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Generator operating frequencies.
19

Generator operating frequencies.

Generator operating frequencies.

(OP)
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.

RE: Generator operating frequencies.

You've got it sort of backwards.

An islanded generator, not connected to the grid, will run at a frequency determined by its governor, or perhaps by its AVR.

When you want to connect that generator to grid, you can't just throw a switch to connect them instantaneously.

Well, okay, you can, but you stand a good chance of destroying the generator or its engine.
... because the transient torque generated by blindly/randomly joining the grid can be enough to break a crankshaft.

To add your islanded generator to the grid, you must:
Adjust the speed so the frequency matches the grid.
Adjust the voltage to match the grid.
Repeat until both are very close.
Monitor the phase difference between generator and grid.
When the phase difference reaches zero, close the connecting breaker.
Once it has joined the grid, the generator will remain in phase with every other generator on the grid. .... or die trying.


Mike Halloran
Pembroke Pines, FL, USA

RE: Generator operating frequencies.

Quote (HH)

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.
The generator will be having an even harder time.
When the generator frequency does not match the grid frequency exactly it will suffer a condition known as "Pole slip".
A serious and destructive condition, both mechanically and electrically.
A generator just has to drop behind by one cycle or less to experience pole slip. If external forces cause the generator to drop behind by 1/2 or 1/4 cycle it may be past the point of no return and continue to slip for the remainder of that cycle.
A possible case for pole slip may be a co-generation setup in a factory a relatively long way from the next system generator.
A local fault may cause pole slip.
The pole slip will cause a serious electrical disturbance which may be dissipated in part by the length of the lines to the next point of generation.

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

RE: Generator operating frequencies.

The requirements for making any connection to the commercial power grid in a "make before break" generator connection is a good example of the complexity toward connecting an islanded generator into a larger generating grid system.

Power company technical requirements are quite stringent with regard to application, proof of technical requirement compliance, on site inspection and facility access by power company at any time, toward approval of the ability to interconnect. Here is one power company's requirement as an example:

Generator Interconnection Technical Requirements: https://www.eversource.com/content/docs/default-so...

RE: Generator operating frequencies.

(OP)
I am talking about over a very large region like the Eastern Interconnect in the U.S.. I am under the impression that all generators aren't operating at the exactly same frequency all the time.

RE: Generator operating frequencies.

They should be pretty damn close in a stable system. How many decimal places are you talking specifically?

RE: Generator operating frequencies.

(OP)
So, let's say that some load is dumped onto the system or a generator trips. The whole region doesn't sag, I believe. Whatever happens in New York isn't perceived in Texas. If not and you have relaying that trip a unit at 57 hz, that would mean the whole region is being effected or you have been islanded. I am not grasping something here.

RE: Generator operating frequencies.

Yes, that's exactly what it means. I've studied faults in Ontario and seen the frequency charts from Florida. They match up. There is only one system frequency. This is why fault clearing time is so critical, especially at higher voltages.

RE: Generator operating frequencies.

Just for the record, you're correct that disturbances in New York will not effect Texas, but this is only because of the way the Texas grid is designed and connected to the northeastern grid.

RE: Generator operating frequencies.

The frequency in a grid is the same except for problems like pole slip. That said, there can be angular differences across the grid, to which is limited by power flow.

Their are generators that run at different speeds from that of the grid, by design. They are induction generators, and it is possible in theory to run other types of generators that way, such as in a field loss. But that is not what they were designed for.

For most generators, being at 180 degrees is equal to a three phase fault level of current, and not very many protection systems will allow that. So for a machine to be at a different frequency, it should trip off very quickly.

RE: Generator operating frequencies.

(OP)
Marks1080,

I was referencing the parts of Texas that are in the eastern interconnect. ERCOT doesn't cover all of Texas.

Maybe, I was looking at this all wrong. So, anytime your frequency drops either you have been islanded or the whole region is in trouble.

If your frequency control failed and something happened in your area to cause the frequency to sag, everyone close theveninly across the system would see the sag and increase generation to bring the system back to 60 hz.

RE: Generator operating frequencies.

I think because of the controls lag for an event, the most sensitive controls will attempt to pick up frequency first.
And true the closest controls should attempt to pick up voltage first, but with other voltage controls likely are more sensitive.

RE: Generator operating frequencies.

That's the basic concept. The main protections that covers this ARE the generator protections, at least for the high voltage grid. Under/Over freq protection is normal at the LV stations as this is where you will see frequencies changes due to load swings/faults, and islanding is more common, but it's usually referred to as a remedial, or special, protection - not a primary protection. Also, LV systems are low inertia systems. The HV systems are high inertia (quite literally meaning lots of big and heavy rotating machines connected). In theory, if you have a generator where pole-slip was impossible than you would see frequency changes for single unit faults or interruptions. The reality is that during these conditions the large inertia of the grid means no single unit is strong enough to overcome the system inertia to have any influence on the frequency, hence that unit is pushed into a pole slip by the inertia of the system its connected to. When analyzing single units in most large systems we just look at the 'system' as an 'infinite bus.'

So if you have legitimate frequency deviations it probably means you have major system interruptions and your system has started to split up and islands are forming. If you have a single generator unit interruption, generally it's only that unit that will become damaged (other adjacent units can also get damaged due to introduced zero-sequence currents). I do not know this for sure, but I've heard, that NPCC/NERC utilities have an unwritten agreement that if any one of the connected utilities goes down, they all go down together. As in they won't island themselves to save parts of the system. If this is actually true (I have good sources saying it is) than it's just one of the many examples of how MBAs are destroying the power system.

RE: Generator operating frequencies.

It might be helpful to think of the electrical connection between the grid and the generator as behaving in a way similar to the mechanical connection between the generator and its drive motor/turbine. Phase angle differences will result in torsion of the drive shaft(generating or motoring). Too much phase angle deviation will result in pole slip or drive shaft breakage, depending on which is stronger: the drive shaft or the magnetic forces coupling the poles of the rotor and stator.

So, over a period of a number of cycles the generator and grid frequencies must always be exactly equal (no pole slip). I believe your comprehension problem comes from thinking of phase angle variation as frequency variation.

RE: Generator operating frequencies.

I think this is possible if the condition is spread across a very wide range... Ie Quebec say 60Hz, Ontario at 59.87, NY at 59.81, NJ at 59.76, Maryland at 59.65, North Carolina at 59.61, ect ect with the frequency dipping down to 59.00Hz at say Mississippi. Power would flow North to South and every generator would be turning slightly slower than the one above it. Not enough to motor or skip poles, but just enough to be lagging physically.

RE: Generator operating frequencies.

HamburgerHelper, you asked an interesting question!

This caused the guys who know to impart their knowledge to those of us who think we know but really don't.
Thanks for the question
Roy

RE: Generator operating frequencies.

How is the frequency measured? On a small time scale a changing phase angle would look like a changing frequency, right?

RE: Generator operating frequencies.

"So, over a period of a number of cycles the generator and grid frequencies must always be exactly equal (no pole slip). I believe your comprehension problem comes from thinking of phase angle variation as frequency variation."

I should have added: Phase angle variation is a frequency variation, but only over a short time period. The phase angle may vary but the phases must stay locked together.

RE: Generator operating frequencies.

The phase angle variation produces a quasi-stable situation. The angle variations need to be accounted for but units with differing angles can all be treated as having the same frequency. As long as nothing goes unstable the frequency of the various units keep together even while individual units rock back and forth relative to the average frequency.

RE: Generator operating frequencies.

Hi cranky108,

Gas turbines are typically the fastest responders to frequency sags, and they can provide emergency support to a falling grid until the bigger steam sets have a chance to open up their governors and increase boiler firing. The downside is that for a baseload GT operating on its thermal limit, the only way the turbine can provide this response is at the expense of over-firing the engine beyond the normal operating limit. The TSO typically pays a decent premium to have a 'frequency correction' mode which makes the governor response to a falling frequency more aggressive and adds perhaps 20°C to the thermal limit, something which absolutely hammers the hot parts life of the engine.

RE: Generator operating frequencies.

Think of a two horse hitch. One horse is traveling at 8 miles per hour. The other horse is traveling at 8.2 miles per hour.
If there is 2 feet of swing in the double tree, how far can they go before the faster horse is dragging the other horse?
The faster horse can only get a few feet ahead of the other horse until there is a wreck.

Or consider an old power skid with two diesel engines geared together at the same ratio to drive one output shaft.
One engine is running at 1800 RPM, the other engine is running at 1750 RPM.
Something's gotta break.

Two generators are running in parallel. They can only get a few mechanical or electrical degrees out of step before bad things happen.
If one generator is running at 60 Hz and the other generator is running at 59 Hz, there will be the equivalent of a phase to phase fault once a second. For a few seconds anyway.

Two parallel generators will be running at exactly the same frequency. One may be a few degrees advanced ahead of the other, rather than matching position exactly.
Then, if the sine waves are superimposed one sine wave will be a few degrees out of step with the other, but at the same frequency.
The generator that is leading will be producing a greater share of the power. That is how loading between generators is controlled.
But, at a 30 degree difference between generators running at the same frequency, breakers will be tripping.
I have seen operators close a synchronizing breaker with a 30 degree difference (faulty synchroscope connections).
The breaker trips free instantly. On that size and type of breaker an instantaneous trip is caused by 10 times rated current.
30/360 = 12, so 1/12 of a cycle out of step is enough to start the light show.
When the grid frequency changes slightly, all the sets change together. The slower responding sets may lag a few degrees while the faster responding sets pick up a greater share of the load.
We are talking about a few percent of a cycle offset at the same frequency.

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

RE: Generator operating frequencies.

"Two parallel generators will be running at exactly the same frequency. One may be a few degrees advanced ahead of the other, rather than matching position exactly.
Then, if the sine waves are superimposed one sine wave will be a few degrees out of step with the other, but at the same frequency"


Perhaps, but what if these were miles apart? Won't the frequency be slightly different at each generator?

RE: Generator operating frequencies.

One system frequency.

RE: Generator operating frequencies.

Sorry... should have added:

IN A STABLE SYSTEM

RE: Generator operating frequencies.

Ok- noted.

RE: Generator operating frequencies.

Scotty, I agree to a point.

The most sensitive units might not be the fastest at overall reaction. I should have said most sensitive controls and reaction timing.

RE: Generator operating frequencies.

davebeach quiteThe phase angle variation produces a quasi-stable situation. The angle variations need to be accounted for but units with differing angles can all be treated as having the same frequency. As long as nothing goes unstable the frequency of the various units keep together even while individual units rock back and forth relative to the average frequency.

I've also been told that power companies will sometimes bring generators online at a phase angle to correct for power factor in the distribution network.

In regard to several posts regarding phase angle measurements throughout the network, with the extreme accuracy of the reference clocks used to drive GPS satellite timing, GPS reference receivers are commonly used for many different types of precision measurement and control e.g. not only for locking RF transmitters and receivers to precise frequency control, but also far more demanding locking into phase synchronized, geographically distributed coherent radar systems, and of interest to this thread, phase angle measurement at various points in the power distribution network.

Although this article points out a possible vulnerability that could occur with GPS signal spoofing... http://gpsworld.com/wirelessinfrastructuregoing-ag... I can't imagine a power company using a GPS signal directly as opposed to using it as a long term corrective factor to some multiple number of independent on site high precision frequency reference standards and would think these systems would be alarmed to the hilt for any irregularities. Even basic antenna hardware is available as a countermeasure to the possibility. http://gpsworld.com/orolia-gpsgnss-passive-anti-ja... Precision reference standards have been around for a long time.. http://www.virhistory.com/navy/freq-equip.htm

RE: Generator operating frequencies.

(OP)
The more I read about frequency control, the more it becomes clear that generation collectively reacts very quickly to correct for frequency deviation. If you happen to be off by 0.1 hz, you have at most 5 seconds for other generation to increase their power output to prevent you from slipping poles. I imagine that if you have a strong disturbance like losing a unit, everything nearby is told immediately to go balls to the wall to ramp up generation. I suppose any lost in frequency also helps with reducing the load imbalance.

What are the ramp rates of various forms of generation?



This I thought was kind funny. Kosovo has been running area imbalances and has been slowing down the clocks in europe this year by as much as 6 minutes between January and march.


http://fortune.com/2018/03/08/kosovo-serbia-energy...

RE: Generator operating frequencies.

Power companies increase the energy into the prime mover to increase the power (kW) output. This may be more steam in a steam turbine, more water in a hydro unit or more fuel in a gas turbine.
The increased power input moves the output a small angle ahead of the other units.
Utilities increase excitation or voltage setting to correct power factor.
Everything runs at the same frequency, regardless of distance.
I have a short SUV. On a straight road, the front wheels turn at the same miles per hour as the back wheels.
Today I just bought a new truck. It is very long (33 feet overall). How long does it have to be before the front wheels can run at a different MPH than the back wheels? (I left it at the frame shop. It will be shortened to a more usable length.)
On the way home, I saw a turnpike double; a semi-trailer with a 53 foot trailer pulling another 40 foot trailer.
The total overall length was probably over 100 feet. Is that long enough that the front wheels can run at a different MPH than the rearmost wheels?
Sometimes size, or distance for that matter, doesn't matter.

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

RE: Generator operating frequencies.

Sometimes, though, the left wheels turn a bit slower than the right wheels while at other times the right wheels turn a bit slower than the left wheels. Most of the time they're pretty much the same speed and even themselves out in the long run.

RE: Generator operating frequencies.

grin.
And to be fair, I have seen and heard of a couple of cases where the front and back wheels did turn at quite different speeds.
It was after a mechanic installed the wrong ratio differential in one end of a four wheel drive vehicle.
Oh and once when a Wiggle Wagon somehow got the front axle in high range with a rear axle still in low range.
Of course it only lasted for a few feet on a hard surface before disaster struck.
I don't know.- Maybe something like parallel generators running at different frequencies, for a very short time (counted in cycles, on the fingers of one hand) before disaster strikes.


Wiggle Wagon.

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

RE: Generator operating frequencies.

(OP)
I read up on frequency generation and talked with a guy familiar with ramp rates. The numbers he was giving me was around 5 to 50 MW per minute I believe. That means when you have a unit trip out, I guess, you are relying heavily on rotational inertia to hold up the system before you can get your generation up to pick up the MW deficit. And because the ramp rates are so slow, the deficit has to be picked up by a whole bunch of units to prevent whatever region or generator from going unstable. I suppose that when your system starts slowing down in frequency, the load decreases as well and helps lessen the problem kind of like how low voltage ,too, helps reduce load.

I thought this was kind of relevant but Kosovo has been slowing down the European grid by causing energy imbalances.

https://arstechnica.com/tech-policy/2018/04/europe...

RE: Generator operating frequencies.

Search this site for "droop".
In a generation system, one and only one set will be the swing set.
The swing set sets the frequency.
All other sets will be running in "droop" mode.
Droop mode allows the sets to follow the lead of the swing set.
Droop mode also allows the grid to continue to operate should the swing set be lost.
There will be frequency deviations with load changes until the swing set is restored to service.
These changes will typically be less than 1 Hz. but in the event of the complete loss of load on a set the frequency deviation will typically be limited to about 3 Hz. That is the limit for a drop from full load to no load. Most load changes will result in much less frequency deviation.
I was system engineer for a time for a very small island utility. We initially had a combined capacity of 2.2 MW with five sets.
The system has always been run in droop mode without a swing set.
A sudden, heavy load may pull the frequency down on the whole grid system, not just on the closest sets.

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

RE: Generator operating frequencies.

On the grid, not all units can pickup/increase generation. Those are called fixed, or renewables. Other units will run below the max capability for what is called spin. Other units will be hot, but remain off-line (or additional spin) for a quick start.

Yes with a decline in frequency, the load will also decline some.

It's the concern of generation ramp rates that renewables are disliked in the utility industry.

RE: Generator operating frequencies.

Frequency events are MUCH slower than voltage changes. For example, the image below shows large frequency excursion on Feb 17th. Note that the time scale is in seconds, with a time to reach the minimum frequency of about 6 seconds. The faults that typically initiate events like this usually take less than 0.06 seconds (~4 cycles). The average rate of frequency change for this event was about 0.03 percent per second.


DanEE-I can certainly imagine utilities being a decade behind in responding to the thread of spoofed GPS signals. Although atomic clocks were available, they are an order or two in magnitude more expensive. There are an awful lot of GPS receivers that were installed for informational purposes like time stamping relay event reports. As the GPS signals are hooked to other things like "information" PMU's or provided SNTP signals, it can be hard to tell which step should trigger the upgrade to an atomic clock.

RE: Generator operating frequencies.

HamburgerHelper -

Loading rates are very dependent on the type of prime mover and in the case of steam sets also the capability of the boiler plant. The fastest loading rates are the hydro schemes - for example the big pumped storage scheme at Dinorwig, Wales can go from sync idle to 1.7GW in less than 20 seconds (all six machines pre-synchronised and spinning in air), or from standstill to full load in just over a minute per machine. No thermal plants can match that kind of loading rate, certainly on a percentage basis.

RE: Generator operating frequencies.

2

Quote:

I think this is possible if the condition is spread across a very wide range... Ie Quebec say 60Hz, Ontario at 59.87...

This may not be well-known, but [other than for one exception which I will explain later] the Hydro-Québec / Trans/Énergie system is only connected asynchronously to the rest of the Eastern Interconnection, i.e. outside of Ottawa, Ontario via the AC-DC-AC converters at the Outaouais station, between Québec and the state of New York at Chateauguay, etc.

This is not to say that synchronous connection has never been made, but it has always been inadvertent and typically will not last much more than five minutes.

The worst case scenario is that, because the connection is unstable, the flow through the tying circuit oscillates ever larger, eventually crescendo-ing to an automatic trip on overload. The other much more preferable scenario is that the inadvertent paralleling of the two systems is recognized and caught by a power system controller who independently separates the two systems as the flow through the tie point pass through zero. Experience has shown this to be the much less impactive option.

The exception mentioned above is explained in part of an e-mail I received:

"The northern Vermont local grid is divided into sub-grids that can be connected to either the Quebec or Vermont power system without interrupting service. The line 1400 is energized from Stanstead and loads are connected at Newport. VELCO regulates deliveries from HQ by switching loads between the Québec and the New England systems. This regulation mode is commonly referred to by us as "block loading".

"Since the HQ and NEPOOL systems are operated asynchronous, loads must be connected to either one or the other, but not both simultaneously (except during switching). The actual transfers are accomplished by an automatic device when HQ and NEPOOL systems momentarily drift into phase. This transfer of loads between HQ and NEPOOL without power interruption is commonly referred to as a "quick switch."

"Quick switches are accomplished by momentarily connecting the load being transferred to both sources "in phase" and then disconnecting one of the sources before the systems drift out of phase. Once initiated, it may take several minutes for the two systems to come into phase and the actual switching (connecting and disconnecting) to take place."

CR

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

RE: Generator operating frequencies.

Thanks for sharing that information CR. That certainly is a special case.
To avoid confusion can we describe this as a tie between two different systems and not parallel generators on the same system?

Quote (CR)

This is not to say that synchronous connection has never been made, but it has always been inadvertent and typically will not last much more than five minutes.

The worst case scenario is that, because the connection is unstable, the flow through the tying circuit oscillates ever larger, eventually crescendo-ing to an automatic trip on overload.
From my observations of generators being paralleled out of phase (Miswired synchroscope) I would suggest that when the two systems drift about 20 or 30 degrees out of phase the automatic trip will happen.
At different frequencies, it won't take long to get 20 degrees or 1/18 of a cycle out of step.

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

RE: Generator operating frequencies.

(OP)
I don't think 20-30 degree is necessarily too much. I have seen neighboring utility tie relays set to block reclosing at 35 degrees out of sync. That utility had a strong grid but you would only see large angles on long tapped lines that were open ended. The current surge due to angular differences is going to be dependent on angle difference between both sources and each end's thevenin impedances to their sources. If you have two generators near each other without much impedance between them, you probably want them closed in close so you don't have a lot of power swinging back and forth.

I thought this was interesting but up in Minnesota, MISO had the issue that they couldn't close in a line due to the angular differences they saw while their system was loaded. They had to sit and wait for a period where the system load decreased enough to allow them to close in the line.

RE: Generator operating frequencies.

Correct, Bill; this is a tie between systems...but the bigger the systems the closer the slip frequency between them typically is before synchronization occurs or is even attempted, most commonly in the order of < 0.01 Hz.

As noted earlier in this thread, once the two systems are tied, their frequency is the same; what causes problems from that point forward is the "stringy-ness" of the HQ system and the subsequent response differential between its units on one side of the tying circuit and the "infinite bus" of the Eastern Interconnection on the other, the combination of which leads to the excessive phase angles you are describing.

CR

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

RE: Generator operating frequencies.

Interesting, HH; I'm curious though as to why the phase angle was of such concern, since other than for closing transients the power flow would immediately re-distribute itself among the available paths. The scenario suggests there was no anticipated loss of either load or generation from placing the line in question on load...

My utility would likely have used its power system analysis tools to calculate the expected flow changes, and provided these were not egregious, I suspect we would have requested authorization from the IESO to load the circuit using synchro-check bypass rather than operate in a less secure state for eight or more hours while waiting for the phase angle to lower on its own so to speak.

CR

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

RE: Generator operating frequencies.

(OP)
Crshears,

There are power swings that occur when you close in two systems with angular differences. The angularly leading generator will initially supply power to the angularly lagging generator and speed it up and then it oscillates back and forth between the two until it settles on the steady state power flows for the new configuration. With Waross talking about close parallel units, the energy would flow quickly due to very little impedance being between them and produce large torques and currents on the generators. The angular difference here on line relaying I believe is so that line relaying doesn't trip on the current surge during the oscillations. You care about this because you set your relaying to handle unstable and stable power swings. Stable ones being the oscillations the generator and system can live with. Unstable power swings are the ones you want to trip for to prevent generator damage or portions of the system losing stability. NERC reliability standard PRC-026 gets into some of this. (https://www.nerc.com/pa/Stand/Reliability%20Standa...). The oscillations basically happen as the system migrates from the existing power flows to its new state. There is a talk that Tom Ernst gave Michigan Tech that explained an issue that he had at the utility he worked at up in Duluth, MN. The system is or was pretty weak by the taconite mines and when they would lose a unit, they would see strong oscillations back and forth as the system settled into very different power flows. I can't get the video but here are his notes on how he sets the blinders for out of step blocking on his SEL-421 line relays to prevent tripping for stable oscillations.

http://www.ece.mtu.edu/faculty/bamork/EE5223/NSM%2...
http://www.ece.mtu.edu/faculty/bamork/EE5223/115kv...
http://www.ece.mtu.edu/faculty/bamork/EE5223/MP%20...


RE: Generator operating frequencies.

I'll have a look at those on the weekend when I'm on nights; no time right now...

CR

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

RE: Generator operating frequencies.

Thank you for the clarification HamburgerHelper.
To put this in perspective for those less familiar with these issues, 30 Degrees out of step represents a voltage difference of 50% of rated voltage.
Yours
Bill

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

RE: Generator operating frequencies.

crshears - this isn't as big an issue in Ontario. The design of the grid pretty much forces standard power flows. There ARE exceptions, and you'll see them where some sort of special power swing protection is deployed (northwest had one I believe). Of course the 2003 blackout is a great example of how bad things can get when standard system impedance's get farked around with. Ontario's grid is amazingly stable when it is treated as a single power system. Since the gub-ment gone and broke it up and now one company owns this part and another company owns that part and some private dick head owns that plant the stability is decreasing. In fact I think it's easy to make the point that the single largest risk in power system security for the past 20 years has been politicians.

RE: Generator operating frequencies.

The issue with large angles isn't always a planned thing, but could have just been a relay limitation. There are two different functionalities of sync check:
* At generation sites and interfaces between planned islands, the sync check has a large allowable slip frequency, but a very small allowed angle difference.
*For other locations, the goal is to disable closing between island without disabling reclosing for a large standing angle. Hence this kind of sync check has approximately zero allowable slip frequency, but a very large allowable angle difference.

Older single function sync relays such as the Basler BE1-25 really do just ONE function. Since the BE1-25 has just checks 1 angle and 1 time delay, a single BE1-25 cannot do both kinds of sync check. In new microprocessor based relays, there are separate settings for each situation.

In response to the Southwest Blackout, transmission operators in the USA now have to address phase angle limitations as part of their next-day studies. Page 114 of the blackout report notes that the relay on the Gila-Hassayampa line was set at 60 degrees but the actual angle was 72 degrees.

RE: Generator operating frequencies.

Does not the rate of angular synchronization between two systems imply the inertia that must be handled during a connection? I would think the rate of approach of synchronizm would be as important as the actual phase angle difference. A large rate means the paralleling network is going to be forced into decelerating or accelerating as compared to little or no "catching up or slowing down" with a near zero degrees/sec connect?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Generator operating frequencies.

I had to go hunting for the answer, but yes, it does; it's referred to as the synchronizing transient, and implies a transfer of energy between the two systems through the tie circuit as the governors on the controlled generation sources self-adjust to find their new equilibrium at the resultant new frequency. In my neck of the woods the slip frequency between islands is never to exceed 0.1 Hz, and is preferably to be as close to zero as is reasonably achievable.

As an aggregate summation of what HH, bacon4life and marks1080 noted, there is a distinction between standing angles when closing loops in power systems and the rotating angle found upon synchronization of islands.

CR

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

RE: Generator operating frequencies.

crshears - I've often heard that when doing angular syncing between two parts of a network that really anything less than 90degrees will close in. It can be a rough 'synchronizing transient' but 90 degrees is the limit. Anything above 90 will result in damage to the switch gear or pole slipping or maybe even relay operation. Based on looking at a voltage/current graph with a 90 degree lead/lag I can kinda believe that statement in theory. Typically I see values between 20 and 35 degrees for syncing, which is obviously much less than 90. Do you have insight on how true that is - syncing at or close to 90 degrees?

RE: Generator operating frequencies.

(OP)
Marks1080,

The syncing is just due to the angular differences in voltages. It doesn't have to do with voltage/current lead and lag. 90 degrees is a lot between two different systems. They might be getting that number from the equal area criteria graph, which peaks at 90 degrees. Any swing beyond the angle where mechanical energy in equals the electrical energy out doesn't have any electrical/mechanical imbalances to pull the motor back to a stable point without generation controls.

RE: Generator operating frequencies.

Thanks HH - yes I'm not sure why i referenced voltage and current there. Just early in the AM i think.

I intended to ask about two voltage angles at 90 degrees.

RE: Generator operating frequencies.

Hey marks, I started composing a reply, but soon concluded that HamburgerHelper's response above covers most of what I was trying to say, and says it better. I therefore defer to his greater expertise.

I did not know that Ontario's grid tends to be more robust than in other places, and as such I may have been arguing from the specific to the general...sorry about that.

I will add though that I've been on shift when both of the 230 kV lines between northwestern Ontario and Manitoba have been out of service simultaneously and it came time to return one to service. There being no other paths because the 115 kV ties in the area were open, once the 230 kV line was on potential it was necessary to do a phase angle check across the breaker to be closed to load the line, then adjust phase shifters and generation around the loop to bring the standing angle to within limits prior to loading the line.

Both side of the open breaker were tied to the Eastern Interconnection, but since there are no grid ties between Sault Ste. Marie, Michigan and Sault Ste. Marie, Ontario, the loop was huge, viz., one side of the breaker at Kenora, Ontario > Whiteshell, Manitoba > Minnesota > Wisconsin > Illinois > Indiana > Detroit, Michigan > back in Ontario, Windsor > Chatham > Longwood > Bruce > Claireville > Barrie > Sudbury > Sault Ste. Marie > Wawa > Marathon > Thunder Bay > Dryden > the other side of the breaker in Kenora.

MISO, Ontario's IESO et al handled the situation masterfully.

CR

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

RE: Generator operating frequencies.

Yes, I am late. But Think that the discussion is quite interesting. If the grid is connected via direct links or transformers, there can not be any frequency differences across the grid. Frequency stays the same. Phase angle shifts, yes. But different frequencies, no.

The following Picture shows what the Swedish grid looked like in late January (high load because of Electric heating being used close to capacity):



There are a few HVDC links to the Danish and European grids, so not sure if the European grid looked the same. But frequency differences between different points in a directly connected grid is physically impossible.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Generator operating frequencies.

Thanks crshears for the answer you gave me above.

Gunnar; Why so many HVDC links? I always assumed they were the goto for jumping frequencies 50<->60Hz but isn't Sweden 50Hz like the rest of Europe?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Generator operating frequencies.

Yes, Smoked. We are all 50 Hz. But there is a lot of water between us and Europe. And you cant put pylons in deep sea. So we have underwater cables instead and cable capacitance is huge so most of the AC current is shunted away. The first HVDC link (with Mercury rectifiers) was installed between Sweden and the Gotland Island in the fifties because DC doesn't leak away in a capacitance. The next one, still ABB, was between France and UK, then came Sicily and, after that, Japan where the two regions (one 50 and one 60 Hz) were tied together with a DC link with zero km "Cable". I was somewhat involved in several of these plants. The first one in France, which probably is the reason why my liver now needs some rest. High-power and high-voltage semiconductors are used today.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Generator operating frequencies.

Ah, most interesting and makes good sense. Thanks Gunnar.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Generator operating frequencies.

(OP)
Skogsgurra,

System generator frequencies don't have to be the same all the time as Davidbeach and others pointed out. I suspect that area control relates closes with slight frequency deviations.

I would have a laugh if the low system frequency you showed for January was partly the result of Kosovo running energy deficits and power being exported from Sweden or being unavailable for import into Sweden.

RE: Generator operating frequencies.

HH I'm confused by that... 'system generator frequencies don't have to be the same all the time'

I kinda think they do... and the very small discrepancies that david was talking about are not discrepancies in actual frequency. its more like a VERY slight lag on a machine. Again, there is only one system freq. :) Generators should actually have a slightly higher than system freq. just before they sync to a grid, but once the sync is done the frequency of the unit has to be the same or else the smoke will get out.

RE: Generator operating frequencies.

(OP)
David was basically saying that as the relative angle swings, its frequency is greater or less than its neighbors. Slipping is a frequency difference. So, a generation that is 60.001 is slowly swinging ahead of his neighbors and controls should stop it slow it down to 60 hz before it deviates angularly too much from the rest.

RE: Generator operating frequencies.

The generated electrical energy connected to the system will have the same frequency as the system. The rotor/prime mover may not. The control system is to protect the generator. the power coming out of the unit has the exact same frequency as the grid its connected to. No slip.

RE: Generator operating frequencies.

Define “Frequency” and specify the means of measurement.

Whether or not different units can have different frequencies depends on definition and means of measurement. If I determine frequency from instantaneous angular velocity of the generator shaft I could find that every unit has a different frequency.

They all have about the same frequency and the maximum variation (assuming a stable system) is small, but they’re not all identical.

Other definitions and measurement techniques will result in different answers for the same system.

RE: Generator operating frequencies.

Define it purely in electrical terms. forget about the mechanics of the generator. When discussing 'system' frequency that is the best way to look at it.

When looking at the particulars of running and individual unit than you need to look at the mechanical aspects, in which case you're correct - every unit has its own shaft frequency - all of which should be very close to the system frequency, otherwise expect unit damage.

Electrically speaking you will measure the same frequency of any 'system' at any point of the system you choose to measure. Exactly the same the frequency.

RE: Generator operating frequencies.

(OP)
Mark1080,

You should look at the voltage angle difference as being an indicator of import or export of real power. The voltage angle between the sending and receive will be:

Power = (magnitude(Vreceiving)*magnitude(Vsending)* sin(angle)0 / impedance.

Sending real power over any impedance produces an angle difference between the sending and receiving voltages.

If you had two island grids connected by a single piece of transmission, the angle of the two islands would be indicative of who is sending and receiving power on the connected transmission. If one island was operating at a slightly different frequency, it would indicate that the voltage angles were continually advancing or lagging the other island. The frequency difference indicates the net import or export is increasing.

RE: Generator operating frequencies.

HH, you are completely wrong. Don't spread misconceptions. It doesn't help us and it doesn't help you. There is no frequency difference. There are small variations in df/dt - you may call it Frequency Modulation - but if you measure the frequency with standard frequency measuring devices, you will see the same frequency everywhere in a connected grid.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Generator operating frequencies.

(OP)
No, I am not wrong. I overstated how much I thought there might be frequency deviation on the system. There are frequency deviations on the system usually on the order of 1/1000. I sit next to a guy who has worked in generation plants for 20+ years and he has said this much. There is always error on the systems otherwise Balancing Authorities would not be using ACE or LCF to try to balance their areas. No one can guess their loads exactly. I am guessing Sweden is so small and tight that deviations would be harder to notice.

RE: Generator operating frequencies.

HH it's time for tough love.
Either you have seriously misunderstood what the guy has tried to tell you or he has given up trying to explain and tells you what you want to hear.

Quote (HH)

System generator frequencies don't have to be the same all the time as Davidbeach and others pointed out.
You have misunderstood this also.
Yes the frequency may vary, but it won't be at different frequencies at different parts of the grid at the same time.
Any frequency deviations affect all the generators on the grid.
We have tried to explain to you politely and diplomatically that generators tied to the same grid must all run at the same frequency.
Diplomacy and politeness just don't cut it with you.
What part of you are wrong don't you understand?


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

RE: Generator operating frequencies.

I worked 22 years on the generation side of an electric utility and have witnessed grid frequency "force" an unsynchronized generator into compliance with grid frequency. On start up, operators would get an outdated manually operated unit as close as possible to synchronous speed and let the grid "lock it in".

The most dramatic was a 280 MW unit that was accidentally motorized from turning gear speed toward synchronous speed... it never made it. Destructive vibration, a hydrogen explosion, and lube oil fire took it out. I did not see this event but participated in the 2 year recovery.

A recent example of a large grid slowly having frequency dragged down has been going on in Europe this year.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Generator operating frequencies.

It's easy to get blinders on in this industry. Happens to me (heavily focused on P&C) all the time. It's helpful to take a step back from time to time and look at Power Systems as a discipline all on its own. If you're experience is 30 years of being in a plant I get that the concept of a single Power System frequency is hard to grasp. Hell, this shit is hard to grasp all by itself.

Step back and look at the Power System as a single entity and it really will become very clear that anything but a single power system frequency just doesn't make any sense at all. This has nothing at all to do with what's going on with the mechanical side of any generator, but ALL generators that are connected to a power system will have an output (or input if motoring) frequency equal to the power system. The only discrepancies in measured frequency will be due to the imperfections of the measuring device itself. If you can actually measure two distinct frequencies it absolutely means you have two separate power systems.

RE: Generator operating frequencies.

Quote (SlideRuleEra)

The most dramatic was a 280 MW unit that was accidentally motorized from turning gear speed toward synchronous speed... it never made it. Destructive vibration, a hydrogen explosion, and lube oil fire took it out. I did not see this event but participated in the 2 year recovery.

That sounds like a really bad day at the plant followed by two years of hell. Ouch!

RE: Generator operating frequencies.

This is how I think of it - Imagine a perfect system where the loads, generation, voltages, currents, and impedances never change. Also imagine the grid is at 60Hz and a connected generator is running at 60.0000000001 Hz and they start out with their voltages exactly in phase.

The period of one cycle for the grid is 1/60

The period of one cycle for the generator is 0.016666666666639 seconds

One degree at 60Hz is equivalent to (1/60)/(1/360) or 1/21600 seconds

For one degree (at 60Hz) of phase difference to accumulate between the grid and generator, the grid and generator would have to run at their respective frequencies for:

(1/21600)/(1/60-1/60.0000000001) = approximately 1.70068 billion seconds, or almost 54 years, and even longer before stability becomes a concern - but it eventually would.


But who cares - we are talking about the real grid not this imaginary one and such precise measurements aren't possible or meaningful since things are constantly changing. Different machines will approach new steady state conditions after system changes at different rates depending on their inertia, but these are transient phenomena and I don't think it makes sense to consider them in the steady state frequency. Again, it all depends on how you define and measure the frequency.

I think everyone (including those arguing) is saying the same thing, actually.

RE: Generator operating frequencies.

Given the fact that grid frequency is constantly changing at least +/- 0.1 Hz, I really cannot see that such an academic example would bring clarity to the thread. And I also disagree that everyone in this thread are saying the same thing. Some know and some think they know. And some have buddies that have told them lies. How to measure frequency is well defined in many text-books. There are, to my knowledge, just two ways of doing it.

There cannot be opinions on this matter. Only facts. Let's put an end to this thread now. Or kill it.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Generator operating frequencies.

Perhaps try looking at this from another perspective:

If measured with sufficient accuracy the angular velocity of any individual generator at a precise instant may not exactly match that of all other machines on the system. For example as a generator's output increases, its load angle relative to the system also increases. For the load angle to increase there has to be a transient increase in angular velocity, i.e. for a finite period the machine is 'faster' than the system until it reaches its new stable load angle for the increased load. As a generator reduces load the load angle relative to the system also reduces and for a finite period the machine runs 'more slowly' than the system until it reaches a new load angle.

Even when machines have a stable power output, changes in load and load-flow on the system itself will cause minute variations in load angle of individual machines. As the load angle changes then by definition the machine has to undergo a transient acceleration or deceleration relative to the system in order to reach the new load angle.

During faults the machines closest to the fault can see significant swings in load angle as the AVR works to keep the machine in synchronism, with the machine initially accelerating and pulling ahead of the system. For acceleration to occur the instantaneous speed of the machine has to be higher than that of the system, but for the machine to stay synchronised the AVR then has to boost the field sufficiently that the acceleration is checked and is followed by a period of deceleration relative to the system. If the machine goes past its stability limit then it breaks synchronism and will pole-slip at a frequency faster than that of the system until hopefully the protection operates and trips the machine.

I don't personally believe that the transient acceleration and deceleration associated with changes in load angle equates to the machine experiencing a change in frequency, but I can see how the confusion might arise.

RE: Generator operating frequencies.

(OP)
If you want to say that the entire system is always dead balls the exact same, that isn't true. Any time load or generation increases, you will see the voltages lead or lag when compared to whatever you want to use as reference. Dispatch couldn't change if voltages couldn't change angles compared to whatever reference. The amount that frequency deviates due to just dispatch isn't much and if you do a back of the napkin calculation with ramp rates and maybe a 90 degree swing in voltage angle (going from load to generation), you can see the order of the frequency change. There has to be frequency change as you add generation or load or the voltage angles couldn't change. If you want to look at something more abrupt, take a look at when load or generation is added or tripped out in blocks. The entire system doesn't see the same frequency excursions as those right next to the event. When there were power swing issues up as I mentioned earlier up by Duluth, the rest of the eastern interconnect wasn't going goofy. In my opinion, generators can do whatever they want frequency-wise as long as they never deviate so much angularly that they start slipping poles. The angle is the issue not frequency. You run fast or slow, you are ok as long as you fix yourself before you go too far. I believe if someone created a contour map with the frequencies of every bus, scaled like from 59.999 to 60.001, someone would be able to clearly see where loads and generation were changing.

RE: Generator operating frequencies.

Frequency changes all the time HH. But it changes equally throughout the grid... because... there can only be a single system frequency in a single system. This has NOTHING to do with any single generator angular anything at all. You can set all the units in your grid to all run at different output frequencies if you so choose, but you still only will measure a single system frequency, and end up with a bunch of destroyed generators. I'm going to refrain from adding comments about harmonics into the discussion, because it's just going to confuse things further.

I've seen such frequency 'maps' that you're referring to for the entire north eastern seaboard. There is one system frequency. The only deviations you will see are measurement errors, and I've never personally encountered measurement errors, only exactly the same frequency readouts throughout different parts of the grid... well except when the grid starts to split into smaller grids... than you see the freq. readouts deviate, but that's because it's no longer a single system.

THE ENTIRE SYSTEM FREQUENCY IS ALWAYS DEAD BALLS THE EXACT SAME. If you're going to claim otherwise please provide references. ;)

RE: Generator operating frequencies.

A couple comments:
Frequency/speed as a turbine setpoint causing confusion.
Some turbine governors use frequency or turbine rpm as the control set point. On the Woodward unit we enter a 3600 rpm setpoint for a 60hZ 2-pole generator to run at synch speed, no load. The internal voltage of the generator and the system voltage are in synch and at 0 degrees apart. To load up the generator you increase the "speed" setpoint, the throttle opens, more steam goes into the turbine, the internal rotor angle increases and power flows to the system. If the unit was set for a 4% droop, the 100% load setpoint would be 3600/1.04= 3744 rpm. Shaft and generator would still be spinning at 3600 rpm and 60Hz, but the control system is calling that throttle position 3744 rpm (62.4Hz).

Hamburger Helper mentioned that load decreases as frequency decreases. That is true if the load is predominantly induction motors driving pumps, fans and other load whose horsepower requirements vary with speed/frequency. The motor slows down and the pump/fan output drops. If load is mostly computer power supplies, servers, electric heat, vehicle chargers and variable speed drives, load stays constant as frequency changes. The individual power conversion modules adjust to maintain the load output. If the power system stability models are based on outdated historical data when industrial motors were the major load, the simulations will overestimate the load drop during a low frequency event.

20-30 years ago, the system was partially self-correcting. During an event, as frequency dropped due to generation/load imbalance, load would drop, helping the correction. The steam driven turbines and hydro units would maintain their MW output until the governors reacted and increased MW to rebalance load/generation. Selected steam turbines were operated at less than 100% throttle to be able to increase output quickly.

Compounding this load vs generation imbalance due to constant loads is the preponderance of combustion turbines in today's generation mix. A combustion turbine's MW output varies with the square of the turbine speed. In the first seconds after a loss of generation event when frequency drops, load stays relatively constant while combustion turbines' outputs drop, exacerbating the problem. Governors eventually kick in and boost generation to pull frequency back but that initial dip is more drastic than a decade ago. (Ignoring other advances in system control).

When coal and nuclear driven steam turbines were the major power source, system operators could count on the droop response of the governors to correct a frequency dip. Many of the steam turbines operating today are combined cycle units running at "valve wide open" to turn every bit of steam from the combustion turbines' exhaust heat into MW. With the throttle already at 100% there is no way to quickly increase steam turbine output in response to a frequency drop. The gas turbines have to increase fuel input, increasing the exhaust heat flow to make more steam to increase the steam turbine output. Only the combustion turbines have a true droop response to a frequency drop.

What sort of droop response can we count on from a solar panel or wind turbine? Zero.

This creates some interesting issues for system operators.


RE: Generator operating frequencies.

I will chime in. After reading reports on many blackouts and watching voltage collapse simulations in power world, it is possible to have differing frequencies in an unstable system with interconnected with AC lines. Maybe not 59 vs 61 HZ, but something like 0.005 is possible.

RE: Generator operating frequencies.

Mbrooke; For how long? Or, for how many angular or electrical degrees?

Quote (First Post)

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.
It can't for more than about 1/4 of a cycle, or 4 milliseconds.

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

RE: Generator operating frequencies.

[edit: entire post re-worked]

As I'm only a power system operator/controller and not an engineer, and as math is not my strong suit, I've had to dumb this down for myself and think of a hugely finite grid - like the Eastern Interconnection - as one honking great matrix of line shafting arranged in loops and spurs, with all of its tapped [synchronous] generation and loads solidly connected to it by either solid shafting or through solid gearing, meaning with no intentional slip built in.

The speed of one great honking line shaft matrix like this 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. 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 the same. The only way for there to be a 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.

If you wish to think of a generator's [or motor's, or grid portion's] speed as "changing" while the amount of twist in its connecting shaft varies, more "power" to you; but I for one don't view it that way.

As to the mechanical analogues for the various components attached to the system, induction generators can be viewed as connected to the matrix by a fluid coupling, with the result that the only way power can be transferred from this generator to the matrix is by increasing its energy input so that a difference in speed develops between the machine and the matrix, else a power transfer will not occur.

The same rule applies to induction motors but in reverse as the speed of an induction motor will deviate from the speed of its supplying grid in direct variation with the applied load.

Phase shifters [ aka quadrature boosters ] are like spline boxes consisting of two concentric cylinders with left- and right-hand slots resembling the “rifling” of a gun barrel cut into them; by means of an axially shifted hydraulically driven pin and collar, the angular relationship between the two sides of the device can be adjusted. Such adjustment alters the amount of energy transmitted via that particular path by varying the torque, but has no effect on overall matrix speed.

In a station tying together two grids operating at different frequencies, all but one of the frequency changers is commonly equipped with stator shift gear to facilitate the sharing of load between the machines.

As has been noted previously, solar and wind farms are like gate-position-controlled sources of energy input to the matrix, and as such have no speed droop capability associated with them, therefore contributing no stability to the system, which is why these resources are the last to be permitted to re-synchronize to the grid during power system rebuilding and restoration following a grid collapse.

CR

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

RE: Generator operating frequencies.

(OP)
If anyone wants to see real synchrophasor data, Dr.Grady puts out reports on synchrophasor data on mostly ERCOT and a little on the other two interconnects.

https://web.ecs.baylor.edu/faculty/grady/Texas_Syn...

The 2017 report is note worthy because wind generation in ERCOT got up to 45% of the total for a few days in January. Aside, from the one unit trip, you can see the effects that a large penetration of wind generation ramping up and down has on his synchrophasor voltage, angle, and frequency measurements at his sites in ERCOT. The frequency measurements differ at each site, mostly during when wind generation is ramping up or down or something else like a unit trips that abruptly change power flows. Other reports and studies make note of things like loss of a DC tie, mysterious frequency ringing, generator trips, and frequency variations during the super bowl in each of the interconnects. It is worth checking out just to see real synchrophasor data.

RE: Generator operating frequencies.

There are people with three arms (not talking about Zaphod B. now) but that doesn't prove that humans in general have three arms.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Generator operating frequencies.

HH,

I suspect that minimum frequency data in that report is measured on a single-cycle basis, so during significant swings in load angle there will be isolated cycles which are slightly short or slightly long giving anomalous frequency readings. The report itself alludes to angle changes in the comment on page 6 relating to the trip event.

I'm not completely in agreement with the author's use of the term 'minimum frequency' to describe these single-cycle events.

RE: Generator operating frequencies.

(OP)
ScottyUk,

Go read the other reports. There is ringing across the system and other events that don't resolve themselves after a quick changes in generation or load. It is interesting reading even if you don't agree with me. There are bunches of cases where the generators keep on oscillating in respect to each other minutes after a quick change. In my opinion, there is likely more odd stuff that happens on the grid that previously went unnoticed or was assumed didn't happen due to not having synchrophasor data, time stamped voltages and currents over large regions. As hard as wind generation can ramp in ERCOT, (+/- 1 GW 10 minute ramps), it isn't very surprising to me that Dr.Grady watches the system closely during high wind penetration.


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 or the whole interconnect being in real trouble. Now I believe that large block changes in load or generation could sag the frequency quickly to beat the generator ramp rates and cause it to under frequency trip without the whole system going down. The oscillography I saw when a load got switched in likewise was not felt the entire region. Maybe, this isn't a profound question but just something I hadn't thought much about before that generation can co-exist at different frequencies ,often very slightly different frequencies, so long as the deviations don't produce an angular difference to cause stability issues. So you can have odd things, like generators ringing for minutes without them causing any issues in the system.


I do find it kind of curious the definition of frequency is somehow the crux of this. If I said the phase angles of the system voltages will sometimes change relative to each other, everyone would be in an agreement. If I said that the frequencies of each generator will be different relative to each other during a change, some I guess will disagree. This is all while most of us are using microprocessor relays that operate using subcycle fourier filters to screen out the fundamental and various harmonics.

RE: Generator operating frequencies.

Grid frequency is a measure of the number of cycles of alternating current in one second.
Each cycles lasts approximately 1/60 seconds or 0.01667 seconds.
Short term measurements of frequency in the order of cycles may be reported as a frequency excursions.
Let's go back to the line shaft analogy:
Imagine that the line shaft drives the various loads through sheaves that have resiliant hubs, so that as the torque increases the resilient hub will allow as much as one quarter of a turn lag as the load/torque on that sheave is increased from zero to full load, the angle of lag increases up to one quarter of a turn.
Now someone applies very sophisticated instrumentation to each sheave to measure the instantaneous speed of each sheave.
He will be able to prove that as the load is increased or decreased the speed of the individual sheaves changes in relation to the speed of the line shaft.
"I just have a hard time grasping why a generator sheave can operate for example at 59 RPM while the rest of the grid line shaft is humming along at 60 RPM."
It may be technically correct to report phase shifts as frequency excursions but it is misleading.
Your synchrophasor data is valid data but please put it into perspective.
Look at the duration and magnitude of the frequency shifts that the synchrophasor reports and calculate how many actual degrees of lead or lag that it relates to.
How many times have you seen accuracy reported as"accuracy plus or minus x.x% plus or minus one count"?
There is a loose analogy here where you are extracting a digital value from analog data.
Not wrong but very misleading.
Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Generator operating frequencies.

<tangent>
My Grandfather had a business card designating him as
Superintendent of Phase Change
for what was then Niagara Mohawk
in upstate New York.

My Dad explained that it wasn't really 'phase' they were changing,
but grid frequency, as a bunch of smaller grids were integrated,
and not coincidentally changed from 25Hz or 50Hz to 60Hz.

When I began to understand what that meant,
I asked about instantaneous frequency
and Dad explained that the system guaranteed
the correct number of cycles in a day,
but not necessarily the correct number of cycles in a given minute.

One of the unobvious operations associated with the 'phase' change
was replacement of affected customers' clock motors.
For reasons of his own,
perhaps just for the amusement of a curious youngster,
Gramps had accumulated several large boxes of useless
but perfectly good Telechron clock motors.

</tangent>

Mike Halloran
Pembroke Pines, FL, USA

RE: Generator operating frequencies.

Hi Mike
I worked for a time in a very old facility in Toronto. We decided to reactivate a part of the plant that had been in disuse for years. I did a survey of the equipment in place.
I found a number of motors still bearing the tags indicating that they had been rewound from 25 Hz to 60 Hz as part of the Canadian harmonization with your Grandfather's project.

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

RE: Generator operating frequencies.

The line shaft may not be the best analogy. Over a large interconnection there can be inter-area oscillations such that one area is slightly slower than the other at one moment. The slower area speeds up and the faster area slows down. Both overshoot. The pattern continues. The line shaft would have a fair amount of torsional flexibility.

Over a long period both areas/ends have the same number of accumulated cycles/revolutions but at any instant they may not (probably won’t) be the same if measured over a sufficiently short time with a sufficiently high accuracy.

Measuring the interval between every positive zero crossing will give a result with more variability over the whole system than measuring the interval between 60 positive zero crossings and calculating frequency from that.

RE: Generator operating frequencies.

"The line shaft would have a fair amount of torsional flexibility."
This was my attempt to address that issue David. "sheaves that have resilient hubs"
I think that we are in complete agreement.

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

RE: Generator operating frequencies.

I just want to observe that this has been quite an interesting discussion - one of the better threads on Eng-Tips. smile

HH -

Being from a generation background I'm familiar with how our machines interact with the larger system; I haven't worked in transmission so I haven't seen the same effect on an inter-area scale although I can see how it could occur in a large system with long interconnectors. In the UK with its geographically small and heavily interconnected grid I'm not sure I would ever see it.

Our relatively small grid can see the effect of a major event such as loss of a large generating station. When the 4GW coal-fired station at Drax tripped in the early 2000's, taking out about 8% of the overall system infeed, we saw the effect right across the country with a significant drop in grid frequency being the result. Our turbine shaft speed measurements tracked the frequency as it fell and slowly recovered.

RE: Generator operating frequencies.

Agreed, Scotty; good thread.

Quote:

So you can have odd things, like generators ringing for minutes without them causing any issues in the system.

What is generator ringing? I am unfamiliar with this terminology.

CR

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

RE: Generator operating frequencies.

Answer: Swinging back and forth (around pole angle zero degrees) after a shock load change.

Re best thread: It could have been an interesting thread if it hadn't started with the question "..one of the generators to be at a lower or high frequency than the rest of the generators in the system.."

That still is not possible. Transient pole angle deviations, yes. Different frequencies, no.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Generator operating frequencies.

Ah! That! I had that happen once when I did a "bad" unit synch; the three units being put on line did this wum-wum-wum thing at gradually decreasing intensity until it disappeared altogether. My instructor said that was "for sure a rough shot; but if it stays in, it's good."

Thanks, Skogs.

Quote:

It could have been an interesting thread if it hadn't started with the question "..one of the generators to be at a lower or high frequency than the rest of the generators in the system.." That still is not possible. Transient pole angle deviations, yes. Different frequencies, no.

Although I agree you do have your facts straight, I must politely disagree with your sentiment; to me what made this thread interesting was the arguments brought to bear in support of the argument, however incorrect the premise was.

CR

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

RE: Generator operating frequencies.

Done that, too!
A 40 MW hydroelectric unit, pressed button when synchronoscope was at noon. No-one had told me that the breaker took a couple of seconds to close. So I was at tea time when it finally happened. But no wum-wum. Only a Big Bang and then silence.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Generator operating frequencies.

(OP)
Crshears,

The initial question of 59 hz or slip frequency was off by a magnitude. I only had relay settings as reference as to what could be allowed. By post four, the question became "why am I under the impression different frequencies can exist on the system?" Any post going back to "59 hz" was basically beating a dead horse since that likely would be too much of a deviation unless the whole grid is slowing down as well. Maybe, that is why the relay settings were so low. Maybe, to give plenty of margin during blackstart when trying to balance load and generation. The only fault of the initial post is the magnitude, the rest is valid.

RE: Generator operating frequencies.

(OP)

http://prorelay.tamu.edu/wp-content/uploads/sites/...

Fig. 06

No trips for various regions to not exacerbate grid problems.

That answers my relaying question. Each interconnect has no trip zones for generation as part of PRC-024 with very wide bands to prevent tripping of generation from making problems worse.

RE: Generator operating frequencies.

I am late for this discussion but I would like to offer my interpretation for my own understanding. If I am not correct I would expect the experts on this site to chime in and correct me and hopefully I can take this opportunity to learn something.

It is my understanding that there can only be one system frequency. What I believe the misunderstanding for HH is the machine rotational frequency vs system electrical frequency. The system electrical frequency is the "infinite bus" since it characteristics do not change regardless of the power supplied or consumed by any device connected to it.

Once the generator is connected to this infinite bus, the stator frequency is fixed. If there is a disturbance in the system, the disturbance will alter the internal magnetic interaction in the machine and cause an increase or decrease in the machine internal power angle. This will result in acceleration or deceleration of the machine rotor only, which is explained by the swing equation. However, this does not mean that the machine terminal frequency has changed. It may be beneficial in this case to separate the two and think of the stator as fixed and unmovable part once connected to a large power system and rotor as controllable part that needs to increase or decrease its speed during disturbances to maintain stability before major damage.

As you also noted HH, it is true that the angle between terminal voltages is largely responsible for real power flow but that does not mean that the frequency is different between the two. In fact, there cannot be any real power flow if the frequencies are not the same.

One simple analogy that may be beneficial is thinking of yourself on a bicycle connecting to a large moving train via a rubber band connection. If you do not want to lose the train than you need to make sure you are at the same speed and moving in the same direction once the connection is made otherwise the band will break and you lose the train. Once connected however, you are not going to change the speed of the train with your bike. If the train starts to move up the hill at its constant speed you need to start to pedal harder since the rubber band will begin to stretch and potentially break. If the terrain makes your bike increase speed than you need to slow down before you crash into the train. But in any case, no matter how hard or slow you pedal you will not move that train.

I hope this help clear at least some of the confusion. Take care!

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic ù and this we know it is, for certain ù then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". û Nikola Tesla

RE: Generator operating frequencies.

3
Nope. The infinite bus is a reasonably useful high level approximation during reasonably steady conditions but does not actually exist.

Again, it all goes back to how "frequency" is defined, there are many possibilities, from instantaneous angular velocity of the generator shafts to averaging a multitude on positive zero crossings. The tighter the measurement, the greater the variation across a large interconnection and the more different "frequencies" within that interconnection. Within a stable system there are limits to how much variation there can be and how the df/dt at various locations must relate to each other. But there are variations. Small, sure (no, not 59Hz in a 60Hz world, there in lies madness), but real none the less.

When Palo Verdi trips the Arizona area slows down more and faster than the Pacific North West. No place is still at 60Hz, or what ever the pre-event frequency was determined to be, but there are a range of frequencies across the interconnection. The South West and the North West probably start oscillating against each other for a while. Each slower and then faster than the other as things come into tighter and tighter stability. As long as none of those frequency changes are enough to push anything out of stability all is good. The ending frequency isn't the starting frequency. It can take seconds or minutes to come to a new equilibrium.

Don't confuse stable with quasi-stable. In a "stable" system, defined as one in which the machines don't lose synch, the system frequency isn't actually stable; it moves all the time and different parts move differently. The system frequency is quasi-stable, it keeps tending back to the mean and doing so with sufficient strength that everything holds together. Lots of calculations can safely ignore the actual frequency and instead assume a fixed system frequency and a variable rotor angle. An extremely useful fiction, makes lots of simulations solvable. But just because it's useful, just because it makes the calcs much easier, doesn't mean it's true. Close enough for all purposes if the system hangs together and if the system falls apart it doesn't matter.

Define frequency as the average number of positive zero crossings per second measured over the past hour on a sliding window, and sure the whole system has the same frequency everywhere to a phenomenal degree of accuracy. Define frequency based on instantaneous angular velocity of the shaft over the past 4 milliseconds and with enough resolution you'll find that every machine on the system has a different frequency, including units sharing the same GSU.

Nobody's been willing to define what frequency is. Is it like the definition of pornography from one former supreme court justice "I know it when I see it" and nobody can actually define it? Or are we willing to arrive at a consensus definition, including measurement techniques and time resolutions?

Much of the back and forth here to fore in this thread could be reduced if we could place one statement in one measurement bin and a different statement in a different measurement bin. We argue like we know what the other posters are talking about, but maybe we don't. Maybe the "there's only one frequency" crowd is is total agreement with the "there's lots of frequencies" crowd because they're talking two different thing. "Well if that's what you mean by frequency, well, then perhaps you have a point...".

We may still be the blind men arguing about how an elephant is to be perceived. It all depends. Well, for whether or not there's one precise frequency or a bunch of different frequencies that all smudge around a common value it just depends. It's both, or it's neither. Or, it's somewhere in between, it just depends.

RE: Generator operating frequencies.

David, should we be talking about frequency error times time?
The greater the frequency error, the less time it may persist without dire consequences.
You will know the exact figure better than I but I suspect that any frequency error that persists until the phase angle difference at the point of common coupling is greater than 90 degrees will be in danger of pole slip.
If we are to consider frequency on a cycle to cycle basis then we must also consider the resulting angular displacement and the time limit until angular displacement becomes too great.
Respectfully

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

RE: Generator operating frequencies.

David, your attempt at philosophy is wasted in this thread. You just created cunfusion and even misled Bill. David, say aftewr me:

There cannot be different frequencies at two ends of a good conductor - no matter how long it is!

The vector's angles can vary. But never the frequency.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Generator operating frequencies.

I'm okay Gunnar. Just having trouble expressing myself.
As the vector angles are changing, the period of the cycles will be either a little more or less. Any instrumentation that determines frequency by measuring the period of each cycle and expressing the reciprocal as frequency will report a short time (very very short time) frequency deviation while the angles are actually changing.
I would characterize this as a phase shift mis-reported as a frequency difference.

How about?
When the frequency is determined by taking the reciprocal of the period of a cycle, there may be small errors in the reported frequency while a vector angle is changing. These errors will be cancelled out by similar errors of opposite sign as the vector angle returns to its original position. The magnitude of the error will be proportional to the degrees of the vector angle shift and inversely proportional to the duration of the shift.

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

RE: Generator operating frequencies.

The vector angle is the thing.
An incoming set may be several cycles off the grid frequency and still sync successfully when connected to the grid. (After which the set frequency will equal the grid frequency.)
If the vector angle is too great, bad things happen.
All of the examples of different frequencies are a snapshot of one instant in time. If that condition continues for more than a handful of milliseconds, bad things will happen.

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

RE: Generator operating frequencies.

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

RE: Generator operating frequencies.

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.

RE: Generator operating frequencies.

3
(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?

RE: Generator operating frequencies.

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... bigsmile

CR

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

RE: Generator operating frequencies.

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?

Quote (Original Post)

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.

Quote (David)

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

RE: Generator operating frequencies.

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

RE: Generator operating frequencies.

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]

RE: Generator operating frequencies.

Quote (HH)

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

RE: Generator operating frequencies.

(OP)
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.


RE: Generator operating frequencies.

Quote (HH)

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

RE: Generator operating frequencies.

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]

RE: Generator operating frequencies.

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.

RE: Generator operating frequencies.

Quote:

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

Yes.

Quote:

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]

RE: Generator operating frequencies.

(OP)
"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.

RE: Generator operating frequencies.

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

CR

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

RE: Generator operating frequencies.

(OP)
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.

RE: Generator operating frequencies.

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

RE: Generator operating frequencies.

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.

RE: Generator operating frequencies.

Quote (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

RE: Generator operating frequencies.

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

RE: Generator operating frequencies.

It has been stated by me and many others that the frequency is constantly changing. Your question was about different frequencies in different places in a connected grid.

Are you doing this only to provoke us?

If that is the case, you have certainly made it. But you have obviously not learned anything from it.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Generator operating frequencies.

Who said anything about nothing changing? Every power grid changes constantly! When a 100 MW electric furnace kicks in on a 20 GW system, a step load of 0.5% is applied and a corresponding speed change of 0.5% will immediately occur...to which each and every speed-droop-governor-controlled power source on that grid will promptly respond to arrest the frequency decline.

Now take that 20 GW system and parallel it with, say 180 GW of other generation; you will now have a 200 GW system which will change speed by 0.05% in response to that same step load, one order of magnitude less, greatly smoothing out that entire interconnection's frequency excursions. What will now be noticed to change when said step load is applied is the tie line flows to neighbouring systems, while the frequency will barely budge from a nominal 60 Hz. This is the reason why all of the entities in an interconnection will always choose to stay connected whenever possible.

The point is that the frequency of the entire grid and all of the synchronous generators and motors connected to it is [macro] changing at once; the nature of the torque angle between each rotor pole and stator pole, viz., that varies directly as the amount of power transferred, does not permit of any other behavior [ see pg. 289, section 16-4, Electric Circuits and Machines, Sixth Edition, by Eugene C. Lister, Chief Electrical Engineer [Retired], Stanley Consultants Ltd., and Life Senior Member of the IEEE ].

If one were to couple a 300 MW steam turbine to a 100 MW alternator, synchronize it to the local power system at the correct voltage and frequency, then attempt to overdrive the alternator, the machine's torque angle would eventually exceed 90°, at which point the machine's pullout torque would be exceeded as well, and it would either go out of step and overspeed or continue to operate as an induction generator [ibid]...until either its protections or its human minders removed it from service.

Hope this helps.

CR

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

RE: Generator operating frequencies.

2
Ok, yes, somewhat provocative, but for a good reason...

Measurement definitions and techniques matter. There have been a lot of absolutest statements about a single frequency on any system of any size without any definition of how that is determined. There are valid definitions that could make that true under essentially all conditions. There are other, equally valid definitions, such as how relays measure frequency, for which that isn't true.

On a system of sufficient size steady state doesn't exist, at least under certain reasonable definitions. If you were to inquire of every line relay in WECC what the system frequency was you'd get a smear, not a point, nor a scatter plot. The smear is a tight grouping of different values, a scatter plot would allow observation of multiple distinct points. To give the smear/dot/scatter plot two dimensions plot df/dt vs. f.

On reasonably sized systems the smear is so small that it can't be distinguished from a point. On larger systems there's always so much variation that it can never be close to looking like a point.

There's an awful lot of running generation that doesn't have any governor response. Run of the river hydro is run based on water flow and ignores system frequency. A lot of base-load generation is run at constant MW output regardless of frequency. Wind and PV just do their own thing and don't give a whit about any needs of the larger system, they just want some, any, system frequency to track. Those units paid to have governor response are the ones to respond to system frequency deviations; the dispersion of governed units is not uniform across the system. Keep in mind, I'm referring to a 150GW (yes GW) system that includes over 121,000 miles of transmission line. Small, islanded, systems need every unit to be governed but on the big system most aren't.

System frequencies, within a tight but ever shifting band, are always tending toward the same value. But different parts of the system are doing different things, some places df/dt is positive and other places df/dt is negative at the same instant. If I plot our system frequency for the past week I get a very wide, very fuzzy "line" that pretty much shades in the whole band between 59.98 and 60.02 with lots of excursions to and beyond 59.96 and 60.04. But if I plot that on a scale from 0 to 100 I see a line that has an occasional pixel that deviates from a straight line right at 60.

There can be no standing differences in frequency, I've never meant to imply that, but (using a relay definition of frequency anyway) there's also not a single frequency everywhere at all times. At least not one that would plot as a nice, neat, dot.

Using definitions other than what the relays do, to measure frequency it might well all be the same frequency. But if the relays all, system wide, always measured the same frequency the approach to under frequency load shedding would have to be very different than what it is.

Same frequency with different phase angles or different frequencies - it's just different paths from the same starting point to the same ending point, all within well defined constraints. A 50ft view vs. a 35000ft view. Just define the criteria.

If you're looking at motor speed at various locations it is certainly one one frequency. If you're analyzing an underfrequency load shedding scheme it's a multitude of frequencies at a multitude of locations. Plot them all and you get that same smear.

The truth isn't dogmatic, but rather pragmatic. Statements that can't be proven to be anything other than true at certain system scales don't hold as well at much larger scales. Einstein proved Newton "wrong" but Newton still gives perfectly good answers within most conditions. Similarly, within lots of systems "one frequency everywhere" is an accurate answer to any degree of precision possible, but that doesn't mean that it still scales to major interconnect levels.

Sure, at steady state it's all one frequency everywhere, but eventually the system is too large to have a steady state. The major interconnects fall into that too large for steady state category.

RE: Generator operating frequencies.

@David- Very well said and I think this ties many posts here together, even those disagreeing.


And while I don't want to get in the middle of the slaps so to speak: I personally enjoy hearing both sides to this argument. I don't see anything elementary or proactive. Rather I am learning and thinking about this in ways I never thought about in depth.


Also, if this forum had a rating system, I would give this particular thread 5 stars.

RE: Generator operating frequencies.

Sorry, David. The OP did not have such subtleties in mind when he put his question. He just couldn't understand how there could be different frequencies, in a very general way. He also gave an example that no-one can stand behind. Quote: "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"

This whole damn thread has confused more than it has helped. For me and, luckily, a few other engineers, it didn't do more than stir up emotions. For many others, engineers or not - I really don't know what to think, it cannot have done any good either. But those participants seem to value fine phrasing more than actual facts and understanding. Are you all under influence of Mr. Trump? Has it gone that far? If so, I feel very sorry.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Generator operating frequencies.

OK, I got somewhat carried away. But, still, there is just one frequency across the whole system. And someone that presumed that my examples from Sweden shows one single frequency because it is such a small grid should be aware that the distance between the two end-points in the Scandinavian grid is like Miami, FL to Ottowa.

I cannot understand what is meant by "I don't see anything elementary or proactive". It is the "proactive" I don't get. I understand proactive as in proactive maintenance. But cannot understand what it means here.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Generator operating frequencies.

My mistake- I meant to type provocative but auto correct failed me again lol.

RE: Generator operating frequencies.

Ah, that makes a lot more sense! I was really confused there.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Generator operating frequencies.

I could certainly understand!

RE: Generator operating frequencies.

Quote:

davidbeach wrote: There's an awful lot of running generation that doesn't have any governor response. Run of the river hydro is run based on water flow and ignores system frequency. A lot of base-load generation is run at constant MW output regardless of frequency.

David, in my experience the turbines of ALL nuclear units I've ever encountered [ and in my world they all operate as base-load plants ] have governors, not valve positioners; and the way it's been explained to me is that when such a unit is in "turbine follows boiler" mode, the turbine governor's speeder gear is automatically and integrally adjusted by steam pressure so as to hold it steady, while the action of the governor itself provides proportional response to system frequency.

In the run-of-the-river plants I know of, all but the very smallest of hydraulic units have governors, most of them manufactured by Woodward. Speeder gear is adjusted to alter the unit water flow, but again the action of the governor itself provides proportional response to system frequency.

All the conventional [ meaning not combined-cycle ] thermal plants I worked in had turbine governors.

Not having worked in combined-cycle plants, I cannot speak to what types of governors or input control such might have.

In my world there is therefore a much larger percentage of governed generation on the system than in yours.

CR

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

RE: Generator operating frequencies.

Just re-read this entire thread; what a ride!

Thought I'd comment on some of the things said along the way...

Quote:

HH wrote: This I thought was kind funny. Kosovo has been running area imbalances and has been slowing down the clocks in Europe this year by as much as 6 minutes between January and March.

Having become cognizant of how important definitions are to this thread, here's my response:

This is a result of an entire power grid's or interconnection's common macro-frequency; if one control area neglects its responsibilities, fails to load up its generation, and instead relies on its neigbors to carry it through peak periods, that entity's ACE [ area control area ] will float into seriously negative territory, that entire interconnection will be under-generated, its frequency will drop below standard, and for the duration of that period its time error will accumulate. Do this enough times in a row, and a cumulative system time error of six minutes is not at all beyond the realm of possibility.

Quote:

HH also wrote: 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.

As it's been explained to me, neither of the foregoing is correct; the cited primary reason for employing UFLS is to save the interconnection by shedding load in response to declining frequency so as to crudely arrest the generation/load mismatch that has developed.

CR

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

RE: Generator operating frequencies.

(OP)
The ERCOT fundementals training manual that is used as operator reference and for training for the ERCOT System Operator Certification Exam has this in it.




http://www.ercot.com/content/wcm/training_courses/...

ERCOT System Operators are in the least told to look changing angles as frequency deviations and that changing power flows require generators to have a "relative accelerations". I think that if they are mentioning small frequency differences during changing power flows, they probably have frequency readings out to several decimal points.


I got a guy to pull up the RPMs on two generators that were being dispatched that were 400-500 miles apart. The two were readings were a few thousandths or hundredths (I can't remember. It was Friday and I had to pick up my daughter) of an RPM apart but I couldn't get them both timestamped at exactly the same time before I had to go. I don't know how accurate the RPM data that I saw was but I know that someplace precise RPM or frequency data is collected and used for grid controls to maintain tie line and countrol area power flows. I have access to this RPM data and it probably would be interesting to watch a unit RPM ramp up and down for the day.

RE: Generator operating frequencies.

Very interesting.
That has been explained several times and now we have a definitive answer.
This explains how a change in power flow will cause a change in power angles.
This in turn will lead to a slightly different frequency only during the time that the power flow is changing.
Where I have a problem is reconciling this with your original statement.

Quote (HH)

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.
To most of us, "humming along" implies a steady state condition at 59 Hz, not a slight variation to about 59.8 Hz lasting for a few seconds.
Steady state versus transient event. One size does not fit all and one explanation does not fit all.

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

RE: Generator operating frequencies.

Governor action. When a unit is in base load configuration, the control could be set to deliver the required output and locked in position.
That would work until the unit lost its load for some reason. Then the locked control (fuel valve, steam valve, water gate, etc.) would cause a runaway.
Instead many base load generators are controlled by governors in droop mode. At 5% droop, they are relatively insensitive to minor frequency variations but become active and limit the speed to 105% or less on loss of load.
With a large change in loading such as CR mentions, 0.5%, all units will initially rspond, and then the swing set(s) will over the next few seconds act to correct the frequency back to 60 Hz.
From a controls perspective in relation to a PID controller, base load sets use Proportional and swing sets use Proportional-Integral.

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

RE: Generator operating frequencies.

(OP)
Waross,

That question was based on underfrequency setpoints I saw that were very low and oscillography for frequency deviation during a block load switching in that I knew wasn't felt across the interconnect. You keep bringing it back to this point of 59 hz after I have already stated earlier that this isn't the question anymore and the magnitude was off. The forth post is the more general question. You are banging away on a point that has long been conceded and I have already said this much and any mention of deviations have included statements that it happens and is allowed provided angular deviations aren't large enough to cause instability.

If you want to bring up steady and transient state, I would be of the opinion that the grid is always changing and never in steady state. Generator controls, AGC, and dispatch are continually trying to limit frequency and generstion and load deviations and maintain tie line flows. These do a pretty good job because sizeable deviations are only seen during block changes. Everything would be deadballs 60 hz steady state only if powerflows in the system quit changing. No changing loads. No changing generation. No changing system configurations. That isn't the case.

RE: Generator operating frequencies.

HH you were (not sure if you still are) implying that changes in system frequency could mean that the system may have different frequencies at different points in the system. This is not the case. Frequency is not constant, but that sure doesn't mean a system with a dynamic frequency isn't stable or steady-state. Minor changes in system load flows (as load changes through the day for example) or minor dynamic changes in frequency do not meet my definition of a transient state. Transients are caused by switching, faults, equipment failures and major load flow swings (not the normal everyday stuff).

RE: Generator operating frequencies.

I'd agree, or maybe even tighten it up a bit, and state, to use NERC-speak, that if there isn't any type of bulk electrical system contingency that requires the system to be re-prepared for the next contingency within 30 minutes, steady-state conditions exist...and please take note that I didn't write "static state".

To rephrase, I would exclude planned [ and sometimes even unplanned ] switching from the criteria that invoke a transient state; switching of static capacitors or even in some case high voltage circuits for voltage control purposes, for example, may cause observable voltage bumps on the system that will nevertheless not come anywhere near the qualifying threshold for declaring a contingency.

Similarly the synchronizing, loading, unloading and disconnection of generating resources in the perpetual pursuit of matching supply to demand involves switching that is, while always executed with vigilance against the unexpected, quite routine.

Likewise, as the load on a higher-voltage autotransformer approaches the point where it would be overloaded by a recognized contingency, nobody panics; either a re-allocation of load onto other available sources of supply, or the designation of specific generators as "must-run" as required to alleviate "the ping" is undertaken as a matter of course.

CR

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

RE: Generator operating frequencies.

crshears,

The UK grid requires that a generator stay on indefinitely at 47.5Hz, maintain a minimum of 95% of rated output at 47Hz, and must remain connected for a minimum of 20 seconds if the frequency dips below below 47Hz.

Under-frequency protection on generating plant serves a couple of purposes: on gas turbines the compressor output drops away quite quickly as the speed falls, so as speed drops the machine will struggle to maintain output. The 95% rated output at 47Hz is a tough demand on a GT - it can be met by roasting the arse off the power turbine through over-firing the engine. On a steam set there are often shaft critical speeds not too far below sync speed, and you can't safely operate too close to them without risking damage to the machine.

RE: Generator operating frequencies.

crshears - yes that is a better way to look at it.

I spent 5 years at a nuclear site. They used the word 'transient' for very different, and much more scary reasons. Basically, if something 'unexpected' happened within the nuclear side of things they would announce a 'system transient'. Absolutely no reference to the electrical side of things.

We always wanted to run away when those announcements happened, but they usually would lock the plant down and we wouldn't be able to leave.

RE: Generator operating frequencies.

Hey Scotty,

Fair enough; but I was referencing HH's mention of UFLS, which is Under Frequency Load Shedding. The generator under-frequency tripping you allude to is indeed another animal entirely; the 60 Hz steam plant I worked in had UFGT @ 57.5 Hz with a time delay of 10 seconds, and @ 57.0 Hz with no intentional time delay.

CR

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

RE: Generator operating frequencies.

Hey crshears, I'm a generation guy. Only our view of the world counts. tongue

I guess UFLS is set to operate slightly before the generators start tripping off to protect themselves. Once the generators start falling away there will be a domino effect as the frequency falls further with each machine trip, leading to system collapse.

RE: Generator operating frequencies.

I used to know what you mean about generation being the only thing that counts...

First I was a steam generation guy, then transmission & distribution, then small hydraulic generation, then more T & D, then really big hydraulic generation, and finally T & D again since 1997.

So, being Canadian and with apologies to Joni Mitchell, I can say, "I've looked at grids from both sides now..."

CR

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

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