Under Frequency Load Shedding scheme 2

[HiSet]

What criterion should be followed for designing Under frequency load shedding scheme i.e. should it be based on security i.e.by having contact of main and check relays in series to issue trip or should it be based towards reliability or dependability i.e. by having contact of main and check relays in parallel for trip?

 

[ScottyUK]

What are you protecting and what are you shedding?

If you are protecting a large high capital value asset like a utility power plant generator I would err toward the reliability / dependability. If it was an emergency generator holding up a critical facility like a hospital or an airport control tower then the generator should hold up until the last possible moment.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[burnt2x]

We based our load shedding from loadflow studies and did scenarios for lots of possibilities. We were able to assign priorities as to which feeder to trip first for each 0.1 Hz drop from the lowest frequency acceptable and cross-referred with the ability of our system voltage to recover after shedding off feeders.
There is no "one scheme fits all" here. You continuously update your system and your criteria changes with every load requirement, i.e. life-safety, communications-critical, etc.

 

[waross]

I am sure that you are aware, but for the information of those following this thread to increase there general knowledge, URFO will help you.
Under Frequency Roll Off is standard on Automatic Voltage regulators up to about one or two Mega Watts. I am not sure above that range. UFRO will allow about 2Hz or 3Hz frequency drop and then start dropping the voltage in proportion to the frequency drop. This helps reduce the load, and maintains the V/Hz ratio so as not to saturate motors and transformers.
I am sure that UFRO was considered when you did your load and recovery planning.

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

 

[slavag]

Hi HiSet.
We need more informastion about your system.
As wrote above , a lot of possibilities for criterions: static, dynamic, adaptive load shedding, with priorities and w/o. It's always depend on the type of load, etc....

Best Regards.
Slava

 

[HiSet]

Guys,
Thanks for you valuable responses.UFLS scheme in question is part of state wide system protection scheme and consists of various stages consisting of rocof and simple under frequency stages.On tripping side it trips mix of residential and industrial loads at appropriate frequency threshold.My query is about philosophy i.e. what is more catastrophic situation i.e.non operation of UFLS scheme or it's maloperation.For your information around 60% of total available load is wired to trip by UFLS scheme and is spilt in 6/7 stages.

 

[ScottyUK]

From the perspective of a major power station where the equipment involved is a multi-million pound turbo-generator with a steam or gas turbine prime mover which I know is likely to be damaged by the under-frequency event and which will take weeks or months to repair at enormous expense then I would dump the load as soon as the requirements of the grid connection code were met and as soon as either relay operated. Protecting the plant is #1 priority; if consumer load is critical then the consumer needs to provide their own standby generation and/or UPS support.

To give some idea why the utilities are so protective of their generators, many big turbines have a limit of just a few seconds operation under load in the whole lifetime of the machine - maybe forty years or more - at a frequency of 94% of nominal or below.

Whether your final load - the one which is left after everything else is shed - is valuable enough to warrant sacrificing your generator in order to protect it really depends on the load. A diesel set is likely to be more resilient in terms of withstanding an under-frequency event and cost less to repair than a turbine.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[waross]

Hi Scotty;
Do I understand correctly that the issue is the turbine running under frequency rather then the generator?
Is this gas turbines, steam turbines or both?
Thanks.

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

 

[ScottyUK]

Hi Bill,

Both have problems with under-frequency. With steam turbines I think it is a resonance problem somewhere on the machine, often the LP rotor. There are quite a few critical speeds between barring and sync speed where the machine can not be allowed to operate for any length of time, and in my experience there's usually one not far below sync speed. Maybe rmw or byrdj is reading this and will comment from a more knowledgable position about the reasons why ST's have such a tough time maintaining load on a falling frequency.

With gas turbines it is compressor-related because the compressor's ability to deliver air at the necessary pressure drops off very rapidly as speed drops. The only way to maintain power output is to burn more fuel by over-firing which means raising the firing temperature of the engine, something which eats up hot parts life at a disturbing rate (i.e. 20 - 30x faster than at normal base load, depending on the level of over-firing). Off-frequency performance is a big problem for CCGT plants trying to demonstrate compliance with the UK's Grid Code and no doubt in other countries too.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[byrdj]

all turbine buckets (blades) have multiple resonance frequency, in addition to shaft critical speeds. this is especially critical for the longer last stages. for example, the running speed would be a frequency between the bucket's 6th and 7th harmonic (~400Hz). operating off frequency will provide a driving stimulus and produce higher bucket stress.

I know the manufacturers are coming out with new designs that allow for poor vacuum, so they could also offer designs to allow wider range for off frequency

Here is example for a particulair unit

Operation Time - Under Frequency / Over Frequency
Unlimited. – 1.5 Hz + 0.5 Hz
90 Minutes – 1.5 Hz to 2.0 Hz / + 0.5 Hz to 1.0 Hz
12 Minutes – 2.0 Hz to 2.5 Hz / + 1.0 Hz to 1.5 Hz
01 Minutes – 2.5 Hz to 3.0 Hz / + 1.5 Hz to 2.0 Hz

these bucket resonance can also cause damage during the steam turbines roll to speed. that is the purpose of the high speed hold wobbulator on GE electronic controls

 

[PHovnanian]

HiSet, your contact/reliability question needs some issues to be considered. What are the consequences of a 'false positive' trip? They shouldn't be too high, or you'll need to consider whether you actually want to dup this particular load or circuit. You also need to consider the consequences of a failure to trip. If this is one of many similar loads in a sheding schedule, no big deal. The next one will drop when the frequency reaches its set point. If its a large single load and the consequences of not dropping it are serious (not enough alternative load to drop or the others are of higher value) then the trip reliability needs to be improved. Possibly at the expense of a false trip from time to time.

 

[slavag]

HiSet.
I would recommend to you ( of course to other also) this data:

http://ieeexplore.ieee.org/Xplore/l...488177/4524335/04524610.pdf&authDecision=-203

It's a very intresting view on the UFLS issue.
Especialy for the UFLS, as system protection. If you have problems with login, I aks those guys send documents to me and will attach to Forum.
From my point of view it's a future of UFLS. Dynamic model of system with lot of options

Im not from this team and not promoter!!!
Best Regards.
Slava

 

[HiSet]

Phonanian and Slavag,
Thanks for for use full tip.I would love to read IEEE document on this subject recommended by Slavag but can't get into IEEE website.Can some one upload this document on this website please?

 

[Sideswiper]

Our older way is to have UF detected on the low side of the transformer. We have UF enable switches for every feeder. At the feeder, the UF trip contact (in series with the UF enable switch) is in parallel with the other sources of trip (overcurrent, etc.).

Our newer way is to let the multifunction digital relays on the individual feeders handle overcurrent, instantaneous, and underfrequency.

Here we only have to put 40% on UFLS (4 steps, 10% of load on each).

How do you define what 40% of the load is? We take a snapshot of the peak load of the previous year, and pretend that the % load of every feeder is constant throughout the year. This assumption is very helpful, I wouldn't want to have to prove that what I had set to trip was actually 40% at any given moment, loads fluctuate!