Frequency drop on the utility

[CarRamrod]

Could somebody please explain why the frequency on the utility goes down when you increase the load too much?

Me and my roommates, all recent EE grads, were discussing it last night and we each have slightly different theories, none of which can be explained clearly. I'd appreciate a solid explanation so I can impress them when I get home.

Thanks

 

[ron]

Yikes, I hope that the utility frequency does not drop unless all h*ll is breaking loose. Then the whole grid would go to pot.

On a islanded system, non-stiff, I can see the frequency dropping when the generator is stressed with an overload, as the prime mover can't keep up. The voltage will drop too..

 

[bphgravity]

Isn't frequency a product of the relationship between the number of poles and RPM's of a generator? How is frequency dependent on load?

 

[CarRamrod]

I live in Alberta, and recently we had to have some temporary rolling blackouts because a lightning strike had taken out a high voltage transmission tower. We lost enough power that apparently the frequency fell close enough to 59.5 Hz that the utility operator told the power company to drop some load. That's what I heard through the grapevine, anyways.

Since we're a big province, and the majority of the population tends to be centralized in a few places, our grid is a bit more sensitive than most places. A major line coming in from the hydro plants in BC was lost, and this had quite an effect. Again, this is through the grapevine, but it came from an engineer who works for a utility.

Anyways, I figured it was because the generators couldn't keep up. I don't suppose someone could elaborate on this though?

 

[CarRamrod]

Yes, in theory the frequency depends on the number of poles and the RPMs of the prime mover. But there is a limit to how much load you can put on the generator, sooner or later there will be consequences, and I think that the frequency will drop if you overload a generator too much. Obviously it should be rare, but it seems like there have been a lot of blackouts recently, so it is not as rare as many would like.

My theory, such as it is, is that as more load is added to the generator, more current is drawn. This increases the losses in the generator, decreasing the terminal voltage. If enough load is added... what happens?

The generator would usually compensate for more load by increasing the field current, thereby increasing the flux, thereby increasing the terminal voltage. But you can only increase the flux so far. At some point the voltage will start falling, and the flux can't increase, so the RPMs fall, changing the frequency.

Or am I out to lunch?

 

[al hildenbrand]

It may be easier to think of a small portable generator, instead of the Power Company.

The PoCo has a lot of safeties built into its system and into its management procedures to insure that the generator is not harmed. They will cloud the answer that I think you might be looking for.

Think of a gasoline internal combustion engine driven 2000 watt generator. The load to be run will be a 100 watt trouble light, a 350 watt drill and a 1800 watt circle saw.

I am using the circle saw, light and drill at a job site and am on the end of a very long extension cord cause I can't stand the din of the generator.

I'm working with the trouble light on, cause the lighting's bad. When I turn on the drill, I see the light dim a little and come back up to mostly the same as it was when the drill wasn't running, just a tiny bit dimmer. I hear the din of the generator fluctuate as the added load of the drill draws more current from the generator, creating more back EMF for the motor to over come, so the motor slows, and the automatic throttle senses the slowing and increases the gas flow into the carburator until the engine RPM comes back to the set point for 60 Hz electricity.

Now I put the drill down and take up the saw. I need to rip a long piece of lumber. First I turn on the saw, not cutting, the light dims a bunch & comes back up, as the gas engine slows and catches back up. But this time, the engine takes longer to recover, in part because the long extension cord is introducing enough voltage drop at the saw that the saw is struggling to get up to no load RPM.

Finally, the saw is at no load RPM, the light is back up, but it is a little dimmer than when the drill was running because the saw has a higher no load running current and more voltage is being lost along the long extension cord.

So, I start to rip my long piece of lumber. I'm slow at first, but as I settle into the work, I just want to get 'er done. . .I push the saw harder.

The trouble light, that was dimming more when I started cutting, now is getting dimmer still.

The saw is trying to do the cut, but is drawing more current to compensate for the voltage drop along the long extension cord, which is creating more voltage drop. . .a vicious cycle. The saw crosses the breakdown point on its torque curve and the current the saw draws increases very fast now.

The light goes out.

Everything is quite.

I realize I have stalled the gas engine because the generator, just like the saw motor, tried to make the current that the saw was drawing, creating more back EMF than the gas engine had horsepower to push.

 

[charlie b]

Take a glass of water and dump it into a kiddie-size swimming pool. Question: does the water level in the pool rise? The answer is yes, but not much. In fact, the change will be so slight that you might not be able to notice it.

Take a 55 gallon barrel of water and dump it into the same pool. Does the water level in the pool rise? The answer is still yes, and it might even be enough to notice it this time.

Start with a perfectly steady state, nation-wide electrical distribution system. For this discussion, lets assume that all generators in the entire country are operating in parallel. Let?s say the load exactly equals the amount generated, and nothing has changed for an hour or so. Now let one person in Iowa turn on a single 100 watt light bulb. Question: will the frequency in every single generator in the entire country drop? Same question, different wording: will every single generator in the entire country slow down? The answer is yes, but not much.

Nothing comes for free. If you add load to a generator, it is going to slow down. Then the governor is going to detect the change, and increase the speed by adding more fuel to the diesel engine (or otherwise increase the power delivered by the prime mover). If you add too much load, the generator will slow down too severely for the governor to be able to control the situation. Then, either the generator breaker will trip on overcurrent or the prime mover will trip on underspeed or some other protective action will take place.

 

[CarRamrod]

Thanks guys, those are very good analogies. I knew I came to the right place.

 

[bphgravity]

Is that really how it is? I would think the generators themselves are isolated or independent of the actual grid. The dynamics of the switching at sub stations would regulate load across the grid while the generators operate at a steady state. I may be wrong - I am not a utility guy.

 

[charlie b]

Switching might take place at substations when there are huge changes in load. With the more routine and relatively minor changes in load, the generators' speed control systems react to keep the frequency within specified limits, and the excitation control systems react to keep voltage within specified limits.

 

[al hildenbrand]

1.21 Gigawatts!! Great Scott.

The power output of the grid and the generators behind it is so massive, by comparison to the loads we imagine being connected, that the generators seem, intuitively, infinite.

But it's not.

I've stood beside the generator room of the downtown power plant, looking out the window into the storm, and as I watched a distribution line start to go up in a fireball a block away, I heard the change, the groan, in the generators behind me, then I felt (and heard) the breakers through the concrete floor under my feet.

--Edit to display the image, Al--

 

[CarRamrod]

That sounds like a line from Blade Runner. Neat.

So, if you removed enough load from a network at one time, would the frequency go up, likewise?

 

[al hildenbrand]

Yup, you got it.

 

[mpross]

frequency deviation

frequency deviation

check out the notes on ch. 11 (the AGC stuff), they were helpful to me. I took this class last semester.

http://vulcan.ece.iastate.edu/~jdm/EE457/ee457schedule.htm

-Matt

 

[don_resqcapt19]

Bryan,

I would think the generators themselves are isolated or independent of the actual grid.

They are directly connected to the grid. There are 3 interconnected grids in the US. East of the Rockies, west of the Rockies and Texas. In each of the grids the energy production must always and instantanously match the energy load because you can really store electricity. The frequency changes slightly to keep the system in balance. Because all of the power lines are connected together in each of the grids, the frequency in each of the grids is the same through out the grid.
Don

 

[mpross]

freq

freq

Is this correct? Should it be the opposite?

The frequency changes slightly to keep the system in balance.

We keep the system in balance to keep the frequency in check.

-Matt

 

[charlie b]

No, Don has it right. The frequency changes in reaction to load changes. So does the voltage. Then the utilitys? various control systems make the adjustments necessary to bring the frequency and voltage back to the desired values.

 

[charlie b]

That sounds like a line from Blade Runner. Neat.

It's a line from the movie "Back to the Future." So too is the image of Christopher Lloyd in the role of Dr. Brown.

 

[charlie b]

Think of it this way. You are riding a bicycle down a flat, smooth road. The work you are putting into the bike is exactly enough to balance out the friction and wind resistance. Therefore, you are maintaining a constant speed. This is like the generator putting out exactly the amount of power needed to equal the power being drawn by the connected loads.

You come to a hill. The amount of work necessary to go up the hill is more than the work you had needed, on the flat road. This is like connecting another load to the generator. The first thing that will happen is that the bike WILL SLOW DOWN. So too would a generator slow down, if you add load. Thus, if you want to keep going at the same speed, you have to pump harder. When your brain gives the command to your legs, it is like the generator?s regulator giving a signal to the speed controller of the prime mover.

One final consideration. What if you were on a tandem bike, with another person helping you pedal at a constant speed. When you come to the same hill, you will still slow down. But with two people doing the work, you won?t slow down as much. Each person will have to work harder to bring the speed back up, but the amount of extra work by each person is less than the work one person would have to do, if they were riding alone. In the same way, with multiple generators connected to the same grid, the addition of load will not cause as much of a drop in frequency, and the extra work that each generator must do to bring you back to rated frequency is not as much as one generator would have to do, if it were working alone.

Does that help?

 

[George Stolz]

Charlie, I thought the Blade Runner comment was regarding this portion of the post:

I've stood beside the generator room of the downtown power plant, looking out the window into the storm, and as I watched a distribution line start to go up in a fireball a block away, I heard the change, the groan, in the generators behind me, then I felt (and heard) the breakers through the concrete floor under my feet.