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CYCLING, LOAD FOLLOWING AND RENEWABLE FOLLOWING ARE NOT THE SAME
By MARK AXFORD
LM6000 gas turbine (GT) genset is in service at the North Pole power station in the City of North Pole, Alaska. It is a suburb of Fairbanks in Central Alaska and home to the Golden Valley Electric Association (GVEA). Running an electric company in Alaska is far from easy. The state population is 740,000, and about 400,000 live in metropolitan Anchorage.
Unlike the lower 48 states, there is not much of a transmission system to move electricity around. There is only one major transmission line in Alaska: The Alaska Railbelt Intertie. This 138kv intertie connects Fairbanks to Anchorage, 170 miles south. It can flow electricity in either direction but has a capacity of only 80 MW.
From an electrical standpoint, Alaska looks like a collection of small islands, each needing to be self-sufficient. GVEA owns six power generation plants, with the North Pole plant as the system hub. It is located adjacent to a refinery that provides diesel fuel, gasoline and jet fuel to Central and Northern Alaska.
In 1976, a pair of GE Frame 7B GT gensets, 60 MW each, were installed as base-load simple cycle units. The 7Bs are still operated today at a capacity factor of about 25%. They have been outfitted with recuperators to improve their efficiency from 25% to 37%. In 2006, GVEA also installed a one-on-one LM6000PC combined cycle unit for base load operation. The steam turbine was oversized in anticipation of a future addition of another LM6000PC.GVEA’s peak load is 230 MW.
“Our peak happens every year on Thanksgiving,” said Paul Morgan, GT plant manager at North Pole. “The delivery of 230 MW requires all three gas turbines, the 28 MW steam turbine, and some imported power from the Anchorage/ Fairbanks intertie.”
All of the GTs at North Pole use liquid diesel or naphtha fuel, which can be expensive. Two other GVEA plants add some fuel and cost diversity to the generation mix: A base load 75 MW clean coal plant at Healy (1998) and the Eva Creek 24 MW wind farm (2012).
The wind turbines have an annual capacity factor of about 33%. While the wind turbines have produced fuel cost savings, they triggered increased maintenance costs on the LM6000 because the operating regime of the LM6000 changed from base load to renewable generation following. After years of steady and reliable operation, the LM6000 has required five combustor change outs since 2014.
“We operated at base load on naphtha for six years and never experienced any premature hot section damage,” said John Kelly, GT plant foreman. “We did our hot section replacements at 16,000 hours every time.”
But soon after wind turbine following began, the hot section experienced severe swirler damage. Ever since then, the plant has seen anywhere between 2,000 to 8,000 hours between hot section intervals. Combustion swirlers pre-mix the fuel and air prior to delivery into the combustion zone.
With renewable generation following, GVEA has observed erosion of the swirlers. If not caught in time, the erosion can cause swirler pieces to break off. On one occasion, a liberated swirler piece caused extensive damage to the high-pressure turbine. TransCanada Turbines and GE worked closely with GVEA to understand the root cause of the problem.
To most power plant operators, cycling means the starting and stopping of a generating unit. Each start from cold (or warm) condition to full heat condition is one thermal cycle. Another cycle is incurred when the genset is shutdown.
Aeroderivative GTs, such as the LM6000 have been favored for many years by utilities that expect daily starting and stopping during the winter and summer peak seasons. Aircraft engines are designed for this purpose as commercial airlines fly several flights per day. Generally speaking, aeroderivative gas turbines incur less cycling-related maintenance than heavy frame gas turbines.
Load following is another operating mode. For utilities not connected to a larger grid, GTs are often used to follow the hourly changes in customer load. GTs are better suited to handle load following than coal or nuclear plants. The change in load is normally gradual and predictable: Higher load during daytime, less load at night.
Also, there are rarely huge changes on a minute-to-minute basis. But with the increasing popularity of wind turbines and solar generation, renewable following is showing itself to be a tougher challenge. Depending on cloud motion and other weather conditions, wind farms and solar power output can change in a matter of seconds.
GTs are called upon to react to this sudden supply change. GVEA has seen its LM6000 ramped up and down between 25 MW and 50 MW dozens of times per day. It would be incorrect to call this cycling or load following. Simply put, renewable following is a whole new type of torture. Who pays?
The answer for GTs required to follow renewable generation may be a new recipe for the swirler and combustor coatings. Or it may be an advanced control system that can change the acoustic signature of the combustion components when high stress is detected.
This problem must be solved as renewable following is not limited to small utilities operating as islands. Many large combined cycle plants rated in the 500 MW to 1,000 MW range were constructed for baseload operation, and are now being impacted by the presence of renewables. Some have already been forced into a new operating regime including being shut down every night and most weekends.
And now they have to deal with renewable following during daylight times. Who pays for the additional maintenance incurred by GTs when new renewable plants are connected to the grid? This is an issue that has to be solved if the viability of many GT plants is to be preserved.