Joint Effort Yields Major Design Change

Electric utilities repeatedly cite corrosion as a prime contributor to the reduced life cycle of padmounted transformers. Although advances in paint materials and coating systems help to extend the life of the units, corrosion often begins when the finish is damaged during shipping, installation, or as the result of vandalism. Corrosion is accelerated by environmental factors such as fertilizers, road salts, irrigation systems, lawn equipment and harsh coastal conditions. Experience at Florida Power & Light Co. (FPL), Juno Beach, Florida, U.S., reveals that many transformers fail prematurely, within five to seven years, because of perforations caused by corrosion of the metal enclosures. Replacement of failed units costs FPL more than US$2 million per year.

Pinpointing the Problem FPL analyzed single-phase transformers that were removed from service during 1988, 1989 and 1990 because of corrosion. The units were single-phase, padmounted and pole-mounted transformer units. The padmounted transformers accounted for almost half the total, which was a significant finding since three times as many overhead transformers are in service on FPL's distribution system as padmounted transformers. In other words, the finding implied that corrosion failure was three times more likely on single-phase padmounted transformers than on overhead units.

A separate evaluation focused on the location of corrosion on single-phase, padmounted units. This study found that on the units that were damaged by some form of corrosion, 50% of the corrosion was concentrated on the hood and sill and another 36% was near the bottom of the tank where there was contact with the concrete pad. Only 14% of the units showed corrosion on the remaining areas of the transformer (Fig. 1).

The FPL project team also discovered that at locations where metal-to-metal contact was likely (such as the hinges and the hood-to-sill interface) the transformer finish was susceptible to damage during transit and operation of the cabinet. Abrasion and paint damage also occurred during installation at areas where the transformer was in contact with the concrete pad. The transformer cable compartment was also subject to condensation, which could lead to accelerated corrosion.

Construction Materials Seeking a solution to this costly problem, the FPL project team reviewed materials available for transformer construction. The research revealed four options.

- Option A-Carbon Steel In regions with minimal corrosion problems, excellent service life can be achieved through use of carbon steel. This is typically the least-cost option available to a utility.

- Option B-Hybrid Design, Ferratic 400 Series Stainless Steel Combined with Carbon Steel This option provides a transformer that has a 400 series enclosure (hood and sill) and a tank made primarily of carbon steel. The corrosion resistance has been improved by welding a 400 series stainless steel strip on the bottom of the tank wall, thus providing 400 series stainless steel for all parts in contact with the concrete pad. This package can meet many corrosion concerns at a much lower cost than a complete stainless steel transformer.

- Option C-Ferratic 400 Series Stainless Steel Extensive testing and observation of units in actual field applications has shown that ferratic 400 series stainless steel provides excellent corrosion resistance at a moderate cost.

- Option D-Austenitic 304 Stainless Steel This option provides the ultimate corrosion protection and will perform for the life of the transformer in the most severe environments. However, because of the high cost of this steel, this option can only be justified in severely corrosive environments.

Joining Forces Seeking alternative solutions to its problem, FPL teamed with the Distribution Transformer Division of ABB Power T&D Co. to address transformer corrosion and to develop a cost-saving, long-term solution. ABB engineers, together with hands-on input from FPL engineers, linemen and operations specialists, embarked on a mission to design a padmounted transformer that would approach the corrosion resistance of stainless steel at a cost more comparable to mild steel. The new design would have to withstand the impact of harsh environmental conditions and abuse found in many areas of the world. The project focused on developing a padmounted transformer enclosure that was:

- Cost effective - Constructed of corrosion-free materials - Highly resistant to scratches and dents - Redesigned to protect susceptible areas from corrosion and damage - Easier to install - Easier to maintain - More aesthetically pleasing in appearance

Early in the design process the team determined that 300 series stainless steel was a good alternative for corrosion resistance, but it was not considered because of its high initial cost. After careful consideration and extensive testing, the project team chose to focus on a new enclosure design that used composite (non-metallic) materials. The various options for construction material, along with each option's relative cost versus performance rating, are shown in Fig. 2.

New Enclosure Design The new enclosure is constructed of a specially formulated composite material consisting of non-conductive, flame-resistant thermoset resin that is reinforced with glass fibers. Additional properties are achieved with additives such as color pigments, flame retardents, weathering agents and ultraviolet inhibitors.

The new design completely eliminates the metal hood and sill components, which are especially vulnerable to corrosion. The new enclosure is a one-piece design that is latched to the unit at a single point and has strategically located stiffening ribs that help to provide the strength, stiffness and flexibility required to meet various design and functional criteria.

A recessed lock pocket and handle are conveniently located at the front top center of the enclosure along with a stainless latch plate and a captive lock bolt. The lock is placed high on the unit to prevent premature corrosion from dirt infiltration, as well as to ease worker back strain. In addition, the handle design is large enough to enable a gloved hand to easily reach under the handle as it is opened.

At the tank and enclosure interface, a tongue and groove arrangement ensures tamper resistance when the unit is closed and locked. The connecting link of the latching mechanism is a molded composite arm with a molded-in locktab/latch nut. The arm is connected to a slotted bracket, which is welded to the tank. The curve in the arm minimizes cable interference and permits the tank attachment to be lower than the lock bolt. This generates both a compression and moment loading, which combine to create adequate holding force at the bottom corners.

FPL's desire for safety and aesthetics resulted in a design with rounded front corners and edges and a latch that is recessed (Fig. 3). To minimize arm interference during installation and maintenance, a slotted bracket on the tank permits the arm to rotate up into a vertical position (Fig. 4).

FPL estimates that the new enclosure will cut installation and maintenance time by 15%-25%.

Testing Criteria The joint project team reviewed input from numerous utility focus groups to ensure that the installation and removal process of the unit met all utility requirements. The enclosure was also required to pass integrity tests (pry bar, push, pull, wire probe) per the American National Standards Institute (ANSI) - ANSI C57.12.28, 1988. The design also met relevant requirements of ANSI C57.12.25, 1990.

Tests were performed to ensure the enclosure would withstand the abuse of string weed trimmers and repeated hammer blows. In addition, a specially designed elbow failure test, with an approximately 4000 A fault current, was performed to ensure the new enclosure's strength and integrity under fault conditions.

Operational Benefits The standard steel-hinged enclosure and the new composite enclosure were evaluated for their ergonomic properties. The composite enclosure weighs 25 lbs (11.3 kg) compared to 50 lbs (22.6 kg) for a typical steel enclosure. Each design was evaluated at the critical operational points: unlocking the unit, opening the unit, and setting the hood on the tank top. The testing was conducted according to the U.S. National Institute for Occupational Safety and Health (NIOSH) standards. Overall, the composite enclosure showed a reduction in resultant stress of 10-14% for all body types and lifting positions compared to traditional steel hood designed units. Because of its light weight, the unit reduces back strain and improves handling.

Because of the built-in lock enclosure, the padlock can't be installed until the lock bolt is sufficiently tightened to secure the enclosure. To enhance removal, the lock bolt is designed to push the enclosure away from the tank when it is loosened. The non-conductive enclosure provides excellent insulation protection from exposed energized cables and bushings inside the cable compartment. It enables workers to pull back the unit and use it as a shield during work procedures.

Most importantly, because the new enclosure design contains no sill, workers can easily access electrical connections, ground wires and ducts. The absence of a sill makes it easier for workers to pull cables, cut off electrical conduit and connect cables and wires. For these reasons, work time is significantly reduced.

Conclusion Corrosion on padmounted transformers is a costly issue for many utilities. Until recently, stainless steels, which require a higher initial cost investment, have been the only guaranteed option to resolve serious cabinet corrosion.

In a unique joint arrangement, FPL and ABB Power T&D have addressed this costly problem and have developed a one-piece compression molded composite hood to replace the traditional metal hood and sill. The composite enclosure improves operation and reliability of future padmounted transformers, as well as improves worker safety and work efficiency.

Miguel A. Valbuena is a principal engineer in the Distribution Group at Florida Power & Light Co. (FPL). He is in responsible for design specifications and field application standards for distribution transformers. Valbuena has the BSEE degree from Florida Atlantic University and joined FPL in 1982 as a design engineer. He has held various positions in marketing, distribution operations and engineering.

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