Turbine Generator Output Control

[121202]

How is the output power of a turbine generator controlled?

Please explain the entire system including reference to

- Control of the gas generator and power turbine

- The control of the fuel\heat rate

- How the electrical output of the generator is controlled

My understanding is pretty basic so please make any explanations explicit.

 

[buzzp]

Start with a general google search. There are lots of sites with very basic info all the way to very detailed.

 

[dpc]

The power output (kW) is controlled by the governor. The reactive power output (kVAR) is controlled by the voltage regulator.

Check

http://www.basler.com

and

http://www.woodward.com

for technical articles on basics of generator control.

 

[byrdj]

Gas generator (compressor section) is like jet engine. Inject more fuel will spin compressor faster and generating more gas. The compressor gas is applied to the power turbine wheel (and the electrical generator). More gas flow will generate more electrical power. If the grid’s electrical MW load is lower than the MWs produced by the generator, the frequency will increase. The governor will sense the speed increase and reduce the fuel to the compressor until the power generated equals grid power consumed. If the grid is large and can use all the power the produced, the power turbine will extract as much work as possible from gas until its exhaust temperature increases to a material design limit and the governor will reduce the fuel to the compressor to limit the exhaust temp.

The generators excitation controls will vary the field current so that the output voltage is maintained at the desired level.

Heat rate is the ratio of power produced to fuel burnt (efficiency) and is more a function of design than control.

 

[121202]

byrdj thanks for your great answer. Just to make sure I'm following you. The gas generator will operate at basically constant speed? If that is true then the air flowrate (mass basis) drawn into the compressor will basically be constant. The only variable that can therefore be adjusted to increase or decrease output is the temperature of the gas downstream of the combustor and this is achieved by increasing or reducing the fuel rate?

Similarly to regulate the supply frequency, the power turbine speed must also be kept constant? Presumably, unless the power turbine is connected to the gas generator by a common shaft, then the speed control of the power turbine relies on closely matching the characteristics of the power turbine to that of the gas generator?

The electrical output is controlled by regulating the exciter?

Thanks in advance.

 

[mersin33]

it is actually more complicated than that. first of all there are two components of electrical output: active (MW) and reactive (MVAr). Active power output is equal to active power input from the turbine, proportional to the gas-regulated by governor. reactive component has got something to do with the excitation and regulated by exciter. Governor controls the turbine, exciter controls the generator.

suppose generator is connected to a grid which is way larger than the capacity of the generator, i.e. your generator is not a significant factor compared to overall grid.

say your generator is producing the active preset power P(MW) at the grid frequeny. governor is controlling the turbine to produce power P at a speed that corresponds to grid frequency. If somehow the grid frequency increases, your frequency will increase too (because you are synchornized with the grid), proportionally you will start producing more power. Governor settings will have to be changed to bring the increased power output to preset P value. Similar thing for the frequency decrease.

governor is relatively easier to understand, because it is dealing with real power. However, there is an exciter system too, pretty much doing the same thing, by under or over exciting the windings, controlling the reactive power to the system, thus the voltage.

In summary you have the power P-made of components V and I. You control P by governor, and V by exciter. Since P is proportional to V and I, by controlling P and V, you control everything.

Hope this helps

 

[mersin33]

correction:

in the above example, if the grid freq. increases, for the same power input to the turbine, electrical power output would decrease, not increase. similarly, for decreased grid frequency, the electrical power output would increase.

sorry for the confusion

 

[byrdj]

to answer your question, Gas generator implies that this is a two shaft unit. the GG will operate at a variable speed, varibale mass. The PT will operate a speed determined by load driven, which there are very detail post. while this page does not detail the controls, it will give detail of the thermodynamics of different configurations(

http://www.gepower.com/prod_serv/products/tech_docs/en/downloads/ger3567h.pdf)

as you see, this is a very interesting and involved question you have asked