Retro?t breakers resolve trip failures
By George L. Hanna, Wyle Laboratories
Power plants can use advance planning and retrofit equipment to upgrade existing systems while avoiding unnecessary downtime
Seven years into the estimated 40-year life of its Shearon Harris Nuclear Power Plant, Carolina Power & Light (CP&L) began retrofitting many of the plant`s original 480-V switchgear circuit breakers. Retrofit replacements for all 19 of the Train A safety-related breakers and 14 non-safety-related breakers for the New Hill, N.C. plant were successfully installed during the Spring 1994 refueling outage and October 1994 maintenance window. Retrofits for the Train B safety-related breakers and 17 more non-safety breakers will be installed this year.
Only 28 breakers service frequently-cycled plant loads. Most are cycled less than 50 times each year. Even though the equipment wasn?t near the end of its design life, CP&L encountered Ofailure to open upon demandO problems during routine maintenance inspections. In addition, this equipment is no longer manufactured and replacement parts are scarce.
A breaker that fails to open challenges the safety features of a nuclear power plant. To ensure reliable operations, the utility selected a comprehensive retrofit program to replace a third of the 200 breakers originally installed at the plant. Under contract, the retrofits?Siemens RLN breakers?were built and qualified for nuclear service by Wyle Laboratories through a special teaming agreement with Siemens Energy & Automation.
The project offers an excellent example of how a utility can use advance planning and retrofit equipment to upgrade its existing systems while avoiding unnecessary downtime.
NRC and an outage schedule
The CP&L staff jointly developed a retrofit breaker program with the supplier and contractor. The team produced a compatible breaker design, delivered equipment ahead of schedule, provided up-front training for CP&L personnel, and ensured efficient, trouble-free installation during the planned maintenance windows. Initial retrofits were completed successfully within the 250 hours available during 3- and 7-day OwindowsO in CP&L?s Spring 1994 refueling outage.
During the Nuclear Regulatory Commission?s (NRC) regular systematic assessment of licensee performance at Shearon Harris in May, the NRC complimented CP&L for the strong engineering support in improving the reliability of safety related equipment by replacing the 480-V circuit breakers with a more reliable design. The Shearon Harris plant received NRC?s highest ratings of superior in all four functional areas.
How CP&L chose retrofits
CP&L?s first corrective action was to report the trip failures to the original equipment manufacturer. Together they modified the LK breakers in an attempt to correct the problem. However, the breakers? failure to open-upon-demand problem persisted. As a result, CP&L considered two options: to replace the entire switchgear buses or retrofit individual breakers.
Replacing the switchgear would simplify the equipment qualification process, but it also would create problems in matching the existing footprint and headroom. Replacing individual breakers would require close cooperation between CP&L and its vendors, plus performing seismic qualification tests.
In September, 1992, CP&L asked several vendors to recommend a solution to the LK breaker situation. In evaluating the economics and plant down time to install the modification, it became obvious that the most expedient and cost-effective solution was to replace individual breakers.
The RLN breaker was the only one that fit into the existing LK breaker enclosure so it was accepted and a favorable retrofit package was developed that included a 10 CFR 50 Appendix B quality assurance program.
As part of its risk assessment process, CP&L also conducted an exhaustive review of the Siemens RLN breaker?s reliability and performance in non-nuclear installations. CP&L then retained the Wyle/Siemens team to retrofit the largest CP&L nuclear plants with the RLN breaker design. With delivery of 68 custom-designed breakers required in fewer than eight months, CP&L established a bonus/penalty contract with the team for early/late delivery.
Building a project team
From the outset, a breaker retrofit team that included CP&L?s project management, crafts, system engineers, maintenance, technical support, design engineering, materials, procurement engineering and purchasing personnel, along with the supplier team, were assigned to the project and met frequently prior to starting the work. Team membership remained constant and direct communications were encouraged at all levels throughout the program.
OThis was a true partnership from the beginning,O said Tony Cockerill, CP&L?s manager for electrical engineering at the Shearon Harris Plant. OWyle, Siemens and CP&L made a total commitment from the top down to getting the job done on time.O
Working out the kinks
The program experienced a few upsets, but up-front efforts paved the way for timely delivery and smooth installation. For example, the initial LK switchgear inspections identified inconsistencies in the breaker cubicle dimensions and electrical wiring which required variations in breaker designs. The width of the 1,600-amp breaker and cradle had to be reduced to fit the existing cells. Some means also was needed to ensure that breaker stabs would connect with the existing vertical feeder bus.
In addition, the installation plan called for 28 retrofit breakers to be installed within the 250 hours during the outage when the breakers? buses would be de-energized. To minimize any surprises during installation, the retrofit team proposed taking one of the plant?s 480-V buses off-line to permit detailed measurements and trial installations.
The plant operations staff agreed to take a bus out of service before the planned plant outage to allow one breaker to be installed on a trial basis. This was so successful in providing additional information about the retrofit project that two more short bus outages were approved prior to the refueling outage. Installation procedures were refined and a slight bus rework reduced the torque required to rack the breakers into position.
Under NRC rules, the plant may operate with one bus down under a limited condition of operation (LCO) for only 72 hours and nuclear plant operations personnel generally avoid LCOs under any circumstances. In this case, retrofit breaker designs were checked out during short bus outages prior to taking the plant down for refueling. These dry runs were a key element in achieving a trouble-free installation while protecting the schedule.
Meanwhile, engineers and technicians were qualifying the retrofit designs to applicable nuclear standards. Initial seismic testing revealed that the commercial grade breaker could change state and come out of its cradle during an earthquake. Engineers then developed a cleating system that keeps the system operational and together under extreme seismic conditions.
Supplier training of the CP&L maintenance staff started four months prior to delivery. Siemens supplied CP&L maintenance with a standard RL breaker immediately after the contract award and prior to final design, and CP&L procured three sections of used LK switchgear to be equipped with breakers for use in maintenance training. That allowed formal training for CP&L personnel to begin well in advance of the scheduled installation.
In addition to time savings achieved by quickly completing the retrofit design, this approach kept engineering costs to a minimum and ensured thorough, complete training before the installation process began.
Advance work pays off
The plan was to work out all kinks before installing the first 28 breakers within the two short windows in March 1994. The plan worked. The early program team meetings, training and trial installations paid off. CP&L maintenance performed the installations without assistance.
The Phase-I breaker replacement project will be followed by retrofits for the Train B safety-related breakers and 22 non-safety breakers during a scheduled refueling outage in the fall of 1995. END
George L. Hanna is the manager of application engineering for electricial equipment retrofits at Wyle Laboratories, Huntsville, Ala. He holds both M.S. and B.S. engineering degrees from Case Institute of Technology, and has more than 30 years of engineering and testing facility management experience.
Why retrofit circuit breakers?
Cash-strapped utilities increasingly stretch capital equipment budgets by retrofitting vacuum circuit breakers in aging metal-clad substation switchgear to give it a new lease on life.
The reason for replacing circuit breakers rather than the switchgear is primarily economics. Fewer changes can save more money. Replacing ancient circuit breakers with new ones makes sense because spare parts are difficult, if not impossible, to find for many units that are no longer being made. Some utilities and industrial plants have tried to keep out-of-date circuit breakers functioning by using spare parts cannibalized from units that were replaced. Others have had spare parts custom made in machine shops, but have found the expense exorbitant.
The cost of unplanned outages resulting from trip-freeze and other mechanical problems brought on by old age also can be prohibitive. For example, we know of a user who had been budgeting $250,000 a year for maintenance to keep eight circuit breakers running. The company had to order some parts 56 weeks in advance to be sure to get them.
In most cases, installing new circuit breakers costs one-half or less than of the price of a new switchgear section. The time cycle from placing the order to putting the refurbished switchgear back on line ranges from eight to 12 weeks.
Retrofitting provides several advantages. If old switchgear remains in place, downtime is reduced. It takes far less time to replace a circuit breaker than to tear out and replace a switchgear lineup.
At a major New Jersey utility, the trucks for 34.5-kV draw-out switchgear from the early 1960s were shipped to Raleigh, N.C., where they were rebuilt with new vacuum breakers. A total of 36 units were done at a cost of $35,000 each, including installation. Replacement switchgear was estimated to cost $70,000 each, not counting installation.
Although most switchgear retrofits are done at the factory, exceptions occur depending on original equipment manufacturer and retrofitter. Siemens turn-keyed field replacement of 18 34.5-kV air blast circuit breakers with vacuum breakers at a large mid-Atlantic utility substation for an installed cost of less than $50,000 each. The reasons behind the retrofit were simple and compelling: There was nothing available with the same footprint, plus we estimated that replacing the 30-year-old switchgear would have cost at least twice as much.
However, some retrofits can be accomplished in days rather than weeks using surplus market circuit breakers. That was the case at a Massachusetts hospital where we retrofitted air blast breakers in the switchgear of a more than 30-year-old outdoor substation owned by the hospital. The substation was fed from the utility at 26 kV and distributed electricity at 4.16 kV.
The decaying switchgear, which had received almost no maintenance for more than 20 years, was revitalized with Siemens 3AF vacuum interrupter circuit breakers, which are designed for 30,000 mechanical operations and up to 100 full current interruptions. The retrofit costs about half that of installing a completely new substation.
An integrated pulp and paper mill in Alabama achieved similar savings during the retrofit of three lineups of obsolete 15-kV, 750-MVA air magnetic switchgear with 39 new vacuum breakers. The retrofit was necessary to enable the paper mill, which was installing 105 MW of new generating capacity, to upgrade the available fault current level to 1,000 MVA.
Because the plant`s Bus No. 3 could not normally be shut down without incurring enormous losses in production revenue, the first step was to locate similar cubicles and breakers on the surplus market and upgrade them. They were then used to replace the obsolete Bus No. 3 breakers during a planned 12-hour mill outage. The obsolete breakers were retrofitted and later used to replace obsolete units in Bus No. 1 and No. 2. Two additional feeders also were added to Bus No. 3 during its brief shutdown. Because space limitations precluded the use of an auxiliary section, cubicles were fabricated by a subcontractor that matched and lined up with the existing switchgear.
For planning purposes, managers should keep in mind that circuit breaker rebuilds at a factory will take about six weeks. Stripping the trucks, shipping them to the factory and then reinstalling them generally adds another two weeks to the project cycle. Field modifications generally can be accomplished in 8 to 12 weeks, from order to installation, if the retrofit is not design intensive.
Quick turnaround and significant time and expense savings have led industries and utilities nationwide to consider retrofitting as a viable option to their obsolete breakers and more expensive switchgear replacement.