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Power Factor Correction: Capacitor Bank

  1. Jun 18, 2015 #1

    rollingstein

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    Typically the power factor penalty that a utility charges is only for inductive lagging power factors or leading ones too? i.e. is the penalty for all PF differences from unity or only on the inductive side?

    e.g. Say I have a lot of motors which bring the PF down & I add a static capacitor bank to correct the PF & now say, at night, 30% of the motors are idle but the capacitor bank is still fully online and hence the PF is now leading. Is that a problem?

    I can imagine it is a problem for capacitor life but are there other concerns? Is overcorrection as bad as having an uncorrected inductive PF?
     
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  3. Jun 19, 2015 #2

    Hesch

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    Yes it is:

    Motors, transformers and other reactive components consume reactive power, while a capacitor is producing reactive power.

    The PF = cos φ, where φ is the phaseshift between voltage and current. The optimal correction of the PF is when φ = 0 → cos φ = 1. When producing more reactive power than is consumed, φ will be negative → cos φ < 1.

    In practice PF is corrected to about 0.85. If it raises overnight to 1.0 it's not a problem, but if it falls again due to that too much reactive power is produced, some of the capacitors will simply be switched off.

    power-factor-003.gif
     
    Last edited: Jun 19, 2015
  4. Jun 19, 2015 #3

    rollingstein

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    Do utilities penalize for only lagging power factors or for both lead / lag?
     
  5. Jun 20, 2015 #4

    Hesch

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    The power factor is a result of leading/lagging phase shift. The power factor itself is just a scalar(?) with no direction.

    In practice the power grid will always consume reactive power, due to motors/transformers connected, so that the phase of the current will lag the phase of the voltage. Capacitors will produce reactive power, and will thus in pratice always increase the power factor.

    In case of leading/lagging phase shift by turn, a synchronous motor could be used ( and are used ) for compensation: Magnetizing it too much, it will produce reactive power and vica versa, when connected to a grid. ( As a stand alone unit, it'll just produce more or less emf ). So in practice you can see a small building along a powerline, containing a synchronous motor with its shaft sawn of. It's doing no mechanical work, it's just there to produce ( or consume ) reactive power. This positive/negative production can be adjusted completely continuous(?).

    Not good models, but anyway:

    http://www.engr.sjsu.edu/ustrasil/spring2006/ee175_sp2005/WebProjects_Sp2005/MatLab_Chen_Howard/EE175WP_Chen_files/image002.jpg

    http://www.engr.sjsu.edu/ustrasil/spring2006/ee175_sp2005/WebProjects_Sp2005/MatLab_Chen_Howard/EE175WP_Chen_files/image004.jpg
     
    Last edited: Jun 20, 2015
  6. Jun 20, 2015 #5
    In general the leading PF can cause local system stability issues (Voltage oscillations, flickering lights) , and increases wear and tear on the systems ( switches, contactors and motors) - the condition of excess capacitance (leading pf) causes large inrush currents during switching events.
    So while I have not heard of a Utility Penalizing for excessive capacitance (in the billing for example) - they will come out to investigate as this will typically cause problems for the neighboring customers.
    The Lagging PF charge is typically handled (I believe - I have been out of this world for 10+ years ) in two ways. Cumulative KVAR Hours - measured just like Watt Hours - but with Vars, VARs (Volt*Amps Reactive) have a polarity ( + leading and - Lagging ) so if the metering is summing + and - the times the PF is leading would be deducting from the times it is lagging - they will not allow this and just like the typical Watt Hour Meter will not "run backwards" the typical VAR Meter will not as well. The other case is Peak VAR demand - in this case the customer is billed a surcharge for the maximum 15 minute period for the month - that seems harsh right? But the whole reason is that the utility has to supply all of the Current to make the VARs - and they are not delivering power. So all of the systems ( generators, cables, transformers etc) need to be sized to accommodate the total current - even though the delivered current is reactive - it does create a voltage drop in the supplying system.
    Note : there are cases where the utility has an agreement with a suppler to provide VARs - similar to an agreement to provide energy (watts) as from a customer's solar array. A lot of the energy storage systems have some form of VAR control where the utility can command the system to export VARs ( look capacitive to the grid). By providing local capacitance the overall utility runs more efficiently and the generators can be maximized for Energy -- not just current.
     
  7. Jun 20, 2015 #6

    Hesch

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    @rollingstein
    There is some confusion as to #4 and #5 ( Leading/lagging , positive/negative ). But don't care about that.

    It's because I'm from Europe/Denmark where we (could) state PF as:

    Cos φ = 0.8reactive by positive reactive power consumption.
    Cos φ = 0.8capacitive by negative reactive power consumption.
    . . . when we want it to be expressed exact.

    It's just a matter of terminology.
     
    Last edited: Jun 20, 2015
  8. Jun 20, 2015 #7
    I prefer the Reacive as negative, why? the current follows the voltage. Most people have a voltage based mindset. Also "lagging" is similar issue. -- while agree PF is typically defined as apparent power / real power. In most real world applications there is some "sign" ( + or - ) applied. As an engineer this is a matter of conversation, you should learn to "think" about power in a 4 quadrant concept. It took me years to adopt that!
     
  9. Jun 20, 2015 #8

    Hesch

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    I have learned it. I'm an electric power engineer. :smile:
     
  10. Jun 20, 2015 #9

    jim hardy

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    At least one charges by power factor irrespective of lead/lag.
    A good friend bought an old industrial building . His electric bill showed a premium for "low power factor" .
    Thinking it was the fluorescent lights he bought some PF correction capacitors. They made PF worse.

    Finally he found when the previous owners had removed the big motors and transformers they'd left the PF correction capacitors behind.
    Finding and disconnecting most of those solved his trouble.

    You'd have thought the utility would be happy to have some free capacitive vars out there.
     
  11. Jun 21, 2015 #10

    rollingstein

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    That's what I'd have thought too. Shouldn't they be happy about the free capacitive vars? When the entire system is so chock full of inductive loads?
     
  12. Jun 21, 2015 #11

    rollingstein

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    Yes. Me too. I find the signs confusing. And I don't deal with this stuff on a daily basis. So I always preface my power factor with inductive / capacitive.
     
  13. Jun 21, 2015 #12

    jim hardy

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    Where i worked we called them "Leading or Lagging" megavars,
    leading(capacitive) we called negative and lagging(inductive) positive. edit i'd flipped those at first - mild dyslexia jh
    I remember re-arranging the megavar meter and the voltage regulator null meter on our control board
    so that when operator raised excitation , everything moved up
    field amps , generator volts, megaver meter, and null meter should all move same direction in response to an action.
    I had to invert the megavar meter. it had leading on the top half so we flipped it.

    That made it more intuitive which way to go during a grid upset.

    If i can find an old photo will post it. Maybe Dan has one.
     
    Last edited: Jun 21, 2015
  14. Jun 21, 2015 #13

    rollingstein

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    Leading and lagging is very clear too. It's when someone uses positive or negative that I get confused.
     
  15. Jun 21, 2015 #14

    rollingstein

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    How does a VAR meter work internally? I mean, Wattmeter are common & I sort of know how they are made internally. They have been around since pre-electronics era.

    But in a non-electronic era was there a trick to measuring VARs? How did one design a purely electro-mechanical device to measure only the reactive component?
     
  16. Jun 21, 2015 #15

    jim hardy

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    Our plant was a 1960's design.
    The megawatts were measured by a "Lincoln Thermal Converter" , a quite accurate device from the 1930's
    that uses voltage and current from the metering transformers to heat a thermopile in a box with precise thermal characteristics.
    That thermopile's millivolt output is proportional to power,
    and its thermal mass gives it a few second time constant low pass response..(old adage "Encourage Mother Nature to help your design work well")



    Realizing that any device that multiplies VIcosθ will report watts
    if we fool such a device by handing it a current signal 90 degrees out of phase
    it'll report the vars instead.
    Our megavar meter was an inexpensive little Hall effect analog multiplier.
    Less accurate than the thermal converter but faster..
    It received phase AC voltage and phase B amps
    which if you draw your phasor diagram, are 90 degrees out of phase.

    Megawatts are the holy grail in a power plant so megavars were not measured with equal precision.
    Megawatts were the prominent display in the control room, displayed on a huge recorder high on the panel.
    Megavars only got a solitary meter near the generator voltage and ammeters.

    hope that helps.

    old jim
     
    Last edited: Jun 21, 2015
  17. Jun 22, 2015 #16

    rollingstein

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    I checked the fine print of my utility. It too penalizes irrespective of lead / lag. Sad.
     
  18. Jun 22, 2015 #17

    rollingstein

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    I've an interesting situation. My utility gives a 5% discount on the bill if power factor is maintained 0.985 or above.

    Say I add a capacitor bank. How much is the parasitic resistance of a correction capacitor bank? I wonder if those kWs will eat up a large chunk of this 5% incentive.

    Commercially / Practically you cannot get a purely capacitive capacitor right?
     
  19. Jun 22, 2015 #18

    anorlunda

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    I doubt that the resistance of the capacitors will eat up much of that discount. If they did, two things would result. They would get very warm and perhaps need some form of external cooling. Nobody would make them for this application because there would be no market.

    BTW what were you thinking using any form of the word "pure" in an engineering forum? :smile:
     
  20. Jun 23, 2015 #19

    rollingstein

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    Cooling fans seem common. e.g. See here:

    https://library.e.abb.com/public/a8eacbc0eb7634be852578df0067d127/1SXP981002D0202.pdf

    OTOH, you are right about low losses. ABB quotes 6 Watts per kVAR. That seems approx. 0.5% power loss.
     
  21. Jun 23, 2015 #20

    Hesch

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    From your attached document:

    Capacitor total losses are less than 0.5 watts per kvar. Auto-
    bank total losses (without reactors), including accessories such
    as Power Factor Controller and contactors are less than 1.5
    watts per kvar.


    So it's not due to losses in the capacitors, that cooling is needed.

    The significant losses, due to reactive power, are in the grid. That's why someone will give you 5% discount.

    Of financially reasons, the PF is in practice only compensated to about 0.85. Capacitor banks are expensive and are worn due to short circuits inside the capacitors. They are made self-healing, but they loose capacitance every time such a short circuit/healing takes place. So within some amount of years you will have to replace them. Your 5% discount must pay this replacement.
     
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