By James Kozak, Cooper Power Systems
Federal Energy Regulatory Commission data indicates that there are more than 132,000 power transformers in service in U.S. substations. A number of utilities have reported to the insurance industry that as much as 40 percent of their substation transformer population is already at or beyond its original life expectancy. As a result, the expected rate of failures of these types of transformers is on the rise. The existing population of distribution transformers paints a similar picture. The percentage of single-phase pad-mounted transformers used in replacement applications is estimated to be 20 percent and rising annually according to several utilities.
And, during this time of year, the problem becomes greatly exacerbated. In the summer months, heat attacks transformers from both the inside and outside, with the most damage being caused by accelerated aging of the insulating paper which leads to dielectric failures. In the results of a study published by the International Association of Engineering Insurers, dielectric failure of the insulating paper was the leading cause of power transformer failure and represented more than 50 percent of the claims paid by insurers. The average age at failure of these transformers was approximately half of the original expected life of the units, according to an analysis of transformer failures reported on in an Association of Engineering Insurers conference in 2003.
Summer heat also leads to peak annual loading in most areas of the country, as people rely on air conditioning to escape the heat. Power and distribution transformers are often driven beyond their rating to keep pace with the increased demand. The higher loading generates losses which are largely dissipated in the form of heat. At the same time a transformer is producing these high losses, its ability to dissipate the heat is usually compromised. The warmer the surrounding air, the more slowly heat is removed or dissipated away from the transformer. The irony is that, under this scenario, a transformer’s ability to dissipate heat is often poorest while its loading is the greatest.
This explains the high transformer replacement rates which occur during the warmest months of the year. Summer sales (July through August) of single-phase transformers have outpaced winter sales (December through January) by 24 percent from 1998 to 2006 at Cooper. While a portion of this may be attributed to higher activity levels in new construction, much of the seasonal difference is driven by the increased demand for replacement transformers. Summer months can take years off a transformer’s life.
Utilities don’t generally want to resign themselves to the fact that rapid deterioration of their transformers’ dielectric system will take place over several months of the year, or all year if they are in warmer climates. Aging power and distribution transformers are a key class of assets which utilities must take an active role in managing-not only to assure reliability of power supply, but also to minimize risk and to develop responsible capital-investment plans.
There is a solution available to combat these harsh thermal effects, while being able to load transformers up to 14 percent more than nameplate rating without accelerated aging of the insulating paper.
Because accelerated aging of a transformer’s insulating paper is directly tied to premature aging of the transformer itself, use of a dielectric fluid is one way to extend the life of the insulating paper.
Extensive testing performed to IEEE standards has shown that insulating paper immersed in mineral oil ages at the same rate as the same paper used in dielectric fluid-filled transformers while running at a 69.8 F (21 C) higher temperature.
For example, the dielectric paper in a mineral oil-filled transformer running steady state at a top oil temperature of 176 F (80 C) would perform as long as the same dielectric fluid-filled transformer running at 213.8 F (101 C). This additional temperature performance affords the owner of the transformer the ability to overload the transformer by 14 percent, while producing the same expected insulating paper life as the more lightly loaded mineral oil-filled transformer.
Research shows that if mineral oil and dielectric fluid-filled transformers are run at the same oil temperature and compared, the insulating paper in the dielectric transformer lasts up to seven times longer than that in a mineral oil-filled transformer.
In addition to the economic benefits, dielectric fluid is more environmentally friendly. Rather than being a petroleum-derived product, it is manufactured from domestically grown soy. The dielectric fluid’s biodegradation rate and completeness meet the U.S. Environmental Protection Agency criteria for the “Ultimate Biodegradability” classification. Standard acute aquatic toxicity and oral LD50 tests have proven the non-toxic qualities of this fluid. (This is especially important in the case of a spill.)
Additionally, the fluid has virtually no fire risk and presents a very small risk to nearby equipment, facilities and personnel. In the event of an explosion due to a low-impedance internal fault in a transformer or shunt reactor, the risk of fire is much lower when vegetable oil is used in place of mineral oils.
Kozak is a field product engineer with Cooper Power Systems.