The life span of the Hartwell Powerplant’s four original generators was expected to be approximately 30 years. So, after 37 years of service, the four original generators underwent a ”rehabilitation” (rehab), or overhaul. Each of the four generators had over 60,000 hours of operating time; this is equivalent to an automobile having over 2,700,000 miles on it (if an automobile averaged 45 mph).Phase I of the rehab work got underway in 1997. There were three major components of the rehab work - generator rewinding/turbine refurbishment, replacing and upgrading circuit breakers, and replacing and upgrading the transformers (this included installation of an oil/water separator). Phase II included more equipment such as replacing all of the switchyard breakers and buswork and updating the powerhouse and Clemson Pumping Station.
The Gantry crane seen here is a permanent part of the dam and is used to move generator parts from the outside deck into a workspace within the powerplant. Before the rehab began, a stress test was performed on the crane because it would be needed to lift and move the rotors that weigh approximately 300 tons (the rotor is the heaviest part of each generator). To do this, water was placed in large balloons to a weight of 375 tons.
Generator Rewind & Turbine Refurbishment
The generator rewind involved stator coil replacements. Coils are the wound wire components in a generator that produce electricity when a magnetic field is passed across them (this is what the rotor spins inside of). Due to technological advancements, the new coils can be rated at a higher power output and the generators were upgraded to 129% of their original ratings.Turbine refurbishment consisted of repairing turbine blades. Over the years, water to blade contact results in very small nicks and general wear to the turbine blades (referred to as “cavitation”). Old paint was sandblasted and blades were welded, sanded, and repainted.In addition to the refurbishment, “hub-baffles” were added to the turbines. Hub baffles are devices designed to pull air into the water flowing through the turbines – this increases dissolved oxygen downstream water and improves water quality conditions for fish.
Replacing and Upgrading Circuit Breakers
The circuit breaker upgrade involved replacing existing air circuit breakers with gas breakers. Gas breakers are more efficient and have a longer life span. Air breakers required bi-annual maintenance, while the new gas breakers only require maintenance every 10 years. Repair work is easier and less expensive. Additionally, the new circuit breakers have a higher current rating to handle the higher output of the generators.
Replacing and Upgrading the Transformers
Transformers have been replaced to handle the additional increase in power being generated. Instead of two banks of three, single-phase transformers, there are now two three-phase transformers (one transformer for units 1 & 2 and one transformer for units 3 & 4; unit 5 has its own transformer). Each new transformer, weighing in at over 242,000 lb. (or over 121 tons) and 32.5 ft. long by 21’ wide, was transported (on separate occasions) by train from St. Louis to the Airline community of Hart County, where it was then placed on a specially designed flatbed trailer and brought to the powerplant. Each transformer requires between 6,000 to 10,000 gallons of insulating oil. As part of the transformer work, an oil/water separator was installed. An oil/ water separator is a secondary containment feature that will prevent oil from entering into the river should a spill occur in relation to the transformer. It is basically an underground holding area, should a spill occur, oil would enter into the holding area where it could then be pumped and properly disposed of.
This diagram gives you a good idea of the location of the major rehab components in relation to one another. The rehab has resulted in a 29.5% capacity increase each for units 1 - 4, or a 22.7% increase in plant capacity. (Capacity is the amount of electricity a generator plant can produce). In other words, due to the increased nameplate rating, we can produce 22.7% more power than before the rehab. The rehab has increased the nameplate rating of 66,000 kW to 85,500 kW. This is equivalent to adding a 6th generator. The rehab is also expected to extend the life of the generators by 35 years. The Corps continues to provide clean energy as the nation’s power demands continue to grow.