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connecting isolated power plant to loads 1
- Thread starter Elec777
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Not really my bailiwick, but a combined cycle plant has a big part that is using waste heat from the primary generation. Using cast-off energy typically puts large time delays into control loops. Large time delays in control loops that have to be agile are very bad for control stability.
Running 'local loads' as compared to feeding into an enormous grid requires agile output control.
Keith Cress
kcress -
Running 'local loads' as compared to feeding into an enormous grid requires agile output control.
Keith Cress
kcress -
Loading will initially be picked up by the primary cycle turbine. Then as the EGT rises the secondary cycle turbine accepts part of the load and the overall efficiency will increase. This leads to the time delays that Keith mentions and restricts block loading to less than the plant total capacity.
Bill
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"Why not the best?"
Jimmy Carter
Bill
--------------------
"Why not the best?"
Jimmy Carter
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1
- #4
rcw retired EE
Electrical
Big difference between connecting to a grid versus trying to start up a load from zero MW and match a varying load up to full plant output. The synchronizing/connecting is easy.
Combined cycle plants have a minimum load requirement for stable operation. At light loads, a steam turbine may overheat because insufficient thermal energy is removed from the high pressure, high temperature steam. A gas turbine can run close to zero load but will probably violate the emissions permits. A combustion turbine is very inefficient at light loads due to the power draw of the inlet air compressor which may take 60% or more of full load fuel input to sit at 0 load and synch speed. These factors make a combined-cycle plant great for base load or 70%-100% load but not economical or emission friendly for extended running below 50% of rating. There are ways to make a combined-cycle work over a wide range and have fast response to load changes. Whether that is economical depends a lot on the type of load, fuel costs, range of load variation, etc.
For an isolated facility there has to be enough load to make the combined-cycle units economical. Multiple units with an N-1 capability are usually needed for reliability. For 100 MW load, you might put in three 1x1 combined-cycle 50MW units with one base loaded and one set up to handle the swing loads and the third in standby. Or get one larger steam turbine and three gas turbines. (Or maybe add 10 MW of solar and 5 MW of wind turbines with fast acting gas turbines and to make up the variations, a micro-grid).
To get an isolated plant started you need another source of power, usually a bank of standby diesel generators that can carry the first few megawatts of load and provide gas turbine starting power.
When the combined cycle plant is synching to a grid, usually start up power is backfed from the grid. The grid also acts as a swing bus to handle load/generation differences. It makes operation much simpler.
Combined cycle plants have a minimum load requirement for stable operation. At light loads, a steam turbine may overheat because insufficient thermal energy is removed from the high pressure, high temperature steam. A gas turbine can run close to zero load but will probably violate the emissions permits. A combustion turbine is very inefficient at light loads due to the power draw of the inlet air compressor which may take 60% or more of full load fuel input to sit at 0 load and synch speed. These factors make a combined-cycle plant great for base load or 70%-100% load but not economical or emission friendly for extended running below 50% of rating. There are ways to make a combined-cycle work over a wide range and have fast response to load changes. Whether that is economical depends a lot on the type of load, fuel costs, range of load variation, etc.
For an isolated facility there has to be enough load to make the combined-cycle units economical. Multiple units with an N-1 capability are usually needed for reliability. For 100 MW load, you might put in three 1x1 combined-cycle 50MW units with one base loaded and one set up to handle the swing loads and the third in standby. Or get one larger steam turbine and three gas turbines. (Or maybe add 10 MW of solar and 5 MW of wind turbines with fast acting gas turbines and to make up the variations, a micro-grid).
To get an isolated plant started you need another source of power, usually a bank of standby diesel generators that can carry the first few megawatts of load and provide gas turbine starting power.
When the combined cycle plant is synching to a grid, usually start up power is backfed from the grid. The grid also acts as a swing bus to handle load/generation differences. It makes operation much simpler.
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