Load shedding

[wuqn]

hi every body
My underfrequency relay is used to remove load out of the generator (load shedding). I want to change the time delay. What delay is suitable ? How do i calculate the delay? the method ?

 

[ZapSib]

Wuqn,

Your generator+ prime mover has a performance curve ( freq. vs time) with a forbidden area.

U/F relay shall be tuned acdg to such curve, taking into account margin to be safe enough.

I belive it may be advisable to use several ( 4 looks good) stage for this kind of scheme.

 

[JensenDrive]

Delays have to be quite small, like 5 cycles or so. Once you hit the target trip frequency, you can assume frequency is dropping fast, so if you delay the trip any longer than a few cycles, it is too late to do any good. The only reason to delay the trip is to help ensure that a fault has not caused a transient wave form that fools the relay into seeing degraded frequency. The time delays associated wtih the performance curve that was mentioned by ZapSib is associated with the time it takes to get from normal frequency to the trip frequency. e.g., suppose you were set to trip at 58hz. What would happen if you actually sat at 58.5 hz for an extended period? The mfr may tell you that is not allowed for more than so many seconds.

 

[ScottyUK]

It depends enormously on the prime mover and the service conditions the machine was designed for. Gas turbines really struggle as frequency falls because the compressor performance drops away so sharply. Diesels are rather more tolerant.

Most grid operators expect a graded response, e.g. for a 50Hz gas turbine a response such as 15 seconds at 47.5Hz prior to load dump with an instantaneous load dump at 47Hz is plausible. Tripping a unit on an under-frequency event (as compared to a load dump) causes a needless delay in returning that unit to service and undue wear & tear on the engine.


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[davidbeach]

In the US, the western grid (WECC) has six frequency/time points at which generation must stay connected. As frequency goes down the times get shorter and shorter, but it is several Hertz down before an instantaneous trip is allowed. (I don't have the required settings at hand, just working from memory.)

 

[ScottyUK]

Hi David,

Do the gas turbine operators have any problems complying with those requirements? From my experience it is tough for a GT station to comply with the UK Grid Code because of the way turbine performance drops away with falling frequency. Typically the requirements of the Grid Code are met by over-firing the engine to maintain power output as frequency falls. This can't be sustained for long because months of engine hot parts life can be consumed in minutes if the engine is pushed right up to its thermal limit. The technology is the same so I guess the same problems exist in the 60Hz world too.

We are lucky in the UK that our grid is pretty stable and major frequency excursions are rare, although whether that continues as the big baseload stations retire and the windmills take over remains to be seen. Perhaps there will be an outbreak of common sense in the Houses of Parliament before the lights go out due to lack of investment in generation plant, but I doubt it.


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Sometimes I only open my mouth to swap feet...

 

[davidbeach]

System frequency excursions that start the timer on the first level are very rare. System frequency excursions that get into the various frequency/time "boxes" will typically fall through the bottom quickly, but there will be an event, sometime, that the settings prevent a system collapse.

 

[rmw]

Scotty,

I think that one difference could be the fact that a 60HZ CT (speaking of frame types as in your case) is turning 3600 RPM while yours, which is basically the same machine (501 vs.701) turning at 3000 RPM. That difference takes away a lot of the saving grace that might tend to let a 60 HZ machine ride through a U/F situation without melting hot gas path parts.

I have seen some islanded systems where the frequency could get several points away from 60 HZ before corrective action was taken and it wasn't taken by the U/F protection either. When the lights start flickering, it is time to trip something.

rmw

 

[Alex68]

Wuqn,
as stated before, the compliance of the grid code is very important, but also the frequency limitations imposed by the turbine manufacturer.

But you asked about a calculation method. The best way is to make some dynamical simulations in which you can try all the frequency sets you like and see the consequences. Obviously a very good model of the whole system is required.

 

[ScottyUK]

rmw,

Was that a wild guess that we run 701s or have you had some dealing with our site? Or have you got an amazingly good memory?

Not being a turbine designer or anything close, I'd have thought that blading changes, especially in the compressor, and differences in the power turbine to optimise performance for 3600rpm would offset any potential benefit the higher frequency would bring. Interesting. Can you expand any further?


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[GTstartup]

Under frequency on a GT is a surge protection issue, not a "melting hot gas parts" issue. This is why under frequency protection for a GT has normally 3 stages,

1.Alarm
2. Trip HV breaker
3. Trip Generator Breaker

The tripping of the GT itself after the above is a function of speed and ambient temperature

 

[ScottyUK]

Under-frequency on a GT which is designed to over-fire by lifting the temperature controller setpoint in order to correct that under-frequency seems to be a 'melting hot parts' issue in opinion of the OEM - it racks up equivalent baseload hours at a disturbing rate.


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Sometimes I only open my mouth to swap feet...

 

[GTstartup]

Scotty,

My point is that under frequency protection on a GT is for surge protection, not for protecting hot gas parts melting. Otherwise the underfrequency protection would only be required at high loads, which is not the case.

What you are described in your last post is peak load due to droop.

No GT OEM will let you operate in a region that will melt hot gas parts. Granted you place additional wear/stress on parts and will rack up EOH and shorten time between inspections.

Under frequency on a GT reduces mass flow and brings the GT closer to the surge line. This can happen at part load or full load, high inlet temperature or lower inlet temperature. This is the main reason for under frequency protection of a GT.

Oh and for the OP, we use 7 cycles delay on the for the first two stages. 1-trip network breaker, 2-trip generator breaker, but it really is a matter for the OEM to guide you.

 

[ScottyUK]

Hi GTStartup,

I'll post the '701 underfrequency settings when I get in to work rather than guess them here, but it is not as complex as seven stages.


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[GTstartup]

Thanks Scotty,

But seven stages? Did I miss something?

 

[ScottyUK]

No, you didn't: I did. Seven cycles, unless someone has done a quick edit to confuse me even further. Just proves I should not type before my first coffee of the day!

Our tripping scheme is:

48.5 Hz: Under-frequency alarm
47.8 Hz: Load dump after 20 seconds
47.5 Hz: Trip engine

Normally the load dump is accompanied by a rapid recovery in speed, to the extent where an over-speed becomes a concern if it occurred while the unit was base loaded. I have never seen a definite time quoted for a trip initiated from the control system but worst case looks like about 180 ms to breaker clearance, i.e. 9 cycles in the UK.


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Sometimes I only open my mouth to swap feet...

 

[davidbeach]

The WECC standards require:
>59.4Hz, continuous operation
<=59.4Hz, 3 minutes
<=58.4Hz, 30 seconds
<=57.8Hz, 7.5 seconds
<=57.3Hz, 45 cycles
<=57Hz, instantaneous trip

On the overfrequency side, the requirements are:
<60.6Hz, continuous operation
>=60.6Hz, 3 minutes
>=61.6Hz, 30 seconds
>61.7Hz, Instantaneous trip.

If it is not possible to maintain the above, the system with the generator is required to trip load equal to the loss of generation.

 

[subodhbhatnagar]

Hey friends!
1.Before providing the solution,we need to know about running of the generator is with grid or in isolation.
2.We should not mix up issues of U/F protection of turbine with load shedding relays since we keep margine between mimimum frequency levels tolerated by turbines and load shedding frequency levels where from shedding commence well before unsafe under frequency reaches.
3.Loss of load in a large grid is taken care by other generators having suitable coordinated governor droop settings for each type of generator and if other generators like Hydel and/or Themal stations have capability to share load and stabilize system.The correction in frequency will be based on studies made by load despatch system management which shall analyse and work out the rate of fall of frequency in various cases of loss of load/generation.
4.Frquency rate should be considered by the load management/shedding while keeping three settings for load shedding relays e.g.alarms and trip at fequency instantenious. frequency with time setting with or with out df/dt setting(rate of change of frequency relays).
5.Therefore, the settings of under frequency relays for load management are set as per requirements/criteria of System Load Despacth Management who may assign setting at various levels of freuency with timesettings and or df/dt of each station for the grid discipline.