Cole-cole representation of transformer oil-paper insulation dielectric response

Information about the condition of oil-paper insulation is of extreme importance for evaluating ageing status and maintenance requirement of transformers. Dielectric testing techniques in both time domain and frequency domain are being assessed in recent years as supplements to traditional techniques by researchers and utilities. Several attempts have thus been made to understand the physical processes of dielectric relaxation taking place during such tests by correlating conventional tests, time domain and frequency domain dielectric measurements, and chemical techniques. In practical dielectric systems, the dipoles are found to relax according to a distribution of elementary Debye relaxation frequencies during the dielectric response measurements. Researchers have proposed models that can simulate such many-body interaction processes with the help of suitable distribution density functions. Frequency domain spectroscopy (FDS) data can further be computed from the distribution density function thus formulated. The present contribution discusses mathematical formulations used for transforming time domain data to distribution domain and further to frequency domain. Findings about the influence on operating temperature, moisture content, charging voltage etc. on the relaxation process of oil-paper insulation have been reported in this contribution. Results of tests on field transformers are also presented. This paper also highlights the possible use of Cole-Cole models and employs the features of Cole-Cole diagrams for analyzing condition of dielectric materials.