(Proceedings on Seminaron Fault-Finding and Life-Assessment of Power Transformers, 25-26 April, 2008, New Delhi, pp 91-96)
Power transformers are critical, capital-intensive assets for utilities and industry. Transformers are extremely reliable; however, many of the transformers in use today have already exceeded their design life. Today transformers are not automatically replaced, if they have reached the end of their life span, but left in service as long as possible. In contradiction to the past power transformers are operated now-a-days at or above rated power. This accelerates the ageing process of the inner insulation, particularly of the insulation paper, which cannot be easily replaced. Investments of new transformers have to be planned in advance because big transformers cannot be bought ready made from the factory. For a systematic replacement and to avoid unexpected breakdowns diagnostic too Is are more and more important.
Reliable assessment of the ageing state of high voltage power transformers is a basic condition for a failsafe and cost-saving service. One ageing indicator is the water content in the solid part of the insulation (paper, pressboard). Water is an ageing product and accelerates the further deterioration of cellulose through de-polymerization. In addition increased water content in oil may cause bubble formation and lead to an electrical breakdown.
It is important to have reliable tools for measuring the water content in the oil-paper-insulation. This way unexpected breakdowns can be avoided and the maintenance, repair or replacement can be scheduled in time.
The paper gives an overview about the latest / measuring technologies and contains interesting case studies for successful insulation diagnosis and fault locations in power transformers.
