, as well as, the effects of electrostatic forces on delaminated stator insulation. As partial discharge activity results in change in the instantaneous capacitance with voltage, it could also be considered as a case of non linear behaviour of the insulation. The paper discusses a generalized approach to study the influence of non-linear behaviour on the variations of capacitance and losses. Case studies are employed to illustrate the usefulness of measurements on the stator windings in service