GENERATION CONTROL
Low-cost maintenance of spinning reserve
To ensure the reliability of electricity delivery, independent system operators (ISOs) must continually balance supply with fluctuating demand while allowing for unplanned outages. To provide a cushion for doing so, power producers hold back a portion of their generating capacity for emergency use. As the Midwest Independent Transmission System Operator (MISO) describes the function in its FERC Electric Tariff: "Spinning Reserve Service is needed to serve load in the event of a system contingency. Spinning Reserve Service may be provided by generating units that are on-line and loaded at less than maximum output, ready to serve additional demand and which can be fully applied in ten minutes."
Prior to deregulation, each utility determined and provided its own level of spinning reserve and passed the costs on to its ratepayers. Now ISOs are responsible for arranging adequate reserve power to meet load peaks. "Typically, prior to deregulation, utilities ran all of their generating assets at somewhat less than full capacity to create a spinning reserve," explains Joseph F. Camean, PE, a senior associate and director of energy services for van Zelm Heywood & Shadford Inc., a mechanical/ electrical engineering firm headquartered in West Hartford, Conn. "Now, that margin is determined by someone else."
The way spinning reserve is managed varies from one ISO to another. For example, the California ISO and PJM Interconnection—the ISOs serving seven eastern states and the District of Columbia, respectively—have ancillary services markets that include the provision of spinning reserve. California's market has been operating since 1999, whereas PJM's has been functional for little over a year. MISO, by contrast, doesn't have an ancillary services market. Instead, it requires each of its regions to purchase a certain amount of spinning reserve—about 1.5% of the capacity needed for point-to-point transmission service.
Money for nothing
MISO also requires all utilities connected to its grid to reserve a specific amount of capacity to ensure that supply and demand can be balanced at all times. Here's an example of how this mandate can affect their bottom lines. MISO requires Great River Energy (GRE)—a nonprofit generation and transmission co-op headquartered in Elk River, Minn.—to keep 50 MW of its 2,500-MW total capacity on standby. The requirement left GRE with two options, neither of which was particularly desirable: reduce the output of one of its two baseload coal-fired plants, and thus the amount of power it could sell, or pay for the electricity needed to keep a peaking turbine-generator unit synched to the grid but not generating.
The main reason why GRE had traditionally chosen the first option was the cost of the second. "Because of drag, it takes 5.5 MW to spin the turbines of one of our peaker units," says Nathan Domyahn, GRE's peaking plants supervisor. That's an expensive proposition, because MISO requires that the spinning reserve be available for an average of 18 hours a day.
A couple of years ago, however, Domyahn had a brainstorm: install clutches between the unit's two turbines and the generator so the latter can stay synched to the grid without having to spin the turbines. "Putting in the clutches decreased our spinning-reserve power consumption to about 0.8 MW," Domyahn says. "Now the cost is so low that we keep the generator ready 24/7. It never shuts down unless it needs maintenance."
The peaker that Domyahn retrofitted with the clutches is a Pratt & Whitney FT4 TwinPac at its plant in St. Bonifacius, Minn. Within the unit (Figure 8) are two oil-fired FP4C aeroderivative turbines that drive a single 50-MW generator. The clutches (Figure 9) disconnect the turbines from the generator. In the new configuration, electricity keeps the generator spinning and synched to the grid while the turbines remain idle. When MISO calls, the turbines are fired up and take over driving the generator.
8. Staying connected. Great River Energy converted a peaking-only gas turbine-generator unit to a 50-MW source of standby power by installing clutches between the unit's two turbines and single generator. Normally, the generator, spun by electricity, remains synched to the grid at all times. When spinning reserve is called for, the turbines are fired up and take over driving the generator.
Courtesy: SSS Clutch Co.
9. Critical clutch. The internals of one of the clutches, after installation.
Courtesy: SSS Clutch Co.
"We found out about doing this though a users' group," says Domyahn. "A gentleman in South Africa was using a Pratt & Whitney TwinPac as a backup power supply for a large nuclear power plant. When we found out how inexpensive the clutches are, we realized they would pay for themselves quickly."
Clutch performer
In May 2003, MISO approved GRE's application to switch its spinning reserve from one of its coal plants to the peaking plant. Although the move gave GRE another 50 MW of inexpensive coal-fired power to sell, the co-op wanted to reduce the cost of spinning the TwinPac. Even with the local cost of electricity at a low 3 cents/kWh, spinning the turbines 18 hours per day would run up an annual bill of more than $1 million. To reduce that expense, GRE selected Size 214T clutches from SSS Clutch Co., Inc. (New Castle, Del.). The company has installed more than 500 clutches on gas turbines worldwide.
"We looked around to see who makes an overrunning clutch that big, and SSS Clutch was the only vendor we found," says Domyahn. "We also have one of their clutches on a different unit and have had very good experience with it."
GRE purchased the clutches in November 2003 and installed them during a scheduled outage. The utility took advantage of this downtime to perform other work, including upgrading the TwinPac's controls. Great River hired a local contractor to do the mechanical work and Wood Group Turbine Control Services (Loveland, Colo.) to install new Woodward Atlas controls on the turbines. But the work actually started prior to the outage (Figure 10).
10. Mission accomplished. Thanks to the clutches, the TwinPac now can supply 50 MW of spinning reserve by starting up its turbines.
Courtesy: SSS Clutch Co.
"The first thing we had to do was extend the unit's foundation by 70 inches on both ends to make room for the clutches," says Ron Haglind, a project manager for Great River. "We could do that while the unit was still on-line, since the work didn't affect the unit's operation."
The foundation work was completed two weeks before the outage began. Once the turbines were taken off-line, GRE removed the generator enclosure, the exhaust stacks, and the turbines themselves and set them off to the side. Next, engineers reworked the existing foundations to accommodate the clutches and the new turbine positions, including moving all anchor bolts. After installing the clutches, they moved the turbines to their new locations along with the enclosure and the stacks. Aligning the turbines took a few days each.
Most of the outage went according to schedule—except for one unexpected issue at the 26-year-old plant.
"When we took the stacks off, we found there was a lot of corrosion on their metal, so we had to make extensive repairs," says Haglind. "That was a big surprise." Despite the setback, the unit was back on-line only a day behind schedule. Domyahn recommends that anyone doing a similar upgrade inspect the exhaust stacks and inlets ahead of time to avoid any surprises.
Quick payback
With the clutches in place, GRE now uses the turbines to bring the generator up to speed and synchronize it to the grid. At that point, fuel to the turbines is cut off and the clutches disconnect the turbines from the generator. The generator then draws power from the grid to keep turning, meeting the utility's requirements for spinning reserve. Because the turbines no longer spin 18 hours a day on standby, the TwinPac's power consumption has dropped by 85%. When peaking power is needed, the turbines fire back up. Once they match speed with the already synchronized generator, the clutches engage and the generator starts providing the needed power.
"By installing the clutches, we took what used to be a seldom-used peaking plant and turned it into a baseload spinning reserve plant that lets us sell an extra 50 MW of very cheap coal-fired electricity," says Domyahn. "The payback has been phenomenal."
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