The Four Dam Pool Power Agency in Alaska replaced the two mechanical governors at its 22.5-MW Tyee Lake plant with digital governors. The resulting improvement in control and stability of the plant’s output makes completion of a transmission line between Tyee Lake and neighboring 22.5-MW Swan Lake feasible. With the two plants connected, excess electricity from Tyee Lake can be used to supplement the Swan Lake output, reducing the need to use expensive diesel-fueled power.
By Harry E. Williams and Terry A. Bauman
The 22.5-MW Tyee Lake plant, which went on line in May 1984, is the primary generation source for an island archipelago in southeast Alaska. However, mechanical governors at the plant, owned by The Four Dam Pool Power Agency (FDPPA), have performed poorly. Minor load variations that occurred several times per year and sometimes as often as once a month would send a governor into unrecoverable oscillations. In addition, the governors could not support rapid load acceptance, which meant the system could not be rebuilt efficiently after an outage. To improve system stability, FDPPA often operated the turbines at a nearby diesel plant, resulting in additional fuel and personnel costs.
In spring 2006, L&S Electric, Inc. replaced the mechanical governors at Tyee Lake with digital governors designed around commercially available components. The new governors allow flexible operation of the system, providing needle sequencing, automatic underfrequency detection, water waste mode capability, and integrated automatic synchronization. The digital governors also stabilize the system, routinely managing load rejections of 2 to 4 MW and avoiding the need to operate expensive diesel generators.
Most importantly to FDPPA, the improved control offered by the new governors is allowing completion of a 57-mile long, 138-kilovolt (kV) transmission line between Tyee Lake and the neighboring 22.5-MW Swan Lake plant. The Swan—Tyee Intertie will allow access to power generated at Tyee Lake by the city of Ketchikan, Alaska, which often uses diesel-generated power because the Swan Lake plant does not generate enough power. The city of Ketchikan will save about $135 million in diesel fuel costs over 30 years (at today’s prices) after the interconnection is complete in 2009. The Swan—Tyee Intertie will be only the second interconnected electrical system in Alaska and the first in the FDPPA system.
Background on the two plants
Alaska is a large state — more than twice the size of Texas — with a small population. Six of the state’s ten most populous cities have fewer than 10,000 residents. Many communities, such as Wrangell and Petersburg, exist in remote locations. These two cities are on islands in southeast Alaska and have a combined population of about 5,000 people. Business and residents in both Wrangell and Petersburg receive power from the Tyee Lake plant on the mainland of Alaska, about 40 miles southeast of Wrangell.
Tyee Lake is supplemented by Petersburg Municipal Power & Light’s 2-MW Crystal Lake plant and some diesel generators, which are used when the Tyee Lake plant is unavailable due to maintenance or repair. A system of power grids and transmission lines, things most residents in the lower 48 states take for granted, does not exist in Wrangell or Petersburg.
The owner of the Tyee Lake plant, FDPPA, is a non-profit wholesale generation and transmission utility. FDPPA is a joint action agency made up of the municipalities of Ketchikan, Petersburg, and Wrangell and the Cooper Valley and Kodiak electric associations. FDPPA owns four hydro plants: Tyee Lake, Swan Lake, 12-MW Solomon Gulch in Valdez, and 22.5-MW Terror Lake in Kodiak. FDPPA also owns and operates the transmission lines that connect the plants to the electric systems of the member utilities. All of the plants were commissioned between 1982 and 1985. They were sold to the newly-created FDPPA in 2002.
Although energy sales from Tyee Lake are within what the utility expected when the plant was built, loads in Wrangell and Petersburg are significantly lower than the plant’s generating capability. As a result, in 1998 the city of Ketchikan began design of and permitting work for a 57-mile long, 138-kV transmission line between the Tyee Lake and Swan Lake plants — the “Swan— Tyee Intertie” — that would allow Ketchikan to access power from Tyee Lake. As owner of these two plants, FDPPA became responsible for the intertie project in 2003.
The most significant benefit of the Swan—Tyee Intertie will be the interconnection of the Ketchikan, Wrangell, and Petersburg electric systems. Integrated operation of the facilities will improve overall use of the available hydro resources. In addition, the intertie will provide the ability to back up hydro units in the system with other hydro units, thereby improving system reliability. Unit maintenance also can be more effectively scheduled and accomplished with an interconnected system.
The state of Alaska provided FDPPA with a $46.2 million grant to replace the governors through the Denali Commission, a federal-state partnership designed to provide critical infrastructure throughout Alaska. Overall, the project is estimated to cost $120 million. As of March 2008, $60 million had been spent on design and construction work. In addition to this grant, money to complete the project will come from grants already received from the Department of Energy and the Denali Commission, as well as $5 million from FDPPA. If needed, money will be borrowed to complete the project.
The Swan—Tyee Intertie also will serve as a significant component of a proposed southeast Alaska electrical transmission system. In the next ten years, it will provide the ability to develop southeast Alaska’s significant hydro potential on a more cost-effective regional basis rather than on a community-by-community basis. The state is evaluating a transmission intertie with BC Hydro, which operates the largest electric system in British Columbia, Canada. This interconnect would be made at the Tyee Lake plant.
The Tyee Lake plant features two vertical shaft, six-jet Pelton impulse turbines. About 70.5 miles of 138-kV transmission line and 11.4 miles of submarine cable interconnect the Tyee Lake plant to Wrangell and Petersburg. Access to the plant is by plane or boat only.
Before 2006, Wrangell and Petersburg experienced about one complete blackout per month because the mechanical governors and servomotors were unable to respond in a stable manner to any load disturbance. For example, if an eagle flew into a transformer, the entire system would go down. Slow governor response, long transmission lines, and highly capacitive submarine cable components combined to make it difficult to bring the system back on line. During an outage, planned or otherwise, operators resorted to the diesel-powered generators to supply the towns. Diesel generation represents the most costly power supply resource in this area.
Deciding to replace the mechanical governors
As inconvenient and expensive as it was to run the diesel-powered generators, what drove FDPPA to replace the mechanical governors at Tyee Lake was its desire to construct the intertie. The existing governors would have compounded the difficulties in sharing loads between the two plants. FDPPA knew it must have reliable governors at the Tyee Lake plant to pick up and share load and to maintain the system frequency.
FDPPA awarded the contract to replace the governors to L&S Electric in October 2005 through a competitive bid process. The contract specified that work on the first turbine had to be completed by July 1, 2006, because the utility needed both its turbines to provide electricity to canneries during the summer months. Therefore, downtime during this period was not an option and work could not resume until after Labor Day. Logistically, this was one of the worst times of the year because of the winter and early-spring wave problems, high winds, and highly variable tides. L&S Electric exceeded FDPPA’s requirements by opting to complete work on both units by the deadline.
Choosing a system with needed features
L&S Electric proposed installing a digital governor solution designed around commercially available components, in particular a control system based on the Modicon Quantum programmable logic controller (PLC), supplied by Schneider Electric.
The digital governor solution would provide many benefits to FDPPA, including improved response characteristics and performance. The new system also would allow for a reduced parts count and fewer spare parts — the automated system is one device that replaced an entire cabinet of parts — and offer better reliability because there are fewer points where it can fail. In addition, the spare parts required for this system are commercially available through Schneider Electric distributors.
Repair time for problems also would be shorter with the digital governor system because it incorporates built-in diagnostic tools and alarms to allow maintenance personnel to easily determine where a fault has occurred. With the mechanical governors, it was difficult to identify the cause of a fault or, in some instances, even to recognize that a fault had occurred until there was a major failure. Additionally, replacing a single electronic component — generally a plug-and-play solution — usually is easier than coping with a mechanical oil blowout or coupling failure. Finally, the PLC platform used offered the ability to add unit control function to the governor or to integrate the governor with a unit controller.
To further make the solution commercially available, L&S Electric kept the existing hydraulic pressure system and used off-the-shelf control valves with individual control loops for each needle servomotor. These valves replaced the custom-made, proprietary, mechanically-linked valves previously used for servomotor control. In the past, valves were fabricated new every time they were needed.
A major component of the project was providing FDPPA with advanced control modes in the control algorithm. Speed sensing, regulation, and stabilization of the governor system are accomplished through this digital control algorithm and electronic circuits that provide signals used to control the turbines.
One advanced control mode L&S Electric included was the ability to control the deflector and needle servomotors virtually independent of each other. If the needle nozzles open or close too quickly, they can create excessive pressure transients in the water tunnel. Independent control allows the deflectors to move rapidly in response to load changes while maintaining stable, controlled operation.
The algorithm that controls the new digital governors includes provisions for needle sequencing, thus giving independent control of the turbine’s six needles. Needle sequencing allows for more efficient operations by running the turbine with fewer needles at lower loads. For example, if only a third of the power is required, it is more efficient to operate a six-needle turbine with two needles at 100 percent flow, rather than six needles at 33 percent flow. Needle sequencing is difficult with a mechanical system because the operator must manually link and unlink the nozzles.
Isochronous load sharing
L&S Electric also incorporated isochronous load sharing into the control for the Tyee Lake plant governors. Isochronous load sharing allows both units to operate with zero droop while sharing load. These two units represent the majority of the generation for the system, so running them in isochronous control eliminates steady state frequency deviations throughout the day.
Typically, only one unit on a system can be operated in isochronous control, and this “swing unit” absorbs all the load changes. By operating two units in isochronous mode, each unit shares the load changes, thus reducing the need for operator intervention to keep the unit away from maximum or minimum generation.
The load sharing feature allows the two units to share the system load requirements either equally or with a predetermined offset. This offset prevents the two units from going through needle transitions at the same time.
Water waste mode
A water waste mode was incorporated into the control for the new governors. Water wasting, which involves operating the unit where the deflectors are cutting some of the water away from the turbine, allows the turbine to react well during load rejections and provide better frequency regulation during load acceptances. Typically, the timing of the needle servomotors is slow — about 20 seconds for closing and 50 seconds for opening — to avoid tunnel, penstock, and equipment damage. Thus, during an underspeed condition, the turbine is slow to provide the additional capacity required.
The underspeed condition can be limited by giving the turbine some spinning reserve so the needles do not have to move quickly to provide more power to the system. Because the needles are opened above the base load requirement, opening the deflectors immediately applies more power to the unit and minimizes underfrequency conditions during load acceptance.
The controller contains two underfrequency criteria. If the controller senses that the system frequency meets either of these thresholds, the underfrequency detection system places the governor into water wasting mode. This allows the unit to accept load as well as reject it rapidly, improving the frequency regulation. It also turns off needle sequencing, bringing all six needles into service and allowing the governor to add power more rapidly than under two- or four-needle operation.
L&S Electric incorporated an automatic synchronizing algorithm so the turbines could be put on line automatically. With the previous system, operators had to go through manual procedures that required constant adjustment of the unit speed and voltage to match the system. The PLC in the digital governor system enables the operator to simply push a button. From there, the governor automatically matches the unit’s frequency and voltage to that of the system. This process removes the risk that the operator is not correctly synchronizing the turbines and generators at the moment of breaker closure.
Installing and commissioning the governors
For L&S Electric, implementing the specialized control modes discussed above was facilitated by the Modicon Quantum PLCs and the non-proprietary Unity software with which the PLCs were programmed. The Modicon Quantum PLC is customizable, flexible, and capable of performing a multitude of operations. In addition, the Unity software is easy to use and gave L&S Electric the ability to write its own function blocks to create a customized solution. The products also gave L&S Electric the ability to monitor and modify the control while the unit was running.
FDPPA did not initially realize what a key factor the automated individual needle control was in the governor system. Seeing some of the processes in action showed FDPPA how much the automated controls, especially needle sequencing and water wasting, simplified its operations while greatly improving performance.
L&S Electric completed engineering work upfront to incorporate as much of the existing field wiring as possible. L&S Electric also built and tested all the equipment at its plant in Wisconsin to ensure that it was wired, built, and programmed correctly before shipping it to Alaska. After it was installed, L&S Electric tested everything again.
During commissioning of the new digital governors, throughout the numerous checks, the system did not experience a single failure. To achieve this result, L&S Electric teamed with Electric Power Systems, the owner’s representative for this project, and the Thomas Bay Power Authority operating personnel to develop thorough test procedures before arriving on site. Thomas Bay Power Authority, a subsidiary of the towns of Wrangell and Petersburg, operates and maintains the plant and line.
The commissioning process involved verifying that the governor was working properly. In addition, L&S Electric spent an entire day per unit generating information for Electric Power Systems to complete an easy-to-understand electrical model of the control system that was integrated with the overall system to ensure everything responded properly. Therefore, L&S Electric spent extra time operating the units at various load levels and with induced disturbances to gather the needed data.
Even with such complex requirements, commissioning lasted only five days for the first unit and three days for the second.
The residents of Wrangell and Petersburg have not experienced a single blackout since the Tyee Lake operators gained familiarity with the new governors. The governors routinely manage load rejections of 2 to 4 MW. The new governing system has provided both towns with stability in their power system and reduced the nuisance factor of the power going out. In addition, with the new system, frequency varies less than plus or minus 0.05 Hertz (Hz) from 60 Hz. Before replacement, a typical frequency swing on any system perturbation was plus or minus 0.75 Hz.
FDPPA expects Wrangell and Petersburg will save $15,000 to $40,000 annually on diesel fuel costs. However, the true future value for the agency is in its ability to operate the Swan-Tyee Intertie. The transmission line is expected to be completed by 2009. The new governors will let FDPPA export power from Tyee Lake to the Swan Lake transmission line and the city of Ketchikan. The value of this power will be a net of $28 million in revenue over the next 30 years for FDPPA. The estimated avoided cost for Ketchikan in diesel fuel bills over the same period is about $135 million at today’s prices. This cost does not include the necessary investment Ketchikan would have made in replacement diesel generators.
Mr. Williams may be reached at Four Dam Pool Power Agency, P.O. Box 110987, 1301 Huffman Road, Suite 201, Anchorage, AK 99511; (1) 907-258-2281; E-mail: firstname.lastname@example.org. Mr. Bauman may be reached at L&S Electric, Engineering Division, 1810 County Highway XX, Rothschild, WI 54474; (1) 715-241-3404; E-mail: tbauman@ lselectric.com.
Ed Williams, P.E., is operations manager for The Four Dam Pool Power Agency. He is responsible for operations management, oversight, and capital improvements for the agency’s four hydro plants and corresponding transmission lines. Terry Bauman, senior controls engineer with L&S Electric, Inc., was the lead control and commission engineer for the installation of the new digital governors at the 22.5-MW Tyee Lake plant.
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