At some older Florida Power & Light Co. (FPL), Juno Beach, Florida, U.S., substations, removing a transformer from service temporarily requires providing a visible gap by operating the high-voltage bus-tie disconnect switch, also known as “the sliding link.” This device consists of a three-phase copper bus pipe capable of sliding into another copper bus pipe of a slightly larger diameter, tying two operating buses together. FPL uses sliding links, which typically are located about 30 ft (9 m) above the ground, in its 69-, 115- and 138-kV stations.
Workers can operate the sliding links from the ground, but the operation is difficult and dangerous (Fig. 1). Workers must avoid energized equipment while maneuvering clumsy switching devices through existing bus work to loosen the clamps that hold the sliding link. Then they must pull the bus pipe apart, one phase at a time, using extension sticks with the switchman on the ground or, worse yet, elevated in a man-lift. The movement of these links is not a smooth motion because of the age of these devices, their infrequent usage and the physical condition of the bus pipe. It normally takes three people three hours to fully open or close three switches.
Since the early 1960s, FPL had been installing these sliding links on structures in many locations. Because they save space and provide a very “compact” substation arrangement, the sliding links installed fairly easy. However, with today's high load demands, the switches are an operational nightmare. These switches possess inherent operational difficulty but are sometimes the limiting factor on the connecting transmission line because of a current carrying capacity limited to 1200 A. FPL no longer includes the switches in its new substation designs, although hundreds are still in service.
The First “Fix”
Over the years, the field personnel who operated these switches developed their own strategies and practices. Their main concerns were safety, reliability and accuracy in the operation. Some of their “fixes” to improve the operation of the sliding links centered not on the switch itself but on the additional use of ropes, hooks, guiding rollers and other attachments. Despite these improvements, the switching remained a difficult, clumsy operation, fraught with safety concerns.
In search of a better switching solution, FPL formed a team of operational engineers, design engineers and substation electricians. The team of substation personnel included members from each area of FPL's service territory to share different methods and experiences. Their challenge was to design a higher-rated switch that one person could operate from the ground in a short period of time along with reassurances of safety, reliability and considerably less effort.
The FPL team sent out requests to different switch manufacturers and partnered with Pacific Air Switch Corp. (PASCOR) of Portland, Oregon, U.S., to take on this challenge.
The main design consideration of the switch was the limited space available for any installation. Because of this restriction, it could not open sideways or vertically and it had to fit on the existing bus supports. The installation had to meet all safety, operational and reliability requirements plus be cost-effective.
A Switch is Born
After several months of design and development by PASCOR and FPL, a new switch was born: The Sliding Link Switch (Fig. 2). The new switch met all the design criteria and opened in the same direction as the bus, therefore meeting all the clearance requirements. It also would fit in the limited space previously occupied by the sliding link. The phases are gang-operated with a hand crank with a pipe linkage that extends from the ground. It provides a safer environment to operate the switch and avoids the need of the switchman to work over existing equipment at the substation. The design meets and exceeds all electrical requirements including a current carrying capacity of 2000 A.
The FPL team members inspected and tested the switch at the factory facilities (Fig. 3). These same team members provided input for changes, modifications and improvements to the first prototype of the switch. PASCOR incorporated all the suggestions and re-tested the switch. The next step was the actual installation at one of FPL's substations.
The Real Test
After careful consideration, the team chose the Miramar Substation, located in Miami, Florida, as the candidate station for the first Sliding Link Switch installation (Figs. 4 and 5). This installation yielded the need for more improvements to the switch design, which were incorporated during this installation. The operation of the prototype switch in field conditions was even better than expected. In less than one minute, one man can operate the switch from the ground.
Sliding Link Replacement
The installation of the first Sliding Link Switch at Miramar Substation started the Sliding Link Switch Replacement plan. FPL replaced eight switches in five stations in the year 2000. The plan calls for FPL to replace 17 switches in eight stations in 2001 and 14 switches in eight stations in 2002.
In the end, the partnership of FPL and PASCOR proved the correct solution to a lingering safety and reliability problem.
Michael J. Brown received the BSEET degree in computer design from Florida International University in 1978. Brown has been employed at Florida Power & Light Co. for 24 years. The last three years he has been working as an Operations Engineer in the Operations Group — Breaker Team. His responsibilities include the approval, operations, maintenance and repair of switches and breakers.
Omar Carrillo received the BSEE degree from Florida International University in 1985. Carrillo has been employed at Florida Power & Light Co. for 15 years. As a project engineer, his responsibilities include the engineering, budgeting and construction monitoring of new substations and power plant switchyards and existing facilities additions/upgrades.