Correlation between Capacitance and Dissipation Factor used for Assessment of Stator Insulation

about internal defects within the insulation. The index k is defined as the proportionality constant between the changes at high voltage of capacitance ∆C and of the dissipation factor ∆tanδ . ∆C and ∆tanδ values were highly correlated when small flat defects were within the insulation and that correlation was lost in the presence of large narrow defects like electrical treeing. The discrimination between small and large defects is made resorting to partial discharge PD phase angle analysis. For the validation of the results, C and tanδ measurements were carried out in a 15MVA 4160V steam turbine turbogenerator placed in a sugar mill. In addition, laboratory test results obtained by other authors were analyzed jointly. In such laboratory tests, model coil bars subjected to thermal cycling resulted highly degraded and ∆C and ∆tanδ values were not correlated. Thus, the index k could not be calculated.