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.

 

[MikeHalloran]

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

 

[waross]

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

 

[DanEE]

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/...uirementsfb378c0f1b5267e39dbdff0000e2e88e.pdf

 

[HamburgerHelper]

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.

 

[marks1080]

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

 

[HamburgerHelper]

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.

 

[marks1080]

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.

 

[marks1080]

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.

 

[cranky108]

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.

 

[HamburgerHelper]

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.

 

[cranky108]

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.

 

[marks1080]

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.

 

[Compositepro]

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.

 

[Mbrooke]

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.

 

[roydm]

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

 

[wroggent]

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

 

[Compositepro]

"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.

 

[davidbeach]

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.

 

[ScottyUK]

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.