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Questions concerning UFLS Calculations

  1. Aug 12, 2014 #1
    I have been interested in finding out more about this topic but I can't seem to find any articles/periodicals that don't assume you to already have 20 years experience with under frequency load shedding in transmission engineering.

    My questions are:

    1.) How is it that you determine the amount of load that has to be dropped? I'm assuming it has something to do with the power angle coming from the generators, but I'm not sure if that's correct.

    2.) What frequency is considered "too low"? In other words: What frequency separates the steps from one another?

    3.) In the case of an under frequency event; how long would it take for the generators to "get back to speed" after the appropriate amount of load has been dropped?

    4.) Is there a point where, no matter how much load is dropped, the generators cannot re-right themselves and they have to be re-started? What would cause such an circumstance?

    Thank you,
    Last edited: Aug 12, 2014
  2. jcsd
  3. Aug 15, 2014 #2
    I'm sorry you are not finding help at the moment. Is there any additional information you can share with us?
  4. Aug 15, 2014 #3

    jim hardy

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    I'm no power systems guy, just worked in a plant.
    Looking over shoulders of our relay guys, i sorta came to understanding a teeny bit about it.

    Rate of frequency decrease tells you the mismatch between generation and load provided you have an idea of system's rotating inertia. Your system guys have the inertia numbers for every machine your utility owns.
    Our system used fixed frequency setpoint relays scattered throughout the system. I don't remember the setpoints, but pretty sure they were in the 59.8 to 58.8 range.
    Search on "underfrequency setpoint load shed"
    here's first two hits it gave me:



    You'll want to shed all the load you need to shed before your generators start disconnecting because of underfrequency. Our nukes allowed 2 minutes at 58hz (blade resonance i think)
    and at 56.1 hz mine would disconnect so as to not pull too much energy out of the reactor coolant pump flywheels .

    Surely the system guys estimate rate of frequency decrease from a 10% generation shortfall

    I can't provide much insight. All i can say is it's a race between shedding enough load and your turbines timing out on underfrequency. Clearly 58 hz is too low. Some system people may know better.

    I looked at some oscillographs of system upsets. It was a matter of tens of seconds.
    When frequency drops the governors open the throttle valves . Now you have a race between boiler pressure dropping(which lowers power output) and getting frequency back up. You want to get frequency back up before plants start dropping from the upset.
    There's another race because power is being imported to the area where generation is insufficient, probably in unusual amounts that overload your lines
    We once lost that race when the HV transmission lines overheated and sagged into some treetops ... There are also time overcurrent relays that can have a say too.

    You bet. When you lose those races i mentioned in (3) the grid voltage collapses as all your customers's induction motors slow down and call for starting current.
    We had one generator fail to disconnect from a collapsing grid and snap its shaft. Ruined it.

    I'm sorry that i cannot give you more academic answers. Hopefully the concept is a little more clear now, though.

    I'd recommend this book:

    Prof Gross was a lowly grad student in 1965 teaching AC circuits and machinery to sophomores, myself included. He was hands down the best explainer i ever encountered. In our company's system department every engineer had a copy and they called it "The Bible".

    Maybe somebody with real system experience has better insight.

    old jim
    Last edited by a moderator: May 6, 2017
  5. Aug 15, 2014 #4


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    For interconnected areas, power "flows" where it's needed and it's up to each area (known as a Balancing Authority) to control their generators to maintain 60 Hz (in US). Generators are raised and lower, usually using Automatic Generation Control (AGC) software, via a real time Area Control Error (ACE).

    Without trying to rewrite all the details, I'll refer you to the North American Electric Reliability Corporation (NERC) technical document Balancing and Frequency Control.

    @ old jim: You may like this NERC "www.physicsforums.com/attachment.php?attachmentid=72197&stc=1&d=1408162899" [Broken] which I attached.

    Attached Files:

    Last edited by a moderator: May 6, 2017
  6. Aug 16, 2014 #5

    jim hardy

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    Don - fascinating documents.
    Especially the Balancing&Frequency control..
    The "declining frequency response" in fig3 is real worrisome, means the whole system is getting less stable.
    Those drivers of it on page 9 ...
    Steam plants have lots of inertia and a boiler that has a reserve of steam to help with initial response.....

    Next couple decades are going to be real interesting for there's fundamental change afoot in electric power.

    old jim
  7. Aug 16, 2014 #6
    My 2 cents from a substation drafter:1. Our UFLS/UVLS are determined by WAPA. We have certain feeder ckts from distribution substations that are dictated by WAPA (and to a certain extent by WECC), to go offline. Our SEL 311 relays are programmed by the engineer to go "off" at around 59.6ish hz. The receiving subs we have will have those same relays (I'm pretty sure), BUT on the TRANSMISSION SIDE (ie 230kv). Obviously, the relay does the computations in <20 cycles, (so, pretty damn fast).

    I'd like to know more of the technical side of how the CT's on the 60kv circuit breakers are configured, ie, are these toroids, regular buss, window, etc. How is that designed/configured? Is that because the manufacturer made it that way? Dunno.

  8. Aug 18, 2014 #7
    Thanks everyone for the fascinating documents and book recommendations; You've all cleared a up a few things I was having a hard time understanding. I've just started to get into this topic and look forward to learning as much as I can.

    @jim hardy... What changes do you foresee in the near future? More generation from nuclear? Implementation of HVDC Lines (seems to be a buzz topic right now)? Obviously, electronic relays have changed things quite a bit in the area of protection, but not much in generation. After 60 - 70 years of remaining the pretty much the same, I suppose it's about time we see a major shift in electric power.
  9. Aug 18, 2014 #8

    jim hardy

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    Hmmm it's dangerous to one's ego to forecast.... Mother Nature loves to swat down a boast.

    Well -
    What worries me today is in those papers Don linked - the whole grid is becoming more "tender"(pardon the borrowed nautical phrase) that is rate of frequency decrease per unit of generation shortfall.
    In other words it's not got the inertia and reserve "oomph" it had a decade ago.
    What that means is a local generation shortfall will pull power from farther out in the grid to make it up.
    The likelihood of power swings between regions is thereby increased. These can easily grow into underdamped or divergent oscillations of power flow like happened in northeast US a couple of times..

    Add to that the administration's drive to shut down coal. Old coal plants tend to be around big load areas like Denver or St Louis , so that power must now come in over long lines adding to the stability challenges.
    That field of study is called "Power System Stability" .
    I honestly expect some huge scale blackouts from system stability issues as the grid becomes increasingly flimsy and power system engineers learn how to maneuver it without losing control.
    Smart Grid might help there by better load shedding . Substations are already getting pretty smart.

    There are some nukes under construction now in Georgia, Florida and someplace in the corn belt
    but i doubt new construction will keep up with retirements in our aging nuclear fleet.

    Automobile industry has moved successful from robust 7 liter V8's to 2 liter 4 cylinder cars. They work okay but at the expense of complexity.
    We'll see something similar in electric grid with similar growing pains as we move from steam turbines to jet engines and windmills.
    I'm too obsolete to help with that, i'm afraid.

    But i'd sure like to make some rooftop solar water heaters. Flat plate collector can't be beat for that application.
    First guy to come up with a practical residential solar airconditioning booster will sell a zillion of them in the sunbelt.
    And those are your two biggest residential loads.

    old jim
  10. Aug 19, 2014 #9

    Sounds like system stability is going to become an even bigger problem in the future. Maybe I can be part of the group of people in my generation that cleans things up a bit. :)
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