A method for the study of the non-linear behaviour of HV stator winding insulation

Measurements performed on stator winding insulation indicate variations in capacitance and tan delta 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 nonlinear 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. The paper discusses a generalized approach to study the influence of nonlinear behaviour on the variations of capacitance and losses. A method is suggested to separate solid and partial discharge losses in the presence of partial discharge activity. Case studies are employed to illustrate the usefulness of measurements on the stator windings in service