Bus Connections: Why localized silver plated deposits make sense

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Bus Connections: Why localized silver plated deposits make sense

In the electric industry, optimizing power flow is a primary concern for the generation, transmission and distribution processes. The key is providing and maintaining low resistance conductive joints through silver plated deposits.

Field experience and laboratory studies have shown that this is especially true in the case of bus bars and bolted high current connections. Specifically, silver plated bus bars outperform unplated bus bars by providing stable contact resistance and a low maximum operating temperature that increase the service life of the bus joint. More importantly, stable contact resistance joints will reduce the need for frequent maintenance, decrease overall downtime of equipment, and greatly reduce the risk of catastrophic failures.

Good industry practices recommend that all bus contacts be silver plated. Most government, IEEE and insurance provider specifications require that all bolted bus connections be plated in accordance with applicable specifications.

One of the key elements to effective bus bar contact plating is applying a uniform deposit of sufficient thickness to provide corrosion protection and a “leveling effect” to increase the surface area of the bus joint.

Brush plating, using pure silver, provides a simple, cost effective solution for in-place plating of bus systems during routine maintenance and can also be of value in upgrading bus, rather than replacing existing bus, when generator or system capacity increases are desired.

The Early Years

In older power plants, the aluminum or copper bus bars were installed. While the performance of an uncoated bus joint may have been sufficient years ago, today’s increasing demands for power, given the limited capacity and economies of the marketplace, are forcing the producers to improve the efficiency and performance of the entire system.

Many early bus connections have not been unbolted since first being installed as far back as 1910. Bus connections have several factors that limit their lifetime performance, including:

  • Irregularities in the mating surfaces
  • Particulate contamination prior to installation
  • Oxidation

These factors, when combined, have an escalating effect of increasing contact resistance and temperature and thereby decreasing the efficiency of the joint over time, potentially causing catastrophic failures. Even when new, imperfections in the bus bar’s copper surface result in only a fraction of that surface coming into direct contact with its connection. By some estimates, that fraction can be as low as ten percent. Although increasing the contact force may flatten out the high areas, the effects are minimal and may even place undesired stress on the fastening system.

The formation of non-conductive surface films due to harmful ambient atmospheric contaminants is also limiting factor of joint reliability – even in bolted contacts. Fritting will reduce the contact resistance of thin surface films, but thicker, more tenacious films may still present a problem that will be magnified by increasing temperature at the joint due to increased resistance. Fritting is the occurrence of the dielectric breakdown of a contact film. A potential gradient of 100V/μm could be sufficient to cause this dielectric breakdown.

The oxidation of the bus material is an ever-present occurrence unless steps are taken to prevent it with a barrier coating. The formation of oxidation layers on the bus material within joints leads to increased resistance, and thereby increased voltage drops and increased local temperatures. It has been reported that the joint resistance across uncoated bus can increase more than 20% due to oxidation formation. Studies have shown that silver plating bus joints significantly reduces bus material oxidation in service.

The combined effects of irregular surfaces, contamination and the formation of non-conductive surface films, and oxidation can create “hot spots” that will further deteriorate the reliability and performance of the joint.

A Better Joint

Testing and field experience has shown that one simple step can be taken to minimize the effects of irregular mating surfaces and the formation of oxides and other surface films on joint performance. That step is plating the joint area with a soft, conductive and corrosion resistant material. The application of a 0.0002” to 0.0005” thick deposit such as silver, nickel or tin can improve the lifetime performance of the joint by as much as 30% and reduce maintenance substantially.

Coating with a soft material such as silver or tin effectively forms a compressive gasket on the surfaces to be connected. The force applied when bolting the surfaces together squeezes the conductive material into the low areas, effectively increasing the contact area and decreasing the overall joint resistance.

Tests have demonstrated that these materials greatly slow down the formation of copper oxide and other surface films, maximizing conductivity and minimizing heat. A silver plated joint allows operation at a higher temperature without joint degradation over the life of the joint. The end result, over time, is significantly increased performance, efficiency, economy and reduced maintenance.

Because of silver’s softness it can be formed more precisely to the contours and crevices of the original piece increasing the actual contact areas. Silver provides a good electrical connection preventing the formation of copper oxide at the mating faces. A nickel coating has shown some cost advantages, but silver displays greater operating performance and efficiencies, exhibiting less resistance and maintaining lower temperatures while in service.

Tank plating the connection using a cyanide silver bath solves the problem of providing a sufficient thickness, but it is costly and time consuming. In the case of bus bars, they must be completely removed from the system and sent offsite for plating. Therefore, the increased outage time required for this method can be unattractive.

Electroplating, as performed using the SIFCO Process®, can be completed on-site with minimal disassembly, during any planned outage without removing the equipment from its location. To ensure safety for the operators and those in the surrounding areas, SIFCO ASC uses non-cyanide silver in its plating process. This portable plating process accurately applies the non-cyanide silver at a rate of 0.020” per hour, producing a smooth even finish. Two mating faces of a 4” x 4” copper bus joint can very easily be masked and plated with 0.0003” of silver in less than 15 minutes.

The desired thickness for each particular application and/or part is calculated prior to plating. SIFCO ASC uses digital ampere-hour meters to accurately control plating thickness to ensure smooth finishes and uniform plating.

A smooth finish and a uniform deposit of sufficient thickness will significantly enhance the reliability and the performance of the electrical joint.

Upgrading the Bus Duct System at TVA’s Fontana Dam

As part of the TVA’s comprehensive hydro modernization program, electrical bus duct systems are sometimes replaced with new systems sufficiently rated for the new higher unit output capacities. This occurs when a generating unit’s output rating is increased beyond the capabilities of its corresponding bus duct system or when the plant’s physical arrangement must change to accommodate other new equipment. TVA has, however, successfully demonstrated that many older bus systems can be re-rated to higher ampacities by using in-place brush plating to increase the current carrying capabilities of the bus. Success is highly dependent upon the modified bolted joints because they must be stable, reliable, and operate at a reduced contact resistance for a wider operating range. All of this can be accomplished at much less cost than bus replacement.

TVA has realized substantial savings by choosing to plate existing bare connections rather than upgrading to a new bus system. Bolted connections were silver plated in-situ during scheduled outages.

Steps in the operation:

  • Mechanically clean the contact surfaces to remove heavy oxides.
  • Solvent clean the contact and adjacent surface to remove any traces of oil or other residue.
  • Mask to define the area to be plated.
  • Electrochemical preparation
  • Plate the part
  • Remove the masking

Conclusion

The performance of contact joints is dependent on maintaining low resistance. The conductivity of the joint will naturally deteriorate over time due to the rigors of service as well as natural forces such as oxidation and moisture.

Testing has shown that plating bus joints with 0.0003” – 0.0005” thickness of silver greatly improves their lifetime performance. On-site selective plating with non-cyanide silver may be the most economical approach because it eliminates the need for component disassembly and transportation to an off-site plating facility, while quickly providing a superior quality, uniformly thick deposit that will stand the test of time.

For more information for your silver plated bus connections, contact us at 800-765-4131 or at info@sifcoasc.com.