By Joseph Carbery, The Ardry Group
March 1, 2004 — More research and improved forensics, along with advances in the development of transformer monitoring and diagnostic techniques, have yielded a much improved understanding in the last several years of the pervasiveness and sometimes crippling effects of moisture accumulation in the paper insulation of power transformers.
It has become clear how moisture continually builds up, and, above even modest thresholds, weakens transformers’ dielectric strength sufficiently to jeopardize electrical performance at rated loads, or their ability to tolerate normal transient system stresses.
But what’s also important in the planning and operation of utility systems is a new-found appreciation for just how quickly elevated moisture levels irreversibly ages the paper insulation. Premature aging can cut a transformer’s potential service life by years or even decades; in the final analysis, its usefulness is entirely contingent on the condition the insulation.
In the context of the average age of power transformers in many power systems, these observations are cause for pause. In the United States, the deployment of power transformers in utility networks peaked in the 1970s; equipment with a design life of typically around 50 years. The number of installations has declined since, and drastically so in the last decade.
Condition-based maintenance seeks to balance the risk and the implications of unexpected electrical failures of transformers against the quite significant costs associated with conventional dehydration methods, deferring the procedures until they appear essential to ensure continued electrical performance. Larger transformers, especially those in exposed network positions, may be equipped with computer-based systems for this purpose that perform diagnostics in real time.
One conventional field method for dehydration involves the use of heat-and-vacuum-generating equipment and tanker trailers brought to the location. All or most of the transformer oil is drained into the tanker trailers, so that the paper insulation becomes exposed and moisture trapped in the paper can be removed by cycling the application of heat and vacuum. The transformer will be out of service from one to several weeks.
Most effective is the vapor phase dryout method, which restores the transformer to its original condition. But the procedure must be performed at a supplier factory or a well-equipped maintenance facility, which makes it more costly, laborious, and time consuming.
Although these off-line methods are effective in restoring electrical reliability, they do little to preserve the life expectancy of transformers. The damage to the paper insulation from aging occurs during years of inevitable moisture accumulation, and the buildup starts as soon as the transformer is brought back into service.
The tons of paper insulation used in transformers have great affinity for moisture and traps some 99 percent of the moisture accumulation. Some of the buildup comes from normal ingression over the years. Older transformers were designed to “breathe” the ambient air. But, as a practical matter, the modern sealed transformer designs are also subject to moisture penetration from the outside environment. And another important source is the aging process itself ; it produces moisture as a byproduct at accelerating rates.
As moisture builds up from these and other sources, the rate of aging eventually begins to increase almost exponentially. At the moisture levels typical in transformers that have been in service for 15 to 25 years, the paper insulation can age 20 times faster than in its factory-supplied condition. And the damage is irreversible.
In South Africa, the challenge with moisture was long compounded by low equipment quality; decades of import restrictions had limited access to leading transformer suppliers. By the mid-1990s, the Eskom Utility, in its operation of all sub-Saharan power networks, faced an unacceptable rate of moisture-related failures of power transformers.
The utility — today the fifth largest in the world — set out to develop a method for dehydration more economical than conventional approaches, and that also would slow the aging process of the paper insulation. The result was a method to continuously dehydrate transformers — to continuously mange the moisture level — while they remain in service. It was developed by Eskom’s transformer refurbishing unit, Rotek Engineering, in collaboration with the Nuclear Energy Corporation of South Africa (NECSA).
In a gentle, passive, and entirely non-intrusive way that preserves all transformer monitoring data, moisture is removed as a permanent function of a transformer’s oil circulation system. Dehydration is continuous, as opposed to prolonged buildup followed by periodic dryouts. Once a wet transformer has been dried, further deterioration of the insulation is stopped. The remaining potential service life is extended, and the dielectric strength of the transformer is improved and stabilized.
Factory installed, the process maintains transformers in a dry state throughout their service life. Introduced in the field on wet transformers, moisture is slowly reduced and then maintained at a predetermined harmless level. Eskom has found that wet transformers can be dried out at one-fifth the cost of the vapor phase factory technique, and one-third the cost of conventional heat/vacuum dehydration in the field. And after that, individual transformers can be kept dry for approximately $600 per year.
Eskom now has introduced the system — DryKeep® — in networks throughout Africa. Utilities in Britain, Canada, Australia, and other countries have adopted it, and, in the United States, the first installations were brought into service last fall. All in all, installations have been in operation for several years on transformers ranging from 10 MVA to 900 MVA. International transformer manufacturer VA Tech T&D offers DryKeep as an option, and other manufactures are evaluating the process. New power transformers for London are factory equipped with the system.
About the author:
Carbery has 35 years of domestic and international experience in the engineering and marketing of products for electric utility networks, most recently as a vice president of The Ardry Group, an international trading company representing leading suppliers of transmission and distribution equipment to electric utilities around the world. A former lieutenant of the US Navy Reserve, Carbery was affiliated with Westinghouse Electric Corporation for 10 years, working on submarine and surface nuclear programs of the US Navy. He holds a B.Mar.E degree from the State University of New York Maritime College, and studied at New York University’s graduate school of business administration.
Carbery is now the president of Ardry’s DryKeep division, which provides sales and technical service to North American electric utilities in the area of transformer moisture removal. He can be contacted directly at 845-429-5128 or at firstname.lastname@example.org. More information on Ardry can be found at www.ardry.com, and on DryKeep at www.drykeep.com.