FEM analysis of the transformer insulation XY model

Dielectric Frequency Response, DFR (also known as Frequency Domain Spectroscopy, FDS), was introduced more than 20 years and has been thoroughly evaluated and proven in a number of research projects as and field tests with good results. DFR data in combination with mathematical modeling of the oil-paper insulation is proven as an excellent tool for understanding insulation properties e.g. moisture content in cellulose insulation and temperature dependence of the insulation system. The traditional XY model is widely known for describing the dielectric response of the paper-oil insulation system. This paper explores the accuracy and limitation of the XY model by comparing it with FEM using COMSOL Multiphysics. The `hump´ phenomenon that sometimes is seen in field measurement and cannot be described by the simple XY-model is also investigated by using the FEM technique. The analysis shows that the deviation in the frequency response can be described and modeled as a semi-conductive layer with a certain conductivity and thickness.