Dielectric frequency modeling - transformer insulation model improvements including contamination effects

Dielectric Frequency Response, DFR (also known as Frequency Domain Spectroscopy, FDS), is proven as an excellent tool for understanding insulation properties e.g. moisture content in cellulose insulation and temperature dependence of the insulation system. Fundamental to DFR analysis is the modeling of the insulation system. The common model used is often described as an XY-model where X and Y represent the amount of oil and paper in the insulation system. The model is a significant simplification of an actual transformer oil-paper insulation system but experience has shown that the analysis and results are quite accurate in most cases. However for some transformers, the dielectric response does not have the expected shape for oil- paper insulation as described by the XY-model. In those cases, the analysis becomes more difficult and sometimes almost impossible. An example is “frequency hump” phenomena sometimes observed in DFR measurements. Another issue is whether a known ”true” geometry for the transformer should be used when modeling the insulation or if the modeling program should be allowed to use X and Y as matching parameters which in many cases gives a modeled geometry that differs from “true/expected” values. In an ongoing project, the XY-model is compared to FEM analysis of the insulation system. With an extended FEM model it is possible to investigate phenomena and parameters not described in the standard XY-model. In this paper, two limitations of the model are presented and discussed; 1) The effect of gaps between winding turns, creating a non-continuous capacitor plate, and 2) The effect of leakage currents on the cellulose surfaces. Both effects have been modeled with FEM and results are compared with the XY-model. For the surface current effect simulations have been complemented with measurements on an actual XY-cell where a “plain” cellulose surface is compared with a cellulose surface covered with a conductive layer. Measurement results on the special test cell are well in line with results from the FEM-analysis.QC 20211005