Paul Cyr, James Price and Andrew Feimster
November 14, 2012 | 1 Comments
In the normal operating position, the Kaplan runner is 5.5 feet above tailwater. The module's two spillgates are located immediately above the elbow-shaped draft tube and are operated by hydraulic cylinders located above the normal pool level just below the generator floor. The spillgates open automatically after a generator load rejection to maintain flow discharges. The gates are also used to increase each module's discharge capacity to a total flow of 1,550 cfs. Each module contains a hydraulic power unit in the generator enclosure to operate the wicket gates, runner blades, spillgates and other auxiliary systems.
A programmable logic controller inside the generator enclosure monitors and controls the various systems and may be operated locally via a touchscreen or remotely via computer control.
Each module has six electrically operated screw jacks at the generator floor that are extended to "lock" the generator floor to the tower slots. These jacks resist the generator's normal running torque of 51,256 foot-pounds and short circuit torque of 243,000 foot-pounds. The jacks are extended before operating the turbine and retracted before raising or lowering the module.
Maintenance and repair of the module is accomplished by raising or lowering as required to provide access to the various components from either the upper or lower work platforms. Major repair of the turbine components requires that the module be disassembled and removed from the bulkhead slots.
The module is raised and lowered only under balanced head conditions, with the tower's service or emergency gate in the closed position and the downstream chamber between the module and gate flooded. Each module is raised and lowered by a hydraulic cylinder located within the discharge tower with a maximum lift rating of 200 tons. In an emergency condition, the cylinder and associated mechanisms are designed to apply 50 tons to assist in closing the module. The cylinder engages the module via a lifting carriage that has two rotating cam arms that engage a series of steel lift blocks located on the downstream face of the module, similar to a forklift.
For maintenance operations, the module can be raised 67 feet to position the turbine runner at the level of the lowest access platform. The lifting cylinder and associated carriage raises and lowers the module in increments of 10 to 12 feet.
The modules are tended by operating personnel while being raised and lowered, but the controls can be automated to allow an individual to raise a module 12 feet (one stroke of the lifting cylinder) with minimal intervention. This initial lift is accomplished by entering a command into one of the touchscreens or the local control computer. The control system automatically shuts down the turbine and applies the generator brakes and retracts the generator floor stabilizing screwjacks. The tower's service or emergency gate is then closed and the module's spillgates opened to flood the chamber downstream of the module. Once the hydrostatic pressures have equalized, the lift system activates to raise the module.
The tower's service gate is then opened as directed by the Corps to control the release of flow from the discharge tower up to the capacity permitted by the 12-foot opening. Higher flows require the module to be dogged off and the lift carriage lowered down to the next set of module lift blocks, then the module is raised again.
Two steel platforms were constructed to support the installation, operation and maintenance of the module and its equipment, and a tower control booth was attached to the side of the discharge tower to house the operating controls and electrical switchgear. A 4.16-kV to 23-kV enclosed pad-mounted step-up transformer was installed inside a concrete containment on the crest of the dam, with all power conductors entering and exiting underground.
In the event of an electrical shortage, an automatic transfer switch inside the tower control booth transfers all station service power to a separate electrical service provided by another utility. If both utility services are lost to the tower, a propane-powered standby generator installed on the west end of the dam provides emergency power.
Tower modification and loads
To accommodate the module's installation, modifications to structural elements of the discharge tower were limited to removal of a concrete beam at the top of the tower along the upstream side and two 13-foot-square by 3-foot-thick underwater sections of concrete that formed the top of the concrete grizzly racks. Structural analyses of the access bridge from the dam crest to the top of the discharge tower indicated the bridge has a structural capacity of 250 tons, more than sufficient to support a maximum load of 42 tons when driving the installation crane onto the tower.
To ensure the modules would fit into the bulkhead slots, a survey of the plumbness and squareness of slots was taken at 1-foot increments over the height of the tower from invert to roof. The survey revealed the distance between piers was narrower than shown on record drawings and the slots were neither plumb nor square to the tower, as a 120-foot-tall structure was never intended to fit into slots. Modifications were made to the module design to accommodate the survey results.
Start-up and operation
The first unit began generation on Jan. 12, 2012, and reached rated capacity at the design head and flow. Unit 2 became operational in early July 2012. Through Aug. 1, 2012, Unit 1 has generated 6,623 MWh. The first unit has operated over a range of flow from 100 to 8,000 cfs, having operated for more than three days with the module raised 3 to 5.5 feet above the tower invert, allowing the release of about 1,600 to 3,000 cfs while still providing a capacity of 1.8 MW.
The Corps expressed early concern about vibrations that could be produced during turbine operation and their possible impact on discharge tower integrity. Vibration is monitored continuously by the control system, and the levels have remained low. Displacement measurements and values aside, vibrations are low. Actually, higher vibrations are recorded when the unit is offline and water is discharged through the existing tower gates. There are no increases in vibration when the unit is operated with discharges through the tower gates.
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