Generator operating frequencies. 27

[HamburgerHelper]

I have thought about this for awhile and I don't understand it. So, let's say you have generation spread out over a large region and there is a disturbance in the system that causes one of the generators to be at a lower or high frequency than the rest of the generators in the system and the controls don't work to bring that generator back up to normal frequency. What happens? I have a hard time understanding this because in my mind if a generator is operating at a different frequency than the rest of the system, that generator or island around the generator is effectively isolated from the rest of the system from a power flow perspective. The rest of the grid is going to try to motor or add generation to it as the phase angle of the different frequency generation slips around the rest of the grid. I just have a hard time grasping why a generator can operate for example at 59 hz while the rest of the grid is humming along at 60 hz.

 

[waross]

Consider synchronizing an incoming generator to the grid.
The grid is at 60 Hz.
The incoming set is at 60.5 Hz.
It will lock in to the grid frequency and pick up enough load to stabilize at 60 Hz, exactly the same as the grid.
Now consider an incoming set at exactly 60 Hz, the same as the grid.
Try synchronizing this set to the grid with too great an vector angle and bad things happen.
A set can only run at a different frequency than the grid (as determined by the reciprocal of the period) until the vector angle becomes too great and that does not take very d--- long.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[marks1080]

I don't understand why we're talking about vector angles for frequency unless you were somehow comparing the frequency between the voltage and current wave forms - in which case it still doesn't matter.

For those who are still stuck in a world where they believe you can have a single system with multiple frequencies (NOT TWO SYSTEMS WAITING TO BE SYNCED TOGETHER WHICH IS THE DAMN EXAMPLE PEOPLE KEEP IMPLYING!) try graphing both frequencies. You can't. because it's physically impossible - that's why the machines will blow up if you try to force the point.

I'm also getting the impression that people are confusing the conversation with changing frequency. System frequency IS NEVER CONSTANT. It is always changing. But it always changes equally through the grid. If you are trying to measure and compare frequencies throughout different parts of a single system make sure you have a sat clock or atomic clock to get a base time reference.

 

[davidbeach]

(Edit - I took a while working on this, and marks1080 posted while I was typing)

Come on guys, look at the scale. If you want to talk about small systems, like an industrial facility or a small city, or even something the size of our distribution service territory, then sure there's one frequency and there's a bit of minor phase angle difference. The two machine model equations work quite well given a few combinations into equivalent machines. That's one scale, that's where a lot of people have concentrated.

But the OP started out asking about "over a large region". So let's look a a large region.

From Williston Lake (Bennett Dam) in northern British Columbia to El Paso Texas is 1840 miles (2960km) as the crow flys (a very tired crow). They are both part of the Western Interconnect (WECC). For reference, Stockholm to Rome is only about 2/3 that distance. Assuming there was a great circle line from Williston Lake to El Paso an electrical impulse would take nearly 10 milliseconds to travel the distance. In the two machine model systems there's not much elasticity and any power transfer across an angle greater than 90 degrees slides off into instability. I've seen phase angle plots where northern British Columbia is leading San Diego by 135 degrees and there are stable power flows north to south across the entire distance. The power marketers can even sell BC Hydro power to SDG&E. It works because it isn't a two machine system; there are hundreds of machines along that path, each doing its part; the flow moves along a series of smaller angular differences. Each one of those nodes introduces a certain amount of elasticity into the system

Over those distances, and resulting times, things can happen. Most of the time you can't tell the difference in frequency between Williston Lake and El Paso, just the rotor angle difference. But now and then the system gets thumped. Frequencies deviate, RAS systems do their thing, system stability is maintained, the system recovers, nothing slips poles. Afterward plots are produced that show frequency (at least the units on the vertical axis are Hz, not degrees of rotor angle) at various locations around the interconnection. The frequencies aren't all the same, but they all get back to the same place eventually.

Maybe it's all one frequency that definitely changes and a whole bunch of rotor angle differences, maybe its different frequencies. It depends. From a practical point of view it's different frequencies and I'd expect underfrequency load shedding schemes to operate in certain areas and not in other areas. The relays that run the UF schemes will see different frequencies based on how they determine frequency.

If we go back many posts to Bill's line shaft analogy, if over a certain period of time the line shaft (a really long one) absorbs 720 degrees of twist before the torsional forces even things out again did the two ends have the same speed the whole time? Or was one end faster than the other for a while before the initially slower end caught up and ran faster to work the twist out of the shaft? Over some period of time the two ends made exactly the same number of revolutions, and the difference in accumulated revolutions was never more than two, but did they always turn at the same speed?

 

[crshears]

I'm one of those "I don't measure speed to six places behind the decimal" kind of guys, so my answer would be, "Close enough for me to say, 'Yes, they were.'"

And for the record, if I'm not mistaken, introduction of the line shaft analogy was my fault; Bill brought in doubletrees, power skids and Wiggle Wagons...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

Thank you very much for your interesting, informative and authoritative post.
Respectfully, may I point out what I see as the basis for the controversy in this thread?

I just have a hard time grasping why a generator can operate for example at 59 hz while the rest of the grid is humming along at 60 hz.

But now and then the system gets thumped. Frequencies deviate

To a lot of us, "humming along" implies a steady state condition, and "the system gets thumped" implies a transient condition.
Go back to each example and determine:
Is this an example of a steady state condition or is this an example of a transient condition?
All of our seeming differences will be resolved.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

On a lighter note, an example was given of the wheels on one side of a car turning faster on a curve. The first answer is that the differences will average out over time.
Then I thought about circle racing. One side continues to gain turns on the other side with no averaging.
How does that fit our analogies?
That would be the shaft speed of an induction generator. grin

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crshears]

Or an induction motor, if you look at the wheel on the other side... [another big smile]

I wish to offer my sincere thanks to all who have participated in this thread; my perspective has been broadened.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

I brought this whole thing up because I saw relaying that was set for very low under frequency tripping. So low, I had a hard time understanding how the system could get there without being islanded

Many years ago I worked on the construction of a large mine mill.
The mill was on an island with local generation but the island system could become "islanded" on the loss of undersea cables.
Part of the control scheme of the mill was two under-frequency relays. As I remember the settings were 56 Hz and 58 Hz.
At the first setting, automatic load shedding was initiated. The mill was able to re-start their machines incrementally when the frequency recovered.
If the frequency dropped to the lower setting, the main incoming 140,000 Volt breaker was tripped and the mill went to emergency lighting on standby generators.
The second under frequency relay could only be reset on with the permission of the supply authority's load dispatcher.
HH was your relaying in a large plant on a system that could become islanded from the main grid?
I would expect that a large plant may have under-frequency trips in such an instance.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[HamburgerHelper]

Waross,

I talked with a guy that has worked in generation for different utilities. The plant I visited was once the largest natural gas combined cycle generation site in the U.S.. I don't know where that puts it now but it was very big to me. The plant I believe would get islanded during large frequency excursions. There is under frequency load shedding (UFLS) that goes on that I don't know the specifics to but I assume that load just starts getting dumped in blocks to help bring the frequency back up and all this load shedding would happen prior to the generator getting tripping. NERC PRC-006 gets into this but I don't know the specifics about it. I asked the generation guy what was the point of setting the underfrequency setting and if it was to protect the generator. He didn't believe that the generator would be damaged if operated at low frequencies and that it might be to protect the customers still connected to the grid during the frequency sag. He said that he has worked at utilities that don't even have underfrequency relaying on their generating units. I suspect that there is very little that would be damaged by a frequency sag of a few hz and maybe you start having lightly transformers and other magnetizing equipment start getting pushed closer towards saturation.

 

[waross]

I suspect that there is very little that would be damaged by a frequency sag of a few hz and maybe you start having lightly transformers and other magnetizing equipment start getting pushed closer towards saturation.

This is a concern with the standby sets that I see most often. The Under-Frequency-Roll-Off feature of the Automatic-Voltage-Regulators used on standby sets reduces the voltage in proportion to a drop in frequency. This avoids saturation at lower frequencies and reduces the kW demand of some loads, aiding in frequency recovery.
UFRO typically allows a few Hz drop in frequency before becoming active.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crshears]

Electrically driven boiler feed pumps in steam turbine power plants suffer adverse effects on supply under-frequency, so if such a plant has them...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Mbrooke]

Ok- I'll toss in this question. Say you had a system that stretched and zig-zaged across the globe starting in Alaska, through Canada, California, Midwest, South, North, over to the UK, Norway, Central Europe, Middle East, Africa, Over to the South America, China, Russia, ect ect now to Australia and then New Zealand. You get the idea- thousands upon of miles. Would anything in the laws of physics stop Canada from starting at 60HZ and New Zealand down to 55 (or less)? Each generator turns slightly slower than the other accumulating in divergence over time and power (over all) moving North to South.

 

[crshears]

Would anything in the laws of physics stop Canada from starting at 60HZ and New Zealand down to 55 (or less)?

Yes.

Each generator turns slightly slower than the other accumulating in divergence over time and power (over all) moving North to South.

No, it/they won't; discounting the nanoscopic "speed changes" that occur to a given machine or system while it develops or sheds angular advances or retardations relative to adjacent components, speeds will all be the same.

Quoting myself, with revisions: "The speed of one great honking line shaft matrix like this" [read: Eastern interconnection] "will definitely fluctuate with imbalances between generation and load, but the entire matrix’s speed – from Key West to Kashechewan, from Sioux City to Sydney - will fluctuate at the same time and by the same amount." Because of their natural resilience, "there will of course be varying degrees of twist in the various shafts as load and or generation are applied and removed, but the speed of all connected components remains" essentially "the same. The only way for there to be a" macro "difference in speed is if a separation occurs somewhere, either by shifting one or more of the intervening gearboxes out of gear, or if a shaft breaks in one or more places."

The same can be envisaged to apply to a world-sized grid; it would just be an up-scaled version of the [North American] Eastern Interconnection.


CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[HamburgerHelper]

"You get the idea- thousands upon of miles. Would anything in the laws of physics stop Canada from starting at 60HZ and New Zealand down to 55 (or less)? Each generator turns slightly slower than the other accumulating in divergence over time and power (over all) moving North to South. "

This is getting academic but I think that is possible. It isn't the length that is going to get you there but the number of generators in series. The slip between the first generator and the second generator is going to give you some time period before it becomes unstable. The slip between the second and third is going to be a slip on a slipping generator. So the last generator will be slipping n*slip-(n-1)*slip with its neighbor. With a lot of generators in series you could get enough slip across the whole start to finish for some period of time. n*(individual generator slip) = slip frequency from start to finish. This probably occurs but not with a 5 hz separation from start to finish. I am guessing when something abrupt happens on the system, this can be seen.

 

[crshears]

Generators in series?!?!? Please explain...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[HamburgerHelper]

They don't have to be in series but I find it easier visualize them that way. Each generator slipping its neighbor very slightly would allow for the hypothetical until the slip produced an angle difference with a neighbor that exceeded the critical stability angle.

 

[Skogsgurra]

NO! For heaven's sake. There is no room for thinking here, or theories.
I can't understand why this seems to be so nebulous to so many engineers. Synchronous generators (or motors) are just that (synchronous, if anyone doesn't know, means "equal in time") so there cannot be any frequency difference in a connected grid. Frequency deviations across the whole grid, yes. But no frequency difference.
You are driving me nuts!

Do you also have "theories" about Euclid's axioms? Or the Atomic numbers. Or simple facts like 2+2=4? (I beg you not to mention that it can be 5, "for large values of two" - that is a joke.

It is really embarrassing that such a basic fact can lead to such a lengthy and improductive "discussion".

I have been a member of EngTips for quite a while and got lots of help. And I hope that I have been able to help others. But now, I feel like leaving.

Incredible!

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[davidbeach]

The classic two machine system is two buses, two generators, with load at each bus and a line between the buses. Many things equivalenced down to those seven elements. Now add another bus with generator and load and connect it with a line to the original two machine system. Now add a whole string of those.

That's what I'm picturing as "generators in series" and what I was picturing as I wrote some of my earlier posts. Draw a circle around any pair of adjacent buses and you've pretty much got a single frequency and a phase angle difference. But given system dynamics all of those phase angles can be changing independently of each other. As long as the phase angle difference across any single line is low enough all is well. Say there's 2 degree difference across each line; double that to 4 degrees. Some units will move much further than others.

But there can't be a continuous increase in angular difference as that would cause some link to become unstable.

 

[waross]

Is this an example of a steady state condition or is this an example of a transient condition?

In a steady state there can be no difference in frequency, period! No difference. Same frequency.

In a transient event there may be very short lived apparent differences in frequency between machines. However these differences in frequency are generally quite small.
Quoting graphs of apparently different frequencies on the same grid is misleading without including some information as to the very short duration of the apparent differences.
Phase shifts are normal as load flow changes. As the angle is changing, the period of one machine's cycles may be slightly shorter or slightly longer.
Look at the line shaft analogy. As a load is placed on a pulley, the resilient hub drops back a few degrees. If you want to you can measure the angular velocity of the pulley and you will see a slight change in speed as the load is being applied.
You can call that a different speed if you wish, or if you just want to stir up controversy.
Gunnar. Be strong my friend. I feel your frustration.
In case I haven't mentioned it; The frequency must be the same over all parts of a grid. Don't be mislead by phase angle shifts due to loading and unloading.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[HamburgerHelper]

Where is this mythical grid that has nothing changing all the time?