Application of Cole–Cole model to transformer oil-paper insulation considering distributed dielectric relaxation

• Main Authors: Sandip Kumar Ojha, Prithwiraj Purkait, Biswendu Chatterjee, Sivaji Chakravorti
• Format: Article
• Language: English
• Published: Wiley 2019-01-01
• Series: High Voltage
• Subjects: transformer oil; power transformer insulation; dielectric relaxation; dielectric materials; paper; Cole–Cole model; transformer oil-paper insulation; dielectric testing techniques; frequency domain; insulation condition assessment; oil-paper insulated transformers; elementary Debye relaxation properties; Cole–Cole diagram; Cole–Cole plots; dielectric mixtures; cable insulating oil; distribution density functions; time domain dielectric response; distributed dielectric relaxation; many-body interaction process; ionic liquids; polar liquids
• Online Access: https://digital-library.theiet.org/content/journals/10.1049/hve.2018.5079

Researchers have been exploring dielectric testing techniques both in time and frequency domain for insulation condition assessment of oil-paper insulated transformers. In a practical dielectric system, dipoles are found to behave according to a distribution of elementary Debye relaxation properties. Suitable distribution density functions have been proposed to characterise such many-body interaction processes. Cole–Cole diagrams can be one of the methods for studying the nature of frequency dependency of dielectric materials of complex structure. Cole–Cole plots are commonly used for characterising different materials such as dielectric mixtures, ionic liquids, cable insulating oil, polar liquids etc. The scope of its application for assessing transformer oil-paper insulation considering distributed relaxation process has not been explored yet. The present contribution discusses mathematical formulations used for transforming the experimentally obtained time domain dielectric response test data to distribution domain and further to frequency domain for obtaining the Cole–Cole plots. Findings about the influence of various operating conditions and insulation status on the Cole–Cole diagram have been reported in this contribution. Results of tests on field transformers are also presented. This paper attempts to employ the features of Cole–Cole diagrams as potential indicators for analysing condition of the oil-paper insulation considering distributed relaxation process.