Generator operating frequencies. 27

[HamburgerHelper]

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

 

[crshears]

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]

 

[HamburgerHelper]

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 Standards/PRC-005-6.pdf).

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 42 Line relay calc sheet.xls

http://www.ece.mtu.edu/faculty/bamork/EE5223/115kv pnl stand logic descrip - NL 1 BK (rev 2).pdf

http://www.ece.mtu.edu/faculty/bamork/EE5223/MP 115 kV Panel Logic Dwgs.pdf

 

[crshears]

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]

 

[waross]

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

 

[marks1080]

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.

 

[bacon4life]

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.

 

[itsmoked]

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

 

[crshears]

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]

 

[marks1080]

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?

 

[HamburgerHelper]

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.

 

[marks1080]

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.

 

[crshears]

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]

 

[Skogsgurra]

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

http://www.gke.org

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

 

[itsmoked]

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

 

[Skogsgurra]

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

http://www.gke.org

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

 

[itsmoked]

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

Keith Cress
kcress -

http://www.flaminsystems.com

 

[HamburgerHelper]

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.

 

[marks1080]

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.

 

[HamburgerHelper]

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.

 

[marks1080]

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.