Rewind at Flatiron11 September 2009
The US Bureau of Reclamation is undertaking refurbishment work at the Flatiron power plant in Colorado
The Flatiron power plant in the US state of Colorado is part of an historic water diversion, storage, and hydroelectric system called the Colorado-Big Thompson project (C-BT). Authorised in 1937, the C-BT was constructed from 1938-1959. While individual parts of the project came on line at different times, the majority of the project’s hydroelectric power generation became operational between 1950 and 1954.
One of the more complicated projects constructed by the US Bureau of Reclamation (USBR), C-BT spans over 884km, diverting water under the US Continental Divide and dropping more than 884m in elevation. It serves nearly 800,000 people and 600,000 irrigated acres, generating approximately 759MkWh of electricity a year. The project has provided these benefits for over 50 years of operation in the face of changing and competing public demands.
Flatiron is a three-unit power plant located on the lower end of the C-BT system of dams, reservoirs, tunnels, canals and power generation. Flatiron operates under an average net head of 322m, with an original nameplate capacity of 71.5MW. The power plant contains two main power-generating units and a reversible 13,000hp pump-turbine unit which lifts water southward from Flatiron reservoir (afterbay) to Carter lake reservoir. This unit is capable of discharging a maximum of 11m3/sec into Carter Lake by pumping up through a 2.25km long connecting pressure tunnel.
The pumping lift through the Carter Lake pressure tunnel ranges from 61-91m, depending on the water surface elevation at Carter Lake. During peak load demands on the project system, water can be released from Carter Lake back down to Flatiron reservoir through unit 3. At such times, the pump-turbine operates in reverse to generate 8.5MW of power.
In 1983, General Electric uprated Flatiron’s two main units from 35MW each to 43MW. Adding the 8.5MW capacity from unit 3, Flatiron today has a total generating capacity of 94.5MWW.
This year, USBR is replacing the stator cores and armature windings in Flatiron’s main generating units 1 and 2, as well as refurbishing the field poles. In 2006, unit 1 had a severe electrical short damaging coils and core laminations, taking the unit out of service. Work was performed to replace the damaged coils and repair the damaged core laminations, and the unit was put back in service at full capacity. Although the repair was successful, unit 1 later experienced another electrical short in a different area of the winding. As a result, it was determined that full replacement of the stator core and armature windings in unit 1 was needed. Through a value engineering study and extensive testing, it was decided to perform the same work on unit 2 because, like unit 1, the generator was nearing the end of its life. To take full advantage of the rewind unit outages, and because the field poles were still originals from the early 1950s, it was decided that the field poles for both units should also be refurbished.
Of particular interest in this rewind project is an innovative approach in coil manufacturing and installation. Contractor National Electric Coil (NEC) is preparing coils with flexible knuckles. Typical armature coils have hard insulation that is cured in the factory. Because both Flatiron units 1 and 2 have a small core diameter, the typical hard, fully cured coils could crack during the installation process. To address this issue, NEC is employing the flexible knuckles which cure after installation, allowing the coil to flex without cracking.
Another challenge to this rewind is the changing and competing public demand associated with the project benefits of supplemental water and power. Water scheduled for diversion and storage through the project is based upon two primary factors: water user demand and electrical demand on the western third of the US power grid.
When the C-BT first began delivering water and generating power in the early 1950s, 85% of the water diverted and delivered was for agricultural use. Over the last 60 years, end-use of the water has changed. Today, more than 65% of the water diverted and delivered goes to municipal and industrial use. Because water is moved through the project to serve water customers on a schedule optimised for power production, the change in end-use impacts the overall operations of the C-BT and how it generates hydro power.
Unlike the eastern part of the country where regular rain provides for agricultural interests, the 17-western US served by Reclamation are very arid, requiring irrigation augmentation. The agricultural season in the West typically runs from late spring into mid-fall. It begins with the snowmelt running off the mountains, down rivers and streams, into the valleys and plains. Reservoirs capture that water and store it for augmentation to irrigation through the rest of the growing season, usually mid-to-late October.
When the use of C-BT water started moving from agriculture to municipal and industrial supply, demand on the project ceased following a seasonal schedule. Instead of seeing an end to the water year wrap up in October, demand for project water is now almost year-round. This not only narrows the maintenance schedule across the C-BT, but also puts additional wear and tear on all of the facilities, including power plants.
For this reason, a specification section was included in the rewind contract with NEC. It requires the contractor to verify that each generating unit can handle continuous operation at 16m3/sec at 322m net head. The turbine on unit 1 had been replaced in the 1980s, and the winding a few years later under separate contract. While both contracts effectively uprated the unit, neither contractor provided documentation regarding the uprated capacity. This lack of documentation constrained operation of the entire C-BT. If one of the two primary units at Flatiron was out of commission, the project could not deliver its full 16m3/sec capacity. By constraining water flow at the plant, power production was also limited. Specifying the documentation will eliminate these kinds of mechanical limitations on future operations.
Additionally, each unit at Flatiron will also undergo an independent mechanical evaluation. Confirmation from the independent evaluation will also help ensure that both units 1 and 2 can meet the continuous 16m3/sec at 322m of net head demand.
The change from an agriculturally driven water year to a municipal-industrial driven water year also narrows the overall maintenance schedule of the C-BT. With this in mind, the rewind contract on both units was modified to fit a scheduled outage during the water diversion time of year.
This meant that new technology was used where possible to expedite the repair process. To accommodate the accelerated work schedule, NEC used a FARO laser tracker to align the stator bore with the centre of the head cover.
Due to the expedited acquisition for the generator repairs, turbine overhaul was not included in the scope of the work. Normally, with the head cover off and the turbine out of the hole, a contractor would drop a plumb line (tight wire) down the centreline of the unit. The centre would be determined from the seal rings which are embedded in concrete just above the draft tube liner. However, being only partially unstacked made this more traditional approach ineffective and a laser tracker was used instead.
Winding and stator
Two engineers completed the stator iron and winding design. Unstacking the unit was performed by a subcontracted millwright crew of four to five. The removal of the stator winding and iron core was completed by a three to four man NEC crew.
While the old winding and stator were being removed, NEC stamped and finished the new stator core laminations. This involved about a half-dozen NEC employees working at a NEC facility. Windings were also fabricated concurrently.
Once the old core was removed, NEC installed new building bolts. The laser tracker was used to locate and verify that the building bolts were concentric and plumb. This work was completed using a four-man crew from NEC and a single person from the alignment subcontractor.
NEC removed the pole keys (pairs of opposed steel wedges that when driven against each other fix the rotor pole to the rotor spider) in order for them to be shipped offsite for electrical insulation replacement—previously they had contained asbestos.
As this article went to press, NEC is testing the new stator iron. The next step will be to install the new winding and, at the same time, install the rebuilt rotor poles. USBR is taking the opportunity provided by the testing to correct some minor misalignments and restore the thrust and guide bearings. This work is being completed under a modification to the original contract so that the overall project can be kept on schedule. The mechanical evaluations are being completed by a one-person unit subcontractor.
Throughout the work, the Flatiron power plant has been kept operational by staging work so that water flow could continue, albeit not at the full capacity of 16m3/sec. Once all work at the plant is completed, these limitations will be overcome and the power plant can move into the future better equipped to meet the demands of a new century.
For more details visit www.usbr.gov
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