Combined Cycle Power Plant Notes
June 2015
The associated graphic show two units side-by-side. Such an installation is near completion just
north of Denver at the Xcel Energy Cherokee Power Plant. I've placed numbers on the various
components which will cross reference to this document. I have spent many years working at the
Cherokee plant before Xcel Energy took everything over.
1) This area is where everything starts. The blue components are the actual combustion turbine
(jet engine) components coupled to the green generator components. The white tubes coming out
of the green generator are known as an isolated phase duct. They protect the three large
conductors, one for each phase, that run from the generator to the voltage step- up transformers.
High pressure natural gas is routed to the turbines where it is burned to drive the turbines. The
conversion of heat to mechanical work is described by the Brayton thermodynamic cycle (Google
and Wikipedia are your friend).
2) Item two shows the step-up transformer assemblies. The white ducts connect directly to the
main tank of the transformer. Front and back are radiators that cool the transformer. The main
tank and radiators are filled with highly flammable insulating oil. The oil is typically under forced
circulation and is used to transfer heat from the main tank to the radiators. The entire transformer
assembly is often placed inside of a concrete revetment structure to limit damage to other
components should the main transformer tank explode due to an internal short circuit. Units I
have worked on have an input voltage in the 14 Kv range. Output voltage typically ranges from 115
Kv to 230 Kv.
3) Item three is a large air filtration unit for the turbine. Just like with commercial airliners, any
debris in the air flow through the turbine can quickly destroy the turbine. The unit can also be used
to condition the air (e.g. humidity) for optimum turbine performance.
4) The exhaust gas from the turbines is very hot and is now used to boil water to produce
steam. Item four shows the boiler structure used to extract heat energy from the turbine exhaust
and convert it into steam. The process of converting heat energy into steam and then driving a
steam turbine to produce mechanical work is described by the Rankine Steam Cycle (Google and
Wikipedia is again your friend).
5) Item five is the exhaust stacks from the boiler. The exhaust (unlike coal) is exceptionally
clean and resembles what you see from your gas range or gas furnace. There is no fly-ash to be
hauled away. There are no significant heavy metals such a mercury to be removed.
6) Item six shows the cooling tower which is used to condense the steam from the steam
turbine back to water. The very pure water is then pumped back to the boiler to be recycled.
Running a Brayton Cycle in tandem with a Rankine Steam Cycle is where the term combined cycle
comes from. Old technology coal plants such as Tri-State's Craig units are lucky to see 30 – 33%
efficiency. Combined cycle plants usually see efficiencies in the 50% or higher range. Combined
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cycle plants also produce approximately half the CO2 on a per energy basis compared to a coal
plant.
7) Item seven shows a typical Rankine Steam Cycle turbine-generator assembly. Nothing
special here. There are thousands of these units operating all over the planet.
8)
Item eight shows the control room for the entire facility.
Summary – All things considered, old school coal plants simply can't compete with combined
cycle power generation. A key element, however, is the cost of the natural gas. Xcel energy is both
an electric and a gas utility. The Cherokee conversion is only possible because Xcel has access to
the reasonably priced high pressure natural gas resource required to run the plant.
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