Transformer Condition Assessment
 

Transformer Condition Assessment

There are approximately 100,000 large power transformers in use in North America today. Worldwide the count is close to 500,000.  Power transformers are expensive assets that generally cost between $100K and $4M each (before installation), depending on their size and application.  In the area of transmission and distribution power transformers typically represent the highest valued assets of the system.  There is no doubt that these assets are some of the most critical components in the electrical grid and the power generation system.  The demand placed on these power transformers continues to increase. Deregulation has led to large increases in generation capacity with little consideration given to transmission capacity.  This has led to a decrease in the reliability of power delivery and an increase in the likelihood of significant blackouts as operating safety margins disappear and the age of these key assets continue to increase.

The direct cost of a failure includes the cost of new equipment, environmental cleanup, and replacement power which typically must be purchased on the spot market; in those areas which support many industrial and large commercial customers, costs can also include penalties or compensation for the unplanned outage.  At the highest level of the electrical infrastructure, reliability is achieved through redundancy.  However, even when redundancy exists there are impacts due to a failure.  The financial impact of a failure can be greater for the consumer than the utility.  Even with redundancy, switching interruptions resulting from a failure can have a major impact on customers.  Process industries may lose data or be forced to shutdown or restart as a result of such transients.  The costs to industrial and financial customers, when long outages occur, are dramatic.  Such costs are now beginning to be discussed in terms of tariffs and associated penalties for utilities not delivering reliable power.

Monitoring the state of health of power transformers, a key component in the path of reliable power, has traditionally been performed using routine laboratory Dissolved Gas Analysis (DGA) tests performed at annual or semi-annual intervals.  The level and rate of change of combustible gases is measured in the transformer insulating oil; this is an accepted industry practice based on the fact that the presence of combustible gases is a reliable diagnostic tool which can be used as indicator of undesirable events occurring inside the transformer, such as hot-spots and electrical arcing; events which may portend a future of unreliable performance by the transformer.

Many transformer failures can be prevented by closely monitoring dissolved gas levels. Prevention is possible if the reason for the presence of the gases is understood and can be either controlled or corrected by means of repairs or prescribed usage profiles.  Correlating transformer gassing events to load, temperature, and external system events can be a difficult or even impossible task when only one or two DGA samples per year are available to perform such analysis.  Daily on-line monitoring of key gases provides the level of detail necessary to relate gassing to external events, a task that cannot be accomplished using traditional infrequent laboratory sampling.