A failing generator circuit breaker compressor caused a turbine-generating unit to trip at Oglethorpe Power Corporation’s 846-MW Rocky Mountain pumped-storage project. Rather than replacing worn-out and costly parts, Oglethorpe personnel replaced the circuit breaker compressor with a general-purpose air compressor. This solution resulted in an inexpensive, reliable air supply for the circuit breakers.
Discovering the problem
Oglethorpe’s three-unit Rocky Mountain plant in Georgia began operating in the summer of 1995. Major plant components include 367-megavolt ampere (MVa), 20-kilovolt (kV) motor-generators from Hitachi that are driven by 353,000-horsepower (hp), 282-MW Francis-type pump-turbines from Hitachi.
The main generator circuit breakers are HEK 3 sulphur hexafluoride from ABB and are rated at 21 kV and 12,500 continuous amps, with a 100,000-amp symmetrical interrupting capacity. The breakers are air-operated in the open and close directions and are equipped with a Ω-hp, 160 pounds per square inch (psi), 1.34 cubic-foot-per-minute reciprocating air compressor. Nominal system pressure is 120 psi.
When the Rocky Mountain units are used as motors to pump water to the upper reservoir, it is impossible to greatly reduce load when shutting down from this mode. Therefore, shutting down requires that the main circuit breakers interrupt about 8,000 to 9,000 amps of load current. This daily occurrence causes a great deal of wear to the internal contact surfaces in the circuit breaker interrupters, requiring annual circuit breaker overhauls. During these overhauls, plant personnel test the circuit breaker air compressors to ensure they meet minimum air flow requirements specified by the original equipment manufacturer.
In the spring of 1998, operations personnel at the plant began receiving low-pressure alarms on one of the main circuit breakers. As a result, personnel declared the unit unavailable for service. Immediately, maintenance crews performed a thorough inspection. They noticed that the main circuit breaker compressor was not replenishing the air in the breaker within the specified time frame. Thus, the compressor was tripping out on excessive run. A closer inspection of the compressor revealed worn piston rings and cylinder gaskets, allowing excessive air leakage around the piston.
Investigating possible solutions
The most obvious remedy was to order replacement rings and gaskets. However, replacement parts cost more than $500 and are manufactured in Switzerland. Oglethorpe personnel deemed this cost to be excessively high. Another option, purchasing a complete spare compressor unit, would have cost more than $15,000. In addition, delivery time for either option would be more than a few days. Oglethorpe needed a quick solution to bring the unit back on line.
One possible fast solution investigated was to run an air hose from an air receiver tank of another unit’s main circuit breaker to the disabled unit, thereby also supplying air to this breaker’s tank. However, because plant personnel were confident the problem was not generic to this one compressor, it could be expected to occur on the other units soon afterward. This would be a temporary fix at best.
One permanent, although perhaps unorthodox, option was discussed. Maintenance personnel believed that purchasing a general-purpose compressor would cost less than replacement parts. Because of the need for a timely solution, maintenance personnel from the electrical, mechanical, and instrumentation and controls departments quickly organized to investigate this option.
Mechanical maintenance personnel identified a general-purpose compressor that would meet the system requirements. This compressor, manufactured by Campbell Hausfeld, could be purchased from the local Home Depot for about $300. The machine consisted of a reciprocating compressor mounted on a 26-gallon air receiver tank. The compressor would deliver 6.8 standard cubic feet per minute (cfm) at 90 psi, with a maximum working pressure of 125 psi. This would exceed the air flow capabilities of the original compressor. The compressor motor was dual-voltage (120/240 volt) and was rated at 2.25 kilowatts.
Installing a general-purpose air compressor
Electrical maintenance personnel modified the existing compressor control circuitry to incorporate the second compressor. A nearby power panel served as a power supply source. Personnel also incorporated a second motor starter with overload and a selector switch, to determine which compressor would be in service, into the compressor control circuitry.
Instrumentation and controls personnel installed the necessary piping from the compressor to an existing port on the main circuit breaker air receiver tank. The piping included an isolation valve, a check valve, an air pressure gage, and an air filter with an automatic blow down feature to eliminate moisture in the air. A manual push button was provided to allow operations personnel to routinely blow down the compressor air receiver tank during their rounds. All work involving installation of the compressor was completed within two weeks after the discovery of the original problem.
Oglethorpe personnel decided the new compressor would be used as “lead,” with the original compressor in “standby” mode. Immediately after installation of the new compressor, personnel noticed during routine breaker overhauls that the air was being replenished much more quickly than ever before.
Based on this result, maintenance personnel quickly modified the other two unit circuit breakers with identical compressors. This work was completed by the end of 1998. Much to the delight of operations and maintenance personnel, the new compressors have been performing their intended function with no problems since their installation.
–By Julio A. Trujillo, P.E., maintenance manager, Oglethorpe Power Corporation, Rocky Mountain Hydroelectric Plant, 4050 Big Texas Valley Road, NW., Rome, GA 30165; (1) 706 290-5428; E-mail: email@example.com.