03_review_moderntech_g - IEEE Transactions on Dielectrics...

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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 10, No. 5; October 2003 903 Review of Modern Diagnostic Techniques for Assessing Insulation Condition in Aged Transformers Tapan K. Saha School of Information Technology & Electrical Engineering University of Queensland, St. Lucia Brisbane, Queensland-4072, Australia ABSTRACT 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 polarisation r depolarisation 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. 1 INTRODUCTION HE insulation system of a power transformer T consists mostly of hydrocarbon oil and paper. Many of these power transformers within electric utilities around the world are approaching the end of their design life. Insulation degradation is a major concern for these aged transformers. Insulation materials in transformers de- grade at higher operating temperatures in the presence of oxygen and moisture. Practicing engineers currently use a number of modern diagnostic techniques to assess the in- sulation condition of aged transformers. Among them moisture analysis in transformer oil, dissolved gas analysis Ž. wx DGA 1 , degree of polymerisation DP measurement and furan analysis by high performance liquid chromatog- raphy HPLC are frequently used 2,3 . Molecular weight studies by single point viscosity mea- surements are of limited value when dealing with a com- plex polymer blend such as Kraft paper as used in trans- Manuscript recei ® ed on 18 No ® ember 2002, in final form 29 May 2003.
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03_review_moderntech_g - IEEE Transactions on Dielectrics...

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