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from the Engineer department...
Why frequency 50 hz...?
Power generation equipment control. topic
Posted by SUHAIL on 30 April, 2010 - 5:28 pm
I work on g.e frame 9 (model 9001E) 100MW gas turbine my question is during synchronization why we need to match frequency.. And why we use frequency of 50 hz particularly why not higher than that. If i don't match the frequency, voltage, phase angle what can be the result.....?


Posted by CSA on 1 May, 2010 - 6:09 am
There is a lot of conjecture about why 50 Hz or 60 Hz or why not some other frequency. I'm sure there wasn't just one reason, but several. But, it is what it is where it is, and we must accept it, because we can't change it.

Quite simply, if you don't match frequency (and likely voltage) and get the phasing within a certain range, the synchronizing relays won't allow synchronization (presuming they work correctly).

There are many good references to synchronization of synchronous generators on the Internet. I believe that one that's been recommended many times before on control.com is candu.canteach.org. It's some information from the Canadian Nuclear Industry that is very, very good. Some of the links and material are for the nuclear industry, specifically, but there is a lot of very good material about generators and synchronization and basic fundamentals about that kind of "stuff" there. Use your favorite Internet search engine and you can find more, it's certain.

A synchronous generator has a very large rotating magnetic field, and for a Frame 9E that rotating magnetic field (the generator rotor) has two poles (a North and a South pole) and it spins at 3000 RPM to produce 50 Hz "electricity."

There's this little formula that relates speed and frequency:

F = (P * N) / 120

where F = Frequency (in Hz)
P = the number of poles of the generator rotor
N = Speed of generator rotor (in RPM)

When a generator is synchronized to a grid with other generators there is current flowing in the stator windings, and what happens when current flows through a coil (in this case, lots of coils with many turns)? A magnetic field is generated, and in this case the magnetic field (three of them actually) are very, Very, VERY strong. So, this is a "second" magnetic field that is present in the generator, along with the rotating magnetic field of the generator rotor.

They call it a synchronous generator because when it's being operated properly, the magnetic forces of the generator rotor are coupled with the magnetic forces of the stator windings, and this magnetic coupling is very, Very, VERY strong.

The turbine actually tries to make the generator rotor go faster than 3000 RPM when the unit is producing power, but the frequency of the grid through that little formula above and the strength of the magnetic forces inside the generator *will NOT* allow the generator rotor to spin any faster than the frequency of the grid. (If the generator breaker were opened when the unit was producing power and the fuel were not quickly reduced, the turbine and generator rotor speed will increase very quickly, in fact, it's likely the unit might trip on overspeed depending on how much power was being produced when the breaker was opened.)

So, when you are synchronizing the generator to the grid with other generators it's very, Very, VERY important to get the frequency and phasing correct so that the magnetic fields are not opposing each other and are attracting each other. Because if the generator breaker is closed and the fields are not closely aligned, there will be a very, Very, VERY big, loud noise and the generator and turbine will likely not be able to produce power for a very long time.

Now, when we talk about speed, or frequency, matching we don't usually match the generator frequency exactly. Usually the turbine and generator frequency is just slightly higher than grid frequency, and the synchroscope is rotating slowly in the clockwise direction. When the needle of the synchroscope approaches the 12 o'clock position then the voltage sine waves of the generator's frequency and the grid frequency are approaching the same point ("in phase") and that's when we like to close the generator breaker.

When the generator breaker closes with the generator frequency a little higher than the grid frequency and current starts to flow in the stator windings the magnetic forces of the fields developed in the stator GRAB the magnetic forces of the field and actually slow the generator rotor (and turbine shaft) down slightly. Because the fuel is not changed the extra torque that was being used to make the generator frequency a little higher than the grid frequency (by spinning the generator rotor a little faster than the speed which would correspond to grid frequency) causes power to be produced by the generator.

Generators are devices for converting torque (which is what makes the generator rotor spin) into amps. And when the generator rotor is locked into synchronism with the grid if the fuel flow-rate to the turbine is increased then the turbine produces more torque. That increased torque would TEND to increase the speed of the turbine and generator rotor, BUT the magnetic forces at work in the generator keeping the generator rotor magnetic poles locked in synchronism with the stator field forces means the generator rotor cannot be spun any faster. But, the generator, being a device that converts torque to amps, turns the extra torque into amps. And when we multiply amps times volts (and throw in the square root of 3 plus the power factor) we end up with power (watts, or rather, millions of watts, megawatts).

In addition, when you are synchronizing the generator to the grid with other generators it's important to match the voltage of the generator closely to the grid voltage. There are a couple of reasons, and one of them is that if there is a large difference in voltage (potential) between the generator terminal voltage and the grid voltage the generator breaker closing mechanism has to work very hard to close the contacts. If the voltage difference is very small, then the contacts close much easier.

The second reason for matching voltage closely (it's not usually matched exactly, though it can be and at some sites they do match voltages exactly) is that when the generator breaker closes when the two voltages are very closely matched there will be very little reactive power "flow" (we have to be careful when we use "flow" to describe reactive power here on control.com, because we can incur the wrath of the Exclamation Pointer). If the generator terminal voltage is much higher than the grid voltage when the generator breaker is closed, then the VAr meter will indicate lots of lagging VArs when the breaker closes. Conversely, if the generator terminal voltage is much less than the grid voltage when the generator breaker is closed then the VAr meter will indicate lots of leading VArs. (Of the two possible conditions, lagging VArs is more preferable, but usually it's not desirable to have a large lagging VAr indication as it can upset the grid operators and grid reactive power and voltage levels.)

It's very, Very, VERY important to "match" frequency and voltage when synchronizing a generator and to make sure the magnetic fields are nearly "in phase" with each other when closing the generator breaker and to try to limit the VAr meter from indicating too many lagging or leading VArs (again, we have to avoid using "flow" when talking about VArs, even though everyone else in the world uses that description).

Think of how much power the Frame 9E turbine produces when it's running at rated output, 100 MW in your case. 100 MW is more than 127,000 horsepower! And the magnetic forces at work inside the generator are keeping that 127,000 horsepower from turning the generator rotor any faster than 3000 RPM. The Speedtronic isn't stopping the speed from increasing, the generator is! (Remember we said that if the generator breaker were suddenly opened when the unit was producing power that the speed would increase very quickly because that torque which was being converted into amps by the generator will then be allowed to increase the shaft speed, and very quickly!)

Now, think about what would happen if you closed the generator breaker when the magnetic fields were 180 degrees out of phase. The magnetic forces of the stator windings are going to spin the rotor VERY fast in one direction or the other to try to make the fields attract each other and then they are going to cause the rotor to STOP when the fields are attracting each other! In fact, if the generator breaker is closed when the magnetic fields are 180 degrees out of alignment then they are trying to repel each other with maximum force at that instant in time! In any case, the resulting mechanical forces from repulsion/attraction would just destroy the load coupling for certain, and probably the turbine- and generator rotors and/or coupling faces as well. It would not be a very pretty sight.

That's why those synchronizing relays mentioned at the beginning are there: to prevent closing the generator breaker when the magnetic fields are not in phase with each other. Because the results could be not only catastrophic, but deadly.

Look around the Internet (the World Wide Web) actually, for more information. This is difficult to describe without pictures and diagrams, but the concepts are the same whether or not there are pictures and diagrams.

Hope this helps!

And I just noticed, you indicated your unit does not have a GE control system. No matter; the turbine control system doesn't control the unit speed when it's synchronized on a grid with other generators and their prime movers. The grid frequency controls the speed by virtue of that little formula at the top of the response.


Posted by suhail on 2 May, 2010 - 9:35 am
Dear csa

Thank for sharing your valuable knowledge onc again


Posted by Youdo on 2 May, 2010 - 6:43 pm
Thanks, that help me out a great deal too. You made it very clear and easy to understand.

Youdo


Posted by G.Rajesh on 12 May, 2010 - 3:42 am
Dear CSA,

We will be grateful for your good work, very excellent explanation, thank you very much. I too learn something about frequency.

Shall you give your introduction, if you like??

Thanks
G.Rajesh.


Posted by CSA on 12 May, 2010 - 12:22 pm
I'm someone who benefited tremendously (both professionally and personally) from the generosity and patience of several very special and wonderful people many years ago that's just trying to share the benefit of their gift to me with others. I learned more than things from them; I learned how to question things, and how to analyze things, and how to interpret things, and how to see things because they gave me more than just answers. They taught to how to find answers. Those people didn't have to take the time to "answer" my questions (and I had a lot of them!) or to help me (and I needed a lot of help in the beginning!), and yet they did, and for that I am eternally grateful. I have tried to let every one of those individuals know how much they shaped my career, and my life.

I also recognize the benefit of forums like this, where individuals can ask questions and can have their questions answered--the same questions that many other people have! And most importantly, where many others can benefit from the same answers. And most importantly the feedback!. (By learning to use the 'Search' function!)

Because without feedback, the information provided here is not very useful. Unless people who read the questions and answers write back to let others know if the information was helpful and useful, or even if it wasn't, no one else reading the posts can know if the problem was solved or the question satisfactorily answered.

So, I wish to thank the founders and the moderators and the sponsors of control.com for making a forum like this available.

And to remind everyone who posts a question here, or who finds useful information here, to take a minute or two to provide some feedback and let others know the information was useful and helpful, or not (as the case may be).

Also, I wish to thank everyone who takes the time to post responses using their experience and knowledge; it helps us all. I have learned a lot from the experience of posting here, and from reading a lot of the other non turbine-related posts and responses here. There is some great knowledge and experience here on control.com in a lot of fields and areas. I believe that if a lot of the questions posted here were phrased differently and had more information than, "It doesn't work; how do I fix it?" that a lot more people would offer their inputs, but sometimes it's just difficult to try to cover every possible base for people who didn't take the time to be specific and provide useful information to get useful information.

Anyway, I'll get off my soapbox now. Thanks for the kind words.

Live long and prosper.

[Seemed appropriate] ;-)


Posted by G.Rajesh on 15 May, 2010 - 6:06 am
Dear CSA,

Thanks and still I am not able guess your good name and profession, you may ask why this much interest to know your name, because the way you answering the questions inspired us.

G.Rajesh


1 out of 1 members thought this post was helpful...
Posted by Phil Corso on 15 May, 2010 - 11:44 am
G.Rajesh... I suggest you read Thread No.:

http://control.com/thread/1246285612

Regards, Phil Corso


1 out of 1 members thought this post was helpful...
Posted by G.Rajesh on 17 May, 2010 - 7:49 am
Dear Phil Corso,

Thanks for the link, I read that and useful.

Thanks
G.Rajesh


Posted by Rup on 24 August, 2011 - 1:51 am
sir,

i wanted to why the frequency is not above 50- 60 hertz and below that ...as you know this are the world wide standards..... is it that out of lots of research we come to conclusion that the range is 50 or 60 hertz?


1 out of 1 members thought this post was helpful...
Posted by Bilsko on 1 September, 2011 - 3:28 pm
So the 50/60Hz question goes back a long ways. I have an old ABB manual from the 1940s that gives the following explanation:

" The Standard freq. in North America is 60 cycles per second. In most foreign countries it is 50 cycles. As a general purpose distribution freq. 60 cycles has an economic advantage over 50 cycles in that it permits a maximum speed of 3600 rpm as against 3000 rpm. Where a large number of distribution transformers are used a considerable economic gain is obtained in that the saving in materials of 60-cycle transformers over 50-cycle transformers may amount to 10 to 15 percent. This is because in a transformer the induced voltage is proportional to the total flux-linkage and the frequency. The higher the frequency, therefore, the smaller the cross-sectional area of the core, and the smaller the core the shorter the length of the coils. There is a saving, therefore, in both iron and copper."


Posted by CSA on 1 September, 2011 - 9:55 pm
Thanks very much for this!

I recall reading something similar in university, but have never been able to find that quote.

Hooray for old manuals and references!

And thanks for posting this!


Posted by Bilsko on 2 September, 2011 - 9:00 am
Happy to share - its such a tiny contribution compared to your explanation on synchronization above!
Here's the link to the book on Google: http://goo.gl/vBsqn

The frequency discussion is early in the book (within the first 15 pages or so).

Have any good sources of explanations for how inverter-based and induction based systems interconnect? I get the general feel, but having a detailed walkthrough - like yours - makes a world of difference. I have some of the source material that I've seen linked to here on Control.com, but haven't found anything quite as accessible as your description.


Posted by CSA on 3 September, 2011 - 9:53 am
Thanks for kind words--and the link to the book!

I'm sorry to say, my experience is almost exclusively with synchronous machines. I have some experience with adjustable frequency drives in regeneration ("braking") but that's about it.


Posted by Namatimangan08 on 15 September, 2011 - 1:39 pm
50 vs 60Hz? Here is one of the deals looking from power plant design and operation.

For the same inertial energy of rotating mass you need smaller turbine generator. Thus you can save materials and capital cost. This is because inertial energy of rotating mass aka kinetic energy of rotating mass is

KE= 0.5Jw^2 (1)

and

J = mk^2 (2)

Where

J =moment of inertia (kgm^2)

w= angular velocity (rad/s)

m = mass (kg)

r= radius of gyration (m^2)

From Eq (1), it is obvious that we need smaller mass to get the same KE if we increase w. If we double the w, we need quarter of the original mass for the same KE. KE is useful for frequency regulation and control. It acts similar to a capacitor. It can absorb excess in supply for a while and or supply temporary to cover generation shortfall.

The trade off is operation cost. Higher w means higher synchronism power, i.e. the power required to rotate a turbine generator at the synchronizing frequency but delivering zero output. The main factor that contributes to this additional cost is windage resistance, which can be expressed as:

Windage resistance = kw^2

One night argue less mass means windage resistance will become less. That probably true. However, I think it will just reduce windage resistance linearly. Whereas windage resistance increases square to the w.

Which one is more economical? The main stream opinion is 60Hz is more economical. I don't have any opinion of my own for this issue. We can conclusively know if we evaluate life time cycle cost. A 60Hz system capitalizes on lesser capital cost. A 50-Hz system capitalizes on lower operation cost.


Posted by Beginner on 25 August, 2011 - 1:46 am
Dear CSA,

Thanx a ton for giving a vivid explanation.

Regards,
Beginner


Posted by manoj kumar on 14 August, 2012 - 8:23 am
suppose let we take two ac gen sets A and B.

A is having 500v,I=10A,1500rpm,p=2 and f=25hz,I=10A ,B is having 500v,3000rpm,p=2 and f=50hz. your load is assume 10kw. let i take 10 houses for each house is rated with 1kw. now you connect two gen sets in parallel to each other with same bus bar,now one machine is rated with 25hz and another is 50hz, then how you sets or fix the frequency rating to your apparatus because the machines having diff frequencies, if you connect two diff frequency rated machines, your apparatus will totally damaged because all our apparatus are rated with some constant frequency and also our transformers (e directly propotional to F).

if you connect opposite phases then their voltages are aidind so voltage are doubled together.

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