Review of modern diagnostic techniques for assessing insulation condition in aged transformers

Cellulosic paper and oil insulation in a transformer degrade at higher operating temperatures. Degradation is accelerated in the presence of oxygen and moisture. Power transformers being expensive items need to be carefully monitored through-out its operation. Well established time-based maintenance and conservative re-placement planning is not feasible in a current market driven electricity industry. Condition based maintenance and online monitoring are now gaining importance. Currently there are varieties of chemical and electrical diagnostic techniques avail-able for insulation condition monitoring of power transformers. This paper pre-sents a description of commonly used chemical diagnostics techniques along with their interpretation schemes. A number of new chemical techniques are also de-scribed in this paper. In recent times a number of electrical diagnostic techniques have gained exceptional importance to the utility professionals. Among these tech-niques polarisationrdepolarisation current measurement, return voltage measure-ment and frequency domain dielectric spectroscopy at low frequencies are the most widely used. This paper describes analyses and interpretation of these techniques for transformer insulation condition assessment. Index Terms — Insulation condition, transformer diagnostics, transformer ageing, dissolved gas analysis, furan analysis, degree of polymerization, oil insula-tion, cellulose paper, return voltage, polarisation current, depolarisation current, online monitoring, condition based monitoring.