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.
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