Steam turbine generator speed control - clarification

We appear to be making a complex topic out of a simple situation. First generators are not smart machines and only obey the laws of physics. When we have a generator running under load and we appy more load the resistance of the system goes down as resitance goes down current goes up. If the generator current goes up and we maintain the same torque the voltage has a tendancy to fall. Exciters (AVRs) correct this but frequency still goes down unless we apply more speed (fuel/torque). If we have have no magnetic field we have no voltage, if we over excite (more voltage) we produce extra VARs if we under excite (less voltage)we accept VARs. The changing of the strength of the magnetic field (excitation) creates a stronger magnetic field which keeps trying to force the two different magnetic poles to stop in one position creating a speed drop (load on the prime mover). We must thefore increase the fuel into the system to maintain the same frequency. If we add more fuel it creates heat and is restricted in its flow out of the system, so we in turn create higher temperatures (the gases need to escape faster). Our modern controllers do all of the adjustments automatically not so complex.
 
That's EXACTLY what a generator does: convert torque into amperes.

This is exactly what electricity is used for: Transmitting torque long distances via thin conductors.

Of course, these days, a lot of electricity is used for lighting, and computers (virtual torque??), but in the early days it was for factories and machines (the Industrial Revolution!).

So, one burns a hydrocarbon-based fuel to produce heat which is converted to torque which is converted to amps which is transmitted via wires to areas where it is reconverted to torque (pumps, air conditioners, elevators, virtual torque (computers), and light and heat).

Actually, when you think about it, the turbine-generator is really doing the work that the pumps and air conditioners and elevators and computers are doing, by providing the torque which is being produced by the motors (and virtual torque motors--microprocessors) driving the pumps and air conditioners and elevators and computers and lights and heaters.

Pretty ingenious, huh?

There's a formula which is hard to reproduce on this forum:

T = K(t) * phi(f) * I(A)

where K(T) is a Torque Constant ("K sub T"), phi(F) is Field Flux ("phi sub F"), and I(A) is Armature Current ("I sub A"). In the formula, K(T) is the physical construction of the synchronous generator--which is fixed and doesn't change as the synchronous generator is operation (diameter, length, windings, etc.). Field Flux is the strength of the magnetic field of the synchronous generator, which is held reasonably constant as the synchronous generator is operated. Which leaves only two variables: torque and armature current.

Solving the above equation for I(A):

I(A) = T / (K(T) * phi(F))

So, it can be seen that if the denominator on the right side of the equation remains relatively constant as the synchronous generator is operated, varying the torque (the numerator) applied to the generator directly varies the armature current flowing in the synchronous generator stator. More torque equals more current.

Power in a three-phase electrical system is: P = V(T) * I(A) * 3^2 * pf,

where P is Power, in watts; V(T) is generator terminal voltage ("V sub T"); I(A) is Armature Current ("I sub A"); 3^2 is the square root of 3; and pf is the power factor of the load. Generator terminal voltage stays fairly constant during synchronous generator operation; the square root of 3 doesn't change as the synchronous generator is loaded/unloaded; and we presume the power factor of a load is stable. So, if the only real variable in the power equation is I(A), which is the same I(A) as in the torque equation, then increasing torque increases armature current which increases power (out of the generator).

It don't get no simpler than this.
 
Thanks. The above information has been extremely helpful and I am greatly appreciative. I am currently participating in a hydropower program and I undergo oral examinations that sometimes exceed four hours in length. My background as a machinist doesn’t help much in the area of electrical power generation. Your comments, however, have helped shed light on what would (for me) otherwise be an odious task.

Once again, Thanks!

S.Hines
 
We aim to please!

Glad to be of help! This seems to be a great site for asking basic questions and getting some decent answers.

But the thing that really makes this site useful is when people write back to say they've learned something or been helped by the information provided. That way, we can all benefit when we know something has been helpful or informative!

Although this topic is titled "Steam turbine generator speed control", a steam turbine is no different from a hydro turbine or a gas turbine or a reciprocating engine or a wind turbine or any kind of torque-producing prime mover driving a synchronous generator: they provide torque that the synchronous generator converts into amperes.

So the principles being discussed here apply to hydro turbine-generators also. Any kind of prime mover, actually, since a prime mover produces torque which is transmitted to the synchronous generator rotor through the load coupling and which the synchronous generator converts into amperes (when connected to a load).
 
Actually, I think 3^2 is three squared, and 3^1/2 is the square root of three. I hope this didn't cause too much confusion.
 
Outstanding explanation of the the difference between a diffusion flame liquid fuel oil fired 7B and a gas fired IGV controlled DLN 7EA gas turbine.

Regards,

CTTech
 
THANKS A LOT for the useful, easy to understand data! I wish you were my Power Systems professor. This is one VERY informative posting!
 
Thank you all for your replies. I've learned a lot by reading your posts.

However, I'd like to ask you to help me understand what is primary and what is secondary control of turbine speed.

If I understood correctly, primary control is spontaneous reaction of turbine's controller, but don't know what secondary control would be.
Please, can you give a detailed explanation?
Thanks.
 
Ok, you've all discussed the connection of a generator to an infinite bus. What about a generator which is feeding its own small island of load (e.g. a wind turbine supplying a remote farm) or feeding into a weak bus? What is the result here when the wind blows stronger and the prime-mover speeds up? Surely now the frequency will change? What happens to the voltage and current magnitudes? I assume the current becomes purely load dependent?

Thanks,
GMS
 
Wind turbines operating a small load are a whole 'nuther topic, really, just for the reason you cited: varying wind speed. There are lots of different systems on the market for such an application, many involve using a DC generator to charge a battery. Some require the conversion of the loads to run on DC; some use inverters to convert the DC to a relatively constant frequency AC at a typical voltage (usually 120- or 220 VAC depending on the geographical location). In this way, some energy can be stored for use (in the battery) when the wind speed isn't very high, and frequency can be controlled by the inverter.

There's a lot of different systems, and a lot of different opinions as to which is better--but, as you suggest, it wouldn't be very practical to hook up an alternator (AC generator) directly driven by a wind turbine to your house/farm which was wired for 60 Hz, 220/120 VAC, and just release the blade. If the wind speed was high and the load low, the frequency would be excessive. If the wind speed was low and the load "high", the frequency would be less than nominal.
 
R

Rahul P Sharma

If there are two GTGs connected in parallel (but not to the grid) to supply the load with one of them in Preselect Mode and other in floating mode, which generator will control the frequency, with none being in Isoc mode?

au revoir
Rahul
 
Thanks for your reply.

I'm very interested in the technical aspects of generation on a small (250kW and less) scale; wind, solar, small hydro, diesel etc etc. Does anyone know of a good source of information on this stuff?
 
Hello,

I'd like to ask for further explanation about primary and secondary control (regulation). If I understood correctly, primary control is spontaneous reaction of turbine's controller, but don't know what secondary control would be. Please, can you give a detailed explanation?

Thanks.
 
It seems you're asking a question that's probably related to a particular turbine manufacturer's control scheme or turbine control manufacturer's control scheme. That may be why no one's responded; they're not familiar with it because it's not a "generic" term applied to prime mover control systems.

Was primary and secondary control ever mentioned in the post prior to the first time you asked about it?

Where did you read or hear this term; can you provide information/details which we could review for comment?
 
I think that problem is in fact due to terminology. I'm not speaking about any particular turbine. I've tried google terms "primary and secondary regulation" and I failed to find something that matches what I need. If I literally translate, then primary and secondary control would be the terms. I'll try to describe what I'm referring to and hopefully someone will recognize what I want.

The article I'm reading and which actually has triggered previous questions mentions primary and secondary control. It's about controlling frequency and active power of one power system. The article says that there is a tight connection between grid's frequency and produced active power on one side and between voltage and reactive power on the other side. Primary regulation means a spontaneous action of primary machine's controllers (turbine controllers) whenever there is grid's frequency to change. But because it is related to turbine's controllers primary regulation is slow and transients disappearing with time constants of cca 10s. Primary regulation has static steady state error and therefore it is needed that secondary regulation be included. Secondary regulation is added to primary regulation in order to eliminate this error. Production units that participate in secondary regulation are often called regulation units....

This is roughly what is stated in the article. If this sounds familiar to someone please offer more appropriate terminology...

Thanks.
 
P

Phil Corso, PE

Responding to Mikas' 21-Aug (16:59)query... perhaps Charles Concordia's IEEE paper can help:

"Effect of Prime-Mover Speed Control Characteristics on Electric Power System Performance"
IEEE Transactions on Power Apparatus and Systems
Vol PAS-88; Issue 5 Part-I; May '69; pgs; 752-756.

Regards, Phil Corso ([email protected])
 
P

Phil Corso, PE

Responding to CSA's 28-Jun-07 (23:15) mis-statement, "yes, transformers are an inductive load on the system." I would be remiss if I let this error pass without comment.

A transformer is not an inductive load! The only "load" a power source would be "charged" with (excuse the pun) are the transformer's losses and magnetizing current. Combined, they're an insignificant "load!"

Regards, Phil Corso ([email protected])
 
I think you might be reading the wonderful UCTE document regarding requirements for connecting to the grid. It seems to refer to primary- and secondary frequency control, and my interpretation is that primary frequency control is most likely regular droop speed control and secondary frequency control is remote adjustment (by some regulatory agency) of turbine speed reference to try to assist with grid frequency disturbances. It's not too clear without reading the entire 92-page document (at least that's the size of the English translation one that was given to me was) and a lot of things seemed to have been "lost in translation."

Without being able to speak directly to people who know exactly what was written and/or who understand exactly what is meant in that document it's difficult to say. There have been a lot of "interpretations" by many different people from many parts of the world that have read some portion of that document (I don't think most of them have read it all; some of them have been simple but most have been obtuse. Some extremely obtuse.
 
Probably you're right, but I cannot tall that, because I'm reading article in my langauge which is not translation of the document you mention (beacause it is not stated that way). If you have that document in electronic form can you send it to me by email to "brobigi at yahoo.com"?
Thanks.
 
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