At the 2007 Conference of Doble Clients, held in March in Boston, Massachusetts, U.S., a symposium was provided to emphasize the success that utility companies are realizing by monitoring dissolved gases-in-oil to save load-tap changer transformers from failure. The technique uses specific gas levels and ratios to determine when a load tap-changer (LTC) has deviated from its normal operation and prompts an internal inspection of the load-tap changer. Dissolved gas analysis (DGA) of oil has become the key diagnostic for condition assessment of LTCs.

Prior to this monitoring technique, LTCs could develop problems that would remain undetected until either customer complaints were received or the transformer tripped off-line because of a failure. These scenarios cause customer outages and could cause the loss of a valuable asset. In response to these problems, utilities would use time-based maintenance programs with short intervals that now have been either significantly extended or eliminated in favor of dissolved gas-in-oil monitoring. Customer power quality is improved, assets are preserved and labor resources are available for other tasks.

To start the symposium, Rick Asche of Portland General Electric, chair of the Doble subcommittee studying this technique, provided an introduction. Asche noted that the five companies in the symposium have been using this technique for five to eight years and have obtained positive results by saving many LTCs from failure. A “save” is considered a situation where dissolved gas data has initiated action to remove the transformer from service for repair prior to a problem becoming a forced outage or a failure. The table summarizes some information about the five companies' DGA programs.

It was noted that most of the companies realized more than two transformer saves per year on their power grids. These five companies overall averaged about one save per year for every 100 LTCs on their systems. The consensus of the participants was that their DGA program easily pays for itself in saved equipment and the operating time that would be lost for an unplanned outage.

This technique works for many types of LTCs. For the symposium, the companies concentrated on their most interesting saves. The most common malady that was detected was contact and collector ring coking. The technique also found problems with loose connections, failed mechanisms and a failed vacuum bottle. The presentations provided the details of each LTC save including the gassing conditions that justified the internal inspection.

The presentations from the symposium are available on the Doble website (www.doble.com) for clients of Doble Engineering Co.