Economical aspects and practical experiences of power transformer on-line monitoring

摘要

The sector of distribution of electricity is confronted with important strategic changes in a market where there is growing competition. These changes will force electric utility companies to increase the use of their equipment and, at the same time, to reduce costs.Power transformers are very expensive components of high-voltage installations. Due to their high cost and to the consequences of a possible anomaly, the electric companies have to ensure a high reliability level during the life cycle of the installed units. Reliability questions concern mainly strategic transformers, transformers which are in service for more than 30 years and units which show some anomalies like an important increase of dissolved gas concentration, noise, unexpected tripping, excessive maintenance of OLTC, etc. The aging of the apparatus is often difficult to predict due to the different local exploitation conditions (overloading, over-voltage) and because each manufacturer uses his own technologies.To answer these important questions, monitoring concepts with modern data processing techniques have been developed. Such monitoring equipment has been in successful operation in Switzerland and in the USA for more than five years.In Switzerland, on-line monitoring has been used for three years mainly on GSU Transformers with success. Experience has proven to be cost-effective, even the number of units under control is still low,approximatively 20 transformers are now connected with stand-alone systems or integrated SCADA systems.In New York City, on line monitoring of transformers with on-load tap changers has been extensively employed. It is Consolidated Edison's experience that a high percentage of transformer failures are related to OLTC problems. The expense and difficulty of replacing transformers in the highly congested areas of New York City make the replacement of a transformer far more expensive than it would otherwise be. Also, the possible loss of service to customers and the potential for oil spills and fires during a catastrophic failure are important considerations. On-line monitoring is being used as a condition based monitoring tool to allow maintenance to be performed as needed rather than on a time or operations based schedule. The observation of monitored parameters has allowed removal from service many units for maintenance before the occurrence of a catastrophic failure.