1.6 MW, North Carolina
Running Gear: Francis turbines and direct connect synchronous generators. Unit 1 is 1000 kW, Unit 2 is 600 kW.
Scope: Replacement of control system, switchgear, and Woodward governors
This project is located in central NC, and was originally built in the 1930s by the local electric utility. By the 1980s, it had been abandoned for some time, and was then redeveloped by a private business owner.
The current owners purchased the project in late 2013. The running gear (generators and turbines) were still in good condition, but the rest of the plant needed rehabilitation in order to operate at its full potential. The controls were all manually operated, the generator breakers were obsolete, and the Woodward governors were troublesome and prone to leakage. The new owners hired PHT to provide a new control system to operate the plant, including new digital controls, vacuum breakers, and replacement of the mechanical governors. Some key functions that will improve production include automatic synchronization and automatic headpond level control.
The existing Payne exciters were reused, but were modified to allow control by the PLC instead of a hand-controlled potentiometer. Because excitation control was previously manually controlled, the generators were difficult to operate at low loads, since the power factor would drift away from its setting when the operator was not present. Now with automatic excitation control, the generators can be operated over a wide range, capturing more energy.
The Woodward governor conversions PHT provided for this plant are very similar to previous conversions done by PHT at other plants. First, the low pressure hydraulic cylinder and flyball valves are removed, leaving the shuttle, connecting links and crank arm. In place of the old cylinder, a bracket holds the new high pressure hydraulic cylinder. A shuttle adapter bolted into the shuttle connects the new cylinder, which is attached to the mounting bracket and shuttle adapter. A position sensor mounted to the base measures the gate opening. Finally, the hydraulic cylinder is connected to the new hydraulic power unit. The HPUs are custom built for PHT, and include an AC motor-powered pump, accumulator, and valve manifold. The accumulator stores enough energy to shut the wicket gates and shut off the plant in case of a power loss. While the HPUs have available manual controls, they are mostly operated by the PLC to move the wicket gates as required for startup, normal operation, and shutdown.
This is the second project completed by PHT for this customer, an investor backed company that owns several hydroelectric sites in the eastern US.
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