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Growing demand for replacement reserves makes a market for fast-start machines
When it comes to cars, zero-to-60 in less than four seconds has long been the line that separates “peppy” acceleration from neck-snapping performance.
Zero-to-100 MW in 10 minutes is becoming the benchmark that power grids seek in order to fulfill a critical element of ancillary power services needed to assure grid reliability. That’s not to say 10-minute cold-start power in lesser quantities from a single unit or bundle of units is not needed and equally welcome. Most grid operators accept bids starting at 1 MW. But the ability to deliver 100 MW or more in 10 minutes from a single generating asset can have as much appeal to a grid operator or asset owner as a Corvette ZO6 or Viper SRT-10 has to an auto enthusiast.
Why the attraction? Because in order to assure reliability a power grid must match exactly, and almost instantaneously, the amount of power being generated with the amount of power being demanded by customers. One of the four so-called “ancillary services” needed to do that is having replacement reserves, namely non-spinning generating resources able to go from zero to maximum output within 10 minutes.
Other ancillary services, which fall into a special class of generation needed to assure system reliability, include: - Regulation Service–Used to maintain instantaneous balance between load and generating resources.
- Responsive Reserve Service–Spinning generation held in reserve to address loss of generating units and large unexpected changes in generation requirements.
- Non-Spinning Reserve Service–Generation resources that can come on line with short (usually 30 minutes) notice to compensate for load forecast errors.
But it’s replacement services that rely exclusively on non-spinning (and therefore fast-start) generating capacity. And manufacturers continue to provide units that start faster, use less fuel, are more reliable and produce fewer emissions.
There is no common ancillary service protocol for all U.S. regional transmission authorities (RTOs) or independent system operators (ISOs). Efforts by the Federal Energy Regulatory Commission to establish standard market design, which would have included standards for such needs, so far have largely failed. One way or another, however, all grids strive to achieve ancillary service generating capacity that provides the best grid reliability for their electric customers.
Quick-start generation is a critical part of a grid’s non-spinning reserves; namely, those reserves that are not spinning and therefore are not synchronized to the grid. Such quick-start replacement reserves have just 10 minutes to be up, running and producing megawatts. That’s not the same as those units that fulfill other non-spinning reserve service, which have 30 minutes to start, become grid-synchronized and start producing megawatts.
Typically, replacement reserve assets are called upon to replace some of the spinning reserve that has been used up or to provide regulation reserve. This is power supplied moment-to-moment by conventional generators and controlled by grid operators. Such replacement reserve assets can generate revenues, also known as capacity payments, just by being ready to run on short notice.
Some grid operators have created separate markets for buying and selling ancillary services. The Midwest Independent Transmission System Operator Inc. (Midwest ISO), which serves more than 40 million people, will integrate an ancillary services market into its existing energy market starting in September, said Roger Harszy, vice president of real time operations.
This will include spinning reserves (which is generation already synchronized to the grid or supplemental reserves that can be brought up to load within 10 minutes) and regulation reserves that allow for the moment-to-moment balancing of generation and load. MISO market design will clear each product every five minutes. Harszy said this will probably result in more value being recognized for each of these as a marketed product once MISO starts its ancillary services market.
Harszy said that up until now, such services have been the responsibility of the 23 balancing areas that already exist in the Midwest ISO footprint. “Once we start the ancillary services market we will become the balancing authority for our entire footprint. So instead of having 23 balancing areas there will be just one: the Midwest ISO. As the grid operator, we will be responsible for acquiring the reserves and deploying them.”
He said the decision to recognize ancillary services as discrete products in the Midwest ISO market could trigger investment in equipment or technologies to meet the requirements for these products. “There might even be individual companies building units or other facilities just for this,” he said. “People can offer these services to the market and if it clears you’ll get paid for it.”
Harszy said the Midwest ISO is creating an ancillary services market because it’s a more efficient way to operate. The Midwest ISO sees savings and value to its customers by doing so, including price transparency.
Siemens Energy offers several gas turbines that can fill the replacement reserve niche. Siemens is supplying Minneapolis-based Great River Energy with a simple cycle gas turbine unit at the Elk River plant in Minnesota. Scheduled for service next year, the Elk River fast-start unit is unique because it can deliver 150 MW to the grid within 10 minutes. The unit is based on Siemens’ SGT6-PAC 5000F gas turbine package, consisting of an SGT6-5000F gas turbine, air-cooled generator, exhaust stack and SPPA-T3000 plant control system. The plant also includes the dual fuel, ultra low NOX combustion system, extended outage interval components and the 10-minute static fast-start system. The fast-start capability of the SGT6-5000F gas turbine will allow Great River Energy to deliver power to the grid more quickly while reducing NOX, carbon monoxide and volatile organic compound emissions.
“We modified the starting system on the SGT6-5000F so we can get 150 MW in 10 minutes and baseload in about 12 minutes, said Rick Antos, director, global product line marketing, 60 Hz. for Siemens. “The engine can be totally cold, but it’s on turning gear and all the safety systems are set. It takes about five minutes to get up to synchronization and we start making megawatts in the sixth minute. By the tenth minute we’ve gone from zero MW to 150.”
Siemens also offers the SCC6-5000F 1 X 1 Flex-Plant 10, which can also deliver 150 MW in 10 minutes and 200 MW in about 12 minutes and the SCC6-5000F 2 X 1 Flex-Plant 30 that can supply about 200 MW in 30 minutes. The Flex Plant 30, rated at 530 MW, is more of a conventional combined cycle with some fast-start capability. It has the once-through Benson HRSG that enables starting the gas turbine and taking it to baseload in 30 minutes. The Flex-Plant 10 offering (275 MW) is a simplified combined cycle configuration. “There is no maintenance penalty with the 10-minute start,” said Antos. “One start counts as one start.”
Antos said he believes the North American market drives such product offerings due to its geographically immense grid and the fact that the U.S. has more peaking needs because of ambient temperature driven loads (such as air conditioning) than in Europe.
One of GE Energy’s entries in the 100-MW-in-10-minutes market is the LMS100, an aeroderivative gas turbine that delivers 100 MW with simple-cycle thermal efficiency from 44 to 46 percent. More than 30 LMS100s have been ordered worldwide to date and the first units have gone into service at Basin Electric Power Cooperative’s Groton Generation Station in South Dakota.
The peaking plant is intended to meet a variety of on-peak power and ancillary needs for Basin Electric’s customers and the Western Area Power Administration grid. Basin bought a third LMS100 for a new peaking plant at Culbertson, Mont.
Other buyers include EPCOR at its Clover Bar Plant in Alberta, Canada and independent power producer TOPAZ Power Group, which purchased two LMS100s that go into operation later this summer in Laredo, Texas. Units in Argentina and Chile are also about to be commissioned. Additional LMS100 units have been sold in Europe, Turkey and New Zealand.
Shaffer said that a large part of what such capacity provides is power to meet transmission loading restrictions (TLRs), which must be dealt with year-round, regardless of when peak loads occur. TLRs mean that when the transmission system is at capacity in some area, power may need to be injected into the system somewhere else.
Click here to enlarge image
Twelve Cummins Power Generation gas generators operate in an interruptible mode during peak demands, cutting 21 MW of TVA power system load. Photo courtesy of Cummins Power Generation
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The smaller LM6000 gas turbine offers most of the same features as the LMS100, except with lower output (42 MW to 50 MW) and slightly lower operating efficiency. More than 730 LM6000s are currently deployed worldwide. GE recently announced an upgrade of the LM-6000, providing a 25 percent simple cycle power increase and an 18 percent boost in exhaust energy for cogeneration applications. The LM6000 PG has a combined cycle power in the range of 65 MW with efficiencies from 52 percent to more than 55 percent, depending on emission control methods. GE also offers the LM2500 aeroderivative that can produce 20 MW to 33 MW in 10 minutes.
“As wind penetration continues to grow in regions or grids where they don’t have sufficient flexible power, there is a growing need for grid balancing where you can tap capacity when you need it and cycle it without maintenance penalties, especially with the ramp rate you can achieve with the LMS100 and LM6000 where you can ramp from 50 percent load to 100 percent load in less than one minute,” said Tom Walker, general manager for GE Energy’s LMS100 product.
Siemens’ Rick Antos said he also sees interest in fast-start assets from independent power producers. “If you’re getting paid to be a non-spinning reserve, you’re making money when these machines aren’t running. In some cases, depending on the ISO rules and the particular circumstances, you could make money just by being ready to run,” he said. The particulars can vary, of course, depending on each negotiated power purchase agreement.
But despite the fact that natural gas-fired combustion turbines generally supply the bulk of non-spinning replacement reserves, there appears to be a growing market for reciprocating engines in the quick-start capacity market.
Case in point: Wärtsilä will supply 202 MW of gas-fired reciprocating engine capacity for South Texas Electric Cooperative’s new Pearsall Power Plant, in part to meet ancillary generating demands for the region’s growth and grid stability. That need has increased due to the large penetration of wind capacity in Texas and reliability issues associated with wind generation. The plant will consist of 24 Wärtsilä 20V34SG engines rated at 8.4 MW bundled together to operate as a single unit or in any number needed. Pearsall is expected to run about 4,000 hours a year.
Pearsall will be able to provide 25 percent of its power in two minutes and reach full plant output in eight minutes. Wärtsilä’s natural gas-fueled technology has the capability to operate efficiently at low loads. The Pearsall Power Plant will be able to generate as little as 8 MW with a competitive heat rate. Wärtsilä also said its power plants are able to supply all commercially traded ancillary services, including black start capabilities.
Cummins is another reciprocating engine maker moving into the ancillary services realm, with both gas and diesel-fired engines. “With market deregulation some of these services can be very attractive financially,” said Paul Stohr, director of Cummins Energy Solutions Business. “The price paid for power provided for a few hours of the day can be three times more than power for the rest of the day.” Cummins is active in South America, including Brazil, where it provides ancillary services. It is also working with several U.S.-based utilities to incorporate gas and diesel-fired reciprocating engines into the non-spinning replacement power market.
“Reciprocating solutions cost less to build and can be brought online faster than gas turbines,” said Stohr. They are cooled by a closed recirculating water system so water is much reduced as an issue. Many such reciprocating engine plants being built in Brazil run only a few hours a year, but generate needed ancillary grid support and provide a good revenue streams for owners.
Two recent Cummins installations are helping Tennessee Valley Authority (TVA) meet peak demand during hot weather while providing ancillary services. One is a 20 MW diesel power plant operated by the McMinnville Electric System (MES). Powered by 11 diesel generators from Cummins Power Generation each producing 1.8 MW, the facility helps TVA meet its peak demand and provides emergency backup power for up to 40 percent of McMinnville’s load should its transmission connection with TVA fail. MES chose diesel generation because it offers rapid availability, low initial cost and block-loading capabilities; the ability of diesel generators to accept full load in one step.
The second system is a 21 MW peaking power plant operated by Atmos Power Systems. Using 12 1,750 kW Cummins lean-burn gas engine generators, the power plant burns natural gas and operates primarily during periods of peak demand on the TVA power system. The Tennessee Department of Environment and Conservation has permitted the plant to operate up to 1,200 hours a year.
Because both TVA applications use multiple generators running in parallel, they improve reliability of the power system even if one generator out of 50 fails to start properly. Even if that did happen, the remaining 49 will still come online. The other advantage is flexibility, since a utility can start only the number of generators it needs, reducing operating costs.
“Another thing about these reciprocating engines,” said Stohr, “is that many parts of the country are under renewable mandates and these reciprocating engines can help them meet those mandates easily, because they can burn gas collected from landfills or digester gas from waste water treatment facilities.”
Comparing today’s fast-start generating technology to what was available not so very long ago is a bit like comparing today’s high-performance automobiles to the muscle cars of the 1960s. As fast as those cars were, today’s performance machines are even quicker, far more reliable, use less fuel and produce fewer emissions. The same can be said of gas turbines and gas-fired reciprocating engines, which have evolved to better respond to today’s evolving energy market demands.
Power Engineering August, 2008
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