How is the turbine speed controlled by Grid but not by itself?

[Chandrakanth_balusa]

Hello People,

Greetings!

i am back again with a confusing question for which I thought I could get a correct answer from here.

so my question is regarding the speed of the turbine. I learned that the speed of a turbine has to be maintained constant in order to maintain generating frequency also constant. so my question is that when my power plant is connected to a Grid which is running already at certain frequency, another big generator is also connected to the same grid supplying comparatively more power than my power plant. so does that generator somehow influence my power plant to maintain turbine speed constant?

I got this doubt since I am using a software for power plant simulation in which the scenario mentioned above is controlling the speed of my turbine though I dint program any speed controlling automation for my power plant. if i disconnect that generator from the grid my turbine runs with very high speed.

hope you understand my question and expecting some valuable responses!

thanks and regards

Chandrakanth Balusa

 

[berkeman]

Are you asking about a wind turbine or a hydro turbine?

In any case, there are different kinds of generators. I believe the older wind turbine generators were synchronous, so all turbines were locked to the grid when they were generating power. I believe that newer ones tend to use DC generation and inverters to more efficiently couple power into the grid.

I'll let @anorlunda and others give better answers...

 

[Chandrakanth_balusa]

Are you asking about a wind turbine or a hydro turbine?

In any case, there are different kinds of generators. I believe the older wind turbine generators were synchronous, so all turbines were locked to the grid when they were generating power. I believe that newer ones tend to use DC generation and inverters to more efficiently couple power into the grid.

I'll let @anorlunda and others give better answers...

thank you for your quick response :-)

no it is not a wind turbine but am speaking about a coal fired power thermal power plant.

 

[JBA]

My only experience is with engine driven generator sets; but, for those, the speed of each generator is controlled by its governor; and, when an additional generator is to be brought on line it is first brought up to the correct speed and then connected to the grid. After which, if the first generator is to be shutdown, it is first disconnected from the grid and then shutdown in a controlled manner.

 

[anorlunda]

You should read two of our Insights articles.

https://www.physicsforums.com/insights/ac-power-analysis-part-1-basics/
https://www.physicsforums.com/insights/ac-power-analysis-part-2-network-analysis/
The simple answer is that synchronous generators send power to the grid proportional to the phase-angle of the rotor. In order for the power to be constant in time (or nearly constant) the phase angle must be constant. To make that happen, the frequency must match the grid frequency exactly.

When you move the turbine valves, it makes more power or less power, that causes the phase angle to change, but the speed must return to grid speed for it to stay steady.

Of course, before the circuit breaker is closed, the turbine and generator are not connected to the grid, so changing power changes speed.

 

[gmax137]

The forces involved can be quite high. Many years ago a co-worker claimed to have witnessed a large turbine generator being connected to the grid without being properly synchronized. He said the stator ripped right out if its foundation as it tried to rotate due to being out of phase. Pretty amazing, if you have ever seen one of these machines -- they are big and heavy (in a 1000 MW generator, the stator alone is 500 tons) with massive foundations.

 

[tech99]

I think that your generator remains locked in frequency at all times, but it will self adjust its rotational phase angle so that the torque of your turbine (decided by the steam pressure) matches the load presented by the grid. The load presented by the grid will change as other generators come on line.

 

[russ_watters]

When you move the turbine valves, it makes more power or less power, that causes the phase angle to change, but the speed must return to grid speed for it to stay steady.

Or from a mechanical perspective, the valve movement changes the torque applied to the shaft. Similarly:

Of course, before the circuit breaker is closed, the turbine and generator are not connected to the grid, so changing power changes speed.

Applying a torque with no counter-torque save for inertia produces angular acceleration.

 

[anorlunda]

For the benefit of the OP as well as others, the following is a linearized model that we use for power system analysis. The principle linearization is around speed. The model is good for small perturbations around nominal speed.

Where:
Pmech= per unit mechanical power (1.0=rated)
Pelect= per unit electrical power (1.0=rated)
SPEED=per unit speed (the linearization process converts kinetic energy to speed, hence the factor 2 in constant K1)
ANGLE=rotor angle in radians
FREQUENCY= per unit grid frequency (1.0=nominal)
K1= constant = 1/2H where H is the per unit moment of inertia of turbine+generator. Typical value for H is 4.
K2= 377 radians/sec for 60 Hz, or 314 radians/sec for 50 Hz.
K3= a constant proportional to the strength of the transmission ties to the grid. There is no typical value for this. Opening the circuit breaker is like setting K3=0.
K4= a constant proportional to friction and windage of the rotating machines. Typically, 0.03

Expressing everything in per unit, is customary in the power industry.

The block diagram, and the 1/s notation for integration should be familiar to those who studied differential equations, and Laplace transforms. But even if you do not know those things, only two things are significant. They are:

1. This block diagram is well known. It represents a damped oscillator. Damping for a typical power plant is very small.
2. In the steady state, by definition, nothing changes. In that case, the points labeled A and B on the diagram must both be zero. That means Pmech-friction=Pelect, and SPEED=FREQUENCY.

That brings us back to the OP question, which was why does SPEED=FREQUENCY?

 

[Dr.D]

My only experience is with engine driven generator sets; but, for those, the speed of each generator is controlled by its governor; and, when an additional generator is to be brought on line it is first brought up to the correct speed and then connected to the grid. After which, if the first generator is to be shutdown, it is first disconnected from the grid and then shutdown in a controlled manner.

Even with engine-driven gen sets, where several set supply a small system, they are effectively locked together by electromagnetic torques. An additional set cannot be brought on line until the speeds are matched and also the phase angles.

A point that does not seem to have been brought out is that a synchronous machines is essentially just a transducer, an energy converter. It can be driven (by a diesel, steam or gas turbine) or it can function as a motor driving whatever is attached (again, a diesel or steam/gas turbine). It all depends on the local machine phase angle with respect to the rest of the grid. Thus if the grid is ahead of the local machine, the synchronous machine will function as a motor driving its intended prime mover (not satisfactory operation, but possible!).

 

[anorlunda]

A point that does not seem to have been brought out is that a synchronous machines is essentially just a transducer, an energy converter.

All your points are correct.

I once posted that beginners would be better to forget about voltage and current, and think of generators and motors as merely energy conversion devices. The EEs in the thread hated me for saying that. I think some of them still don't forgive me.

 

[JBA]

An additional set cannot be brought on line until the speeds are matched and also the phase angles.

You are obviously correct, it was an omission on my part while recalling the process from some decades past. In our installation, once the the engine governor set the engine speed, we then used a phase meter and a quick button push to connect an additional generator to the grid.

 

[berkeman]

As one of my favorite professors used to say (he ran the laser lab at the University), "Then you have the graduate student thrown the big power switch on the wall to turn on the high-power converters..."

 

[Dr.D]

In our installation, once the the engine governor set the engine speed, we then used a phase meter and a quick button push to connect an additional generator to the grid.

Sounds about like a place I used to work some 44 years ago.

 

[JBA]

Sounds about like a place I used to work some 44 years ago.

I almost hate to admit it; but, mine was 6 years before that.

 

[Chandrakanth_balusa]

thank you all so much for your valuable information.

so with your discussion, I conclude my understanding in this way that the generator can be connected to grid if it is running at same frequency of the Grid. and this can be controlled only by the governing system of the turbine. but once after the generator is synced, when there is a variation of load on the generator the throttle valve tries to maintain the speed constant by increasing or decreasing the steam flow with which torque on turbine can be varied.

my question, if the speed is also controlled by grid is wrong. the generator speed must and should match the grid frequency and that is done only by the governing system.

Glad to be in this Forum! and surprised to have such a quick responses!

thanks a lot!

regards

Chandu

 

[tech99]

I think (in principle) that the generator will stay in lock without the need for a governor and throttle valve. Its mechanical phase angle will adjust itself so that the load equals the supplied torque.

 

[Chandrakanth_balusa]

I think (in principle) that the generator will stay in lock without the need for a governor and throttle valve. Its mechanical phase angle will adjust itself so that the load equals the supplied torque.

could you please explain me in detail how the speed is locked without governor and throttle valve.
I am a little bit confused.

 

[anorlunda]

could you please explain me in detail how the speed is locked without governor and throttle valve.
I am a little bit confused.

The answer to that is in the Insight article I linked earlier. Did you read it?

https://www.physicsforums.com/insights/ac-power-analysis-part-2-network-analysis/
It is also implicit in the model I posted in #9 where it says SPEED=FREQUENCY. I seems that you did not understand that either.

I don't mean to confuse you, but speed governors still play a role even when synchronized. The collective actions of the speed of all the generators on the grid collectively, act to control grid frequency to the 60 (or 50) Hz ideal. That is explained in another Insights article. I think it is kind of magical because it allowed grid frequency control since the 1880s without any electronics or telecommunications. That kind of frequency control is not centralized, it is distributed. That is described in another Insights article.

https://www.physicsforums.com/insights/what-happens-when-you-flip-the-light-switch/

 

[tech99]

Let's assume the alternator is running at the correct speed, so it is in synchronism. It is being powered by steam, so it will try to rotate forward a few degrees in an endeavour to speed up. But when it does this, it encounters increased opposition from the load. It finds itself having to do more work, and is actually supplying energy to the grid. If it moves forward a little too much, the mechanical load of the grid is greater than the torque of the turbine and the generator will fall back. In this way the alternator finds a mechanical phase angle where it is supplying all its output to the grid. The torque of the turbine then matches that of the load.
Imagine walking around a ship's capstan when the anchor is being raised. You wil be walking at the same speed as the other people but when you try to speed up, the load on you increases such that it equals your own effort. Then you are contributing all your available power to the system. If you try to speed up or slow down, the load you feel will alter to keep you in synchronism.

 

[russ_watters]

could you please explain me in detail how the speed is locked without governor and throttle valve.
I am a little bit confused.

The generator pushes back against the prime mover. If the prime mover "tries" to spin faster, the generator torque increases to prevent it.

 

[cosmik debris]

could you please explain me in detail how the speed is locked without governor and throttle valve.
I am a little bit confused.

Its inertia of the whole generator system that prevents one generator from getting out of phase.

Cheers

 

[nsaspook]

The forces involved can be quite high. Many years ago a co-worker claimed to have witnessed a large turbine generator being connected to the grid without being properly synchronized. He said the stator ripped right out if its foundation as it tried to rotate due to being out of phase. Pretty amazing, if you have ever seen one of these machines -- they are big and heavy (in a 1000 MW generator, the stator alone is 500 tons) with massive foundations.

http://archive.boston.com/bigpicture/2009/09/the_sayanoshushenskaya_dam_acc.html

 

[cjl]

could you please explain me in detail how the speed is locked without governor and throttle valve.
I am a little bit confused.

Maybe it would be easier to imagine mechanical power rather than electrical. Rather than an electric grid, imagine we had a vast network of spinning driveshafts all linked together, and you could use those for power. Generating plants have turbines linked to these driveshafts, and when you connect a load, it is powered by them. Once you hook a generator to this "grid", it doesn't matter what the throttle setting is - it will always spin at the same speed as the grid because it is physically connected to the grid. If it is running full throttle, it will be adding a lot of energy to the grid, but it won't spin any faster. If it's off, it'll be taking energy from the grid (effectively acting as a motor), but it'll still keep spinning the same speed.

If you imagine the shaft used to connect it to the grid has a bit of "springiness", you can even envision the phase lead or lag. If the generator is running at high power, the shaft will twist a bit under the load and the generator will be ahead of the grid, but still spinning at the same speed. If you apply a brake to the generator and run as a motor, the shaft will twist the other way and it will lag behind a bit. The amount of shaft twist is proportional to the amount of energy the generator is adding or removing from the grid.

Generators work similarly, but this "springy shaft" isn't physical, it's electromagnetic. It's very strong though, and locks the generator in with grid frequency similarly to how an actual, physical shaft would. A generator making a lot of power will twist this connection a bit, giving the phase lead, but it will still keep spinning at the exact same speed.

 

[anorlunda]

I think the OP's question has been more than adequately answered.
If anyone has more to add, please click on my name and send me a PM.

Thread closed.