Understanding the correlation between dielectric response measurement results and equivalent circuit pole locations of a transformer oil paper insulation system

Lifetime of a transformer is determined by the condition of its oil paper insulation. Presence of moisture in the insulation reduces the dielectric strength and hence the remaining life. During the entire period of operation of transformer, moisture diffusion between oil and paper takes place, this causes the transformer to operate in a state of thermodynamic non equilibrium. Paper insulation of a transformer serves as the prime source of water in transformer. Oil moisture content can be directly measured using Karl Fischer Titration method. But determination of paper moisture will involve the collecting of paper sample from the transformer which will make the test destructive. Hence paper moisture is estimated from the standard equilibrium curves. Recently, pole values determined from the traditional linear model of the insulation structure have been used to serve as a basic guide for assessing the condition of oil paper insulation system of a transformer. Theoretically the RC branches of the linear equivalent model with lower time constant reflects the condition of insulation of oil and higher time constant branch reflects the condition of solid insulation. From this basic idea pole location of different branches are calculated and reported in this article. This present work attempts to assess the condition of insulation by locating position of the poles, and how pole locations vary with paper and oil moisture as well as temperature.