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What does a leading/lagging angle mean on the grid?

  1. Feb 27, 2015 #1
    I ran across a project at the Univ. of Tennessee that has deployed a network of frequency meters around the power grid in the US. see http://fnetpublic.utk.edu/anglecontour.html

    I understand the frequency excursions may indicate unexpected events like generator trips or loss of load. What do the angles mean? Are these indications of possible unexpected generation fed from wind farms? Are there other possible explanations for these angles?
     
  2. jcsd
  3. Feb 27, 2015 #2
    Same thing as angle between voltage and current phasors in any AC circuit.
     
  4. Feb 27, 2015 #3

    jim hardy

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    Wow that is a most amazing display. I hope it's accurate.....

    I gather you're not a power guy. I've only looked over the shoulders of genuine power guys enough to have an inkling of what they do..
    But at a beginner's level here is what's up:

    You need to understand that synchronous generators inter connected by wires will remain in step, just as if they were connected by an arrangements of solid shafts.

    When you send power down a shaft, it twists under the torsion.
    The receiving end is some number of degrees behind the sending end,
    and the more limber the shaft the greater the angular displacement between its ends.

    Same is true of an electrical power line. Its receiving end will be a little behind its sending end.

    So as the entire US electric grid hums along at 60 cycles
    power flows between regions according to each region's surplus or shortfall of generation versus consumption.
    That shows up as angular displacement between regions on this map
    http://fnetpublic.utk.edu/anglecontour.html
    which right now shows region along Mississippi River (lots of power plants there) well ahead(orange) of Northeast(blue)
    so power is going in that direction - from heartland toward Northeast..
    That display appears static, i dont know how often it changes.

    This one however shows clearly the power "sloshing" between regions
    http://fnetpublic.utk.edu/gradientmap.html [Broken]
    it reports minute frequency variations that will integrate to angular displacements.

    I have wanted for decades to see something like this for a big region.
    I have stood alongside my generator with a stroboscope on the shaft and observed our generator swinging a few degrees against the grid at 2/3 hz..... call it "torsional displacement in the grid"... but only the power system guys downtown could see our whole state swing against our neighbors.
    {Incidentally - that generator shaft itself twists ~3.2 degrees when transmitting full power)


    As you can imagine, since there is both a displacing force(generation mismatch) and a restoring force(governors to admit more or less steam), the possibility of harmonic oscillation is very real. It shows up as low frequency power swings between regions at frequencies below about 1 hz. Power system guys have to keep the system stable by allocating generation and transmission resources.

    Thanks SO VERY MUCH for that fascinating link.
    You might read up on "Power System Stability".


    old jim
     
    Last edited by a moderator: May 7, 2017
  5. Feb 27, 2015 #4
    Have pity on a simple economist who is trying to understand the principles here.

    I'm curious why when I look this map the angles are very different in the upper midwest versus the Northeast. Does this imply something about the regional balance of generation and load or some other phenomenon? Or is this just some very local phenomenon related to the building this meter is plugged into?
    Am I grasping here to think there is a larger grid operational condition represented in these angles across regions?
    Thank you in advance.
     
  6. Feb 27, 2015 #5
    Thank so much! You made my day. I suspected there was some power sloshing signal in here. Your explanation makes a lot of sense to me.
     
    Last edited by a moderator: May 7, 2017
  7. Feb 28, 2015 #6
    How much do you know about AC circuits and phasors used for their analysis? If you don't know anything, my advise is you should first learn fundamentals of it. Otherwise, you will hardly be able to understand principles. Despite its' complexity and large physical size, the AC grid, voltage and current electrical states in every part of it, can be represented by phasors. Just like in every AC circuit/network at fixed frequency. For example , you can take a voltage phasor of any grid point as the reference and compare how phasors' angles of other, distant, grid points change with respect to it as you move through the grid. Generally, considering angle between two points in the grid, various things affect it: distance effects, power (load) flows effects, transformers' groups effects, etc.
     
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