Measured and modeled capacitance, loss and harmonics in stator insulation with nonlinear stress control

Diagnostic measurements on stator insulation suffer disturbance from nonlinear resistive stress-control in the end corona protection (ECP). The extent of this effect is studied here, by measurements and modeling. Frequency-domain dielectric response measurements on bars with epoxy-mica insulation in new condition are presented as capacitance, loss and current harmonics, for the guarded insulation in the slot region, the end regions alone, and the combination. The voltages are from 0.3 up to 14.4 kV, at frequencies from 100 Hz down to 0.1 mHz, to cover the range relevant to common diagnostic methods. The ECP approximately doubles the change of capacitance with frequency, and has a much stronger relative effect on voltage-dependence and on current harmonics in an unaged insulation system. The ECP currents are modeled numerically, using material properties from further measurements on samples of the silicon-carbide based ECP tape. A nonlinear 1-dimensional model gives a fair fit to the measurements except at the highest frequencies. Reduction of the ECP disturbance in measurements by subtraction of modeled values is discussed. The main practical limitation of the model is likely to be uncertainty of the input parameters describing the geometry and material, rather than a need of more detail in the model.