Improvements of the transformer insulation XY model including effect of contamination

Dielectric Frequency Response, DFR (also known as Frequency Domain Spectroscopy, FDS), was introduced more than 20 years ago and has been thoroughly evaluated and proven in a number of research projects 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 model used to describe the insulation system inside transformers and bushings is called an XY-model. The model works well for most transformers but there are sometimes measurements showing an irregular response and not the expected shape for oil-paper insulation. In those cases, the moisture analysis becomes more difficult and sometimes almost impossible. One example is “frequency hump” phenomena sometimes observed in DFR measurements. A model of the internal creep currents from e.g. contamination has been developed and by applying it in moisture assessment software, better interpretation is obtained. Simulations are complemented with measurements on an actual XY-cell with a conductive layer on the surface of the insulation. Results support the simulation model.