Steam Turbine generator work done

[JSBeckton]

I will start my junior year as a ME major in the fall, I currently am working for a power company on a co-op. I work with the Predictive Maintance department corporate so I am not at the plant all
the time but have a question about the turbine that might be more of a generator question.

If the turbine always spins at 3600 rpm (or very close), how is it that the generator outputs a varried amount of power depending on demand? It seems that it takes the same amount of work to spin the turbine so shouldn't you get the same amount of power out of it?

 

[Cyrus]

I don't know for sure, but I will stab a blind guess at it.

Edit(1): Because the turbine rotates at a constant angular velocity of 3600RPM, the work associated to maintain this speed (ignoring the generator) is a constant. This is because the generator does not increase the frictional losses due to the bearings, viscous effects of the steam, and energy stored in the rotation of the mass of the turbine blades and shaft itself.

Now I am not 100% sure about below, but someone can fill in the blanks or make corrections if need be.

The output of the generator is going to be a voltage and a current. This will be a function of the torque provided by the turbine. The more voltage and current you produce as the demand increases, the more torque or angular velocity will be required to meet this power demand. This is due to the fact that P=Tw P-power, T-torque, w-angular velocity. This is nothing new to you. Now it is obvious we are running at a fixed angular velocity and so it is the torque that must increase or decrease to deliver the power as the demands change.

(hmm still thinking out-loud to myself, ignore above for now, I need to go back and look at a book on what parameters affect a motors output voltage and current).


Don't be shy, ask these questions to your co-workers. They will give you better anwsers than anyone here can.

 

[FredGarvin]

What drives you to make the statement that the same amount of work is being done despite the load? As the load requirement increases on the generator, the load on the turbin must increase. That is compensated for by the "fuel control."

 

[turbo]

In a typical steam turbine, the turbine-generator has an optimal operating speed and a governor system to keep it near that speed. When the electrical load on the turbine increases, the spool valves feeding steam to the various turbine stages will open to allow more steam to the stages to keep up with the work load. Turbine-generators have fairly complex control systems that can balance the steam feed based on the load, the draw at extraction stages, and many other parameters. Even older, simpler T-G sets in the 30-40 Mw range can have quite complex control systems.

 

[JSBeckton]

I guess it would be more helpful if someone could better explain a generator. As I understand it, its has to do with the shaft work output by the turbine, being transferred to electricity by a process involving magnets. I guess it seems to me that the determining factor of the output is the shaft work put in. Does the force to spin the shaft increase with load because of a higher electromagnetic force in the generator? And if so how does this work?

PS, I did ask my coworkers but none of them knew exactly, we mostly work with vibration, oil analysis, thermography, and motor circuit testing. Power companies have turbine experts that better understand this but they are hard to find!

 

[Danger]

The work required does indeed rise with the amount of demand on the generator. My friends and I (amateurs) just called it 'electrodynamic drag'. Others have referred to it as 'reverse EMF', but I don't know if that's the proper technical term. It stems from the intensity of the EM field increasing with load. You can think of it almost as a 'thickening' of the field; sort of the way oil puts more drag on a propeller than water does. Not a very technical answer, I know, but I hope it's helpful.

 

[turbo]

I guess it would be more helpful if someone could better explain a generator. As I understand it, its has to do with the shaft work output by the turbine, being transferred to electricity by a process involving magnets. I guess it seems to me that the determining factor of the output is the shaft work put in. Does the force to spin the shaft increase with load because of a higher electromagnetic force in the generator? And if so how does this work?

PS, I did ask my coworkers but none of them knew exactly, we mostly work with vibration, oil analysis, thermography, and motor circuit testing. Power companies have turbine experts that better understand this but they are hard to find!

Instead of permanent magnets, envision electromagnets, the field strength of which can be varied. When a large turbine is off-line with no external source of electricity available, it is "bootstrapped". In other words, the turbine is spinning the rotor of the generator, and a huge bank of batteries is used to "flash" these electromagnetic coils and produce the magnetic fields, so that the generator can start producing power. Thereafter, the generator can provide the electricity to keep those coils magnetized. Does that help? You should be able to search Wikipedia, etc, and get a much more detailed explanation.

 

[NateTG]

If the turbine always spins at 3600 rpm (or very close), how is it that the generator outputs a varried amount of power depending on demand? It seems that it takes the same amount of work to spin the turbine so shouldn't you get the same amount of power out of it?

Well, if the pressure (or pressure difference) is larger, then the turbine is going to be producing more power at the same rpm.

 

[FredGarvin]

You are rotating a coil through a magnetic field. There are going to be forces generated that depend on the strength of the magnetic field.

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magfor.html#c2

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/motorac.html#c2

 

[Astronuc]

Adding to what Fred and Turbo mentioned, the turbine has a control system that increases steam flow or combustion to meet the demands (load) on the generator. In a steam system, the turbine control valves would control the steam flow. There is a nominal design setting at which the turbines maintain at a rated power (torque). Baseload TG sets do not vary the load.

The generator has an 'exciter' which provide the currents, which produce the magnetic fields, which induce the load voltage/current in the generator.

The control system monitors the load (or turbine speed and change in turbine speed) and adjusts the steam flow or combustion in the turbine to maintain the speed of the turbine. The AC frequency is dependent on the rotational speed of the generator.