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Power factor correction

  1. Apr 23, 2012 #1
    I am new at this energy management stuff:

    My question is, we have a new Central chiller plant where I work that feeds 5 buildings for there A/C. The Power factor at this building is 99% when the chillers are running (outside temp above 50 degrees) but when the chillers are not running the power factor is 10% or below. When the PF is at 10% the KW is 6.75 so the KVA = 60+. The utility charges us for KVA demand charge which is almost $10 per KVA. Would it be smart to put capacitors in at this location? or Is the KVA Demand charge only at Peak KW usage? It is hard to see what this individual building is being billed by the Utility because it is not the only building on this circuit.
     
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  3. Apr 23, 2012 #2

    psparky

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    Wait.....if you turn off the chillers......are you saying the power factor drops to 10% capacative? Or maybe you mean 90% capacative.

    Your chillers are largely inductive so they are using capacitors to bring your inductive circuit closer to unity.

    10% power factor? That doesn't sound right.

    Are you saying that the chillers have a 10% power factor before corrected? This also doesn't sound right.

    Maybe my questions or assumptions are wrong....but we need to start somewhere.
     
  4. Apr 23, 2012 #3
    I should not use the term "off". The Chillers are never really turned "off" they need power constantly to Heat the and cool the liquid oil inside to operate. I've talked to the HVAC people and that is the answer I got.

    When the Chillers are operating and chilling water the PF is almost unity 99%
    KW is around 300-400KW during this time of operation

    When the Chillers are idle and not chilling water the PF is around 10% for the building.
    KW is around 6-8KW when everything is idle.

    I have a 480V EMON monitoring the system that we put on the main secondary of most buildings to read electric consumption. Thats where I am getting these numbers.
     
  5. Apr 23, 2012 #4

    psparky

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    Now I'm really confused.

    You spend money on 60 KVA to make 6-8KW when the chillers are off?

    This to me, says that your chillers are gigantic capacitors that correct a ridiculously low power factor from .1 to .99. This is obviously wrong.

    So you say the chillers use 300 to 400 KW when on with a PF of .99.

    So you must agree that the chillers use 297 to 396 KVA when running?

    You are also implying that there is a ton of reactive power when the chillers are turned off. Where is this reactive power coming from? Typically, the chillers would be the one creating the reactive power. That's why it doesn't make sense when you say power factor goes way down when chillers are off.

    I'll be glad to help you in any way.....but I need to get to the bottom of what you are talking about first.
     
    Last edited: Apr 23, 2012
  6. Apr 23, 2012 #5
    The reactive power is coming from the heaters within the chiller unit. The oil has to be heated to keep the water out then the oil must be cooled before the chiller can use it. I do not understand any of that part really, I am not a chiller technician or mechanical engineer.

    What I know: I tested the feed with an amp meter on 1 of the 3 legs of A phase, I was reading 40 amps, so that 120 amps per phase at 480V when the chiller is not cooling water. I dont remember the PF at this particular time. There are 2 of these units in the building, I know this sounds crazy, I could not believe it either, but when these units kick on the sound is so loud it makes you nauseous.

    I think the PF goes way down because you still have the almost all KVA the chillers are using for there Oil heating and Cooling but the KW is gone. Thats why when the KW picks up the PF is almost unity. So the way I think about it is with the Power Triangle. Does this help???
     
  7. Apr 23, 2012 #6

    psparky

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    Heaters are almost always purely resistive.....KW....watts.

    40 amps on one leg is 40 amps per phase at 480 volt.

    40*480*1.73=33.2 KVA

    Two units......66.4 KVA when the chiller is off which agrees with your first post.
    Where do you get the 6 KW from?

    When the chiller is off is there some main breaker digital read out that shows 66KVA.......6KW and .1 PF? Or have you obtained one of these numbers some other way? You mentioned the ampmeter which was good.

    Actually, would you mind saying where you got all the numbers you mentioned? Whether it be digital readout, ampmeter, nameplate, specs...etc. And if you are reading or measuring.....mention exactly where please.
     
    Last edited: Apr 23, 2012
  8. Apr 23, 2012 #7
    We use a device called Emon. On the face of the device it has 4 line display that scrolls through and shows me: kWh, kWh/Demand (with peak date and time) Power factor per Phase, Real-time load in kW, Amps per Phase, Volts per Phase. I can only view these items when looking at the face of the device. When I use the EMon software on my laptop I can view everything real time: (kw, kva, pf, kvar, amp, and volts per phase and the sum of all three phases for each item listed previous.) The only reading I physical checked were amps with my fluke.
     
  9. Apr 23, 2012 #8

    russ_watters

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    E-mon D-mon sub-meter, sure. Sounds to me like it is wired wrong because there isn't anything anywhere that runs at 10% pf. Even a bad, old fluorescent light is something like 60% iirc. My guess is the volts and amps aren't matched to the same phases.
     
  10. Apr 23, 2012 #9
    Here's my guess: you already have capacitors. They're what you see as the highly reactive load when the chiller shuts off.

    Otherwise the motors on the chillers would read around 0.8 P.F.
     
  11. Apr 23, 2012 #10

    russ_watters

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    Yeah, that's a possibility...though typically the capacitors would be wired-in with the chillers so they don't affect the rest of the building when off.

    Does the actual electric bill show this power factor penalty?
     
  12. Apr 24, 2012 #11
    we do not pay a PF charge but we do pay a KVA demand charge, so the closer we get to unity the less our KVA is going to be.
     
  13. Apr 24, 2012 #12

    psparky

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    Thanks guys.....I thought I was losing my mind there for a second!!!!

    Something is not making sense here............
     
  14. Apr 24, 2012 #13
    I would say that the EMON is wired correctly. I have over 150 of these devices at various locations. I am not getting an errors saying the CT's are backwards or installed incorrect
     
  15. Apr 24, 2012 #14

    psparky

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    You can't have a .1 PF though. Wouldn't you agree?

    BTW.....is the .1 PF leading or lagging on your meter?

    Perhaps a factor of 10 is somehow screwed up on you CT?

    A .1 PF is basically a giant inductor lagging current (or giant cap leading current). That does not make sense.
     
  16. Apr 24, 2012 #15
    This is all the information I have. It shows the data for Month to date on KW, PF trending, and real-time PF
     

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  17. Apr 24, 2012 #16

    psparky

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    Wow.....we certainly see how you get your numbers.

    Nonetheless.....something is messed up. You either need to fix your meter readings....or something in your system.

    It almost seems like if you disconnect that big inductor or cap when the chillers aren't running you save a ton of cash.

    You need to find out what is causing that .1 power factor effect!
     
  18. Apr 24, 2012 #17
    The City has a power quality meter that we are going to hook up to the transformer on the secondary side to see what readings we are truly getting.

    I'm glad you guys see what I am talking about now. We have poor power factor in a lot of buildings and I am trying to correct that, but this is a brand new building and should have been engineered correctly
     
  19. Apr 24, 2012 #18

    psparky

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    Sounds good.

    Let us know what you find.
     
  20. Apr 24, 2012 #19
    It sure sounds like someone tried to correct the pf for when the chillers were on and ended up making the problem worse. Here is an article from Wikipedia suggesting that synchronous motors may have some advantages over switched capacitor banks.

    From: http://en.wikipedia.org/wiki/Power_factor
    Instead of using a set of switched capacitors, an unloaded synchronous motor can supply reactive power. The reactive power drawn by the synchronous motor is a function of its field excitation. This is referred to as a synchronous condenser. It is started and connected to the electrical network. It operates at a leading power factor and puts vars onto the network as required to support a system’s voltage or to maintain the system power factor at a specified level.

    The condenser’s installation and operation are identical to large electric motors. Its principal advantage is the ease with which the amount of correction can be adjusted; it behaves like an electrically variable capacitor. Unlike capacitors, the amount of reactive power supplied is proportional to voltage, not the square of voltage; this improves voltage stability on large networks. Synchronous condensors are often used in connection with high-voltage direct-current transmission projects or in large industrial plants such as steel mills.

    Instead of relying of PF (Physics Forums) for information about pf (power factor), a good electrical contractor could probably give you much better advice.
     
  21. Apr 25, 2012 #20
    skeptic2
     
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