Posting a message to the forum will remove the above advertisement

Discuss Checking PFC capacitors. in the Industrial Electrician Talk area at ElectrciansForums.co.uk.

Please make sure you checkout our forum sponsors, many do discounts for members and, they keep the forum free to use.
  1. Tiny Spark
    Offline

    Tiny Spark Electrician's Arms

    Location:
    Telford
    Hey guys,

    Hoping someone can point me in the right direction here.

    So I have aquired the responsibility of looking after the PFC units on site, Problem being nobody knows anything about testing the caps including me!

    So basicly they are 3 phase units with anywhere between 5 and 12 stages depending on the unit.

    The caps are all the same ABB 50 kvar 15kv 415v units.

    They are conected through a contactor and 100A fuses.

    Now checking fuses and contactors is fine but I need to know how to properly test the caps.

    A rather unhelpfull contractor just told me to calculate the expected uF value and test it with my meter.

    I found a formula:
    Mf = kvar/(kv x kv x .314)

    Which gave me 0.7.

    But if I test the caps phase to earth my meter is reading around 2nF.
    If I test between phase I just get OL which I assume is because of the shorting resistors on the caps.

    So yeah ime in need of some guidance please guys :)
     
  2. Lucien Nunes
    Offline

    Lucien Nunes Mercury Arc Rectifier Trusted Advisor

    Location:
    London
    I would suggest contacting ABB for information as they probably have a recommended test procedure specific to that model of PFC unit. E.g. to check capacitance and insulation resistance of each bank / step being within a certain tolerance according to age etc. It may be necessary to disconnect the discharge resistors and/or subdivide the banks to determine whether the individual capacitors in the unit are good, all of which they should be able to advise.
     
    • Like Like x 1
    • Agree Agree x 1
  3. Tiny Spark
    Offline

    Tiny Spark Electrician's Arms

    Location:
    Telford
    Thanks Lucien, must admit I never considered calling them!!
     
  4. LankyWill
    Offline

    LankyWill Electrician's Arms

    Location:
    Northants
    What's a PFC unit?
     
  5. Lucien Nunes
    Offline

    Lucien Nunes Mercury Arc Rectifier Trusted Advisor

    Location:
    London
    Power Factor Correction. The control unit monitors the load power factor (assumed to be lagging) and adds more or less shunt capacitance to increase the pf to near unity.
     
    • Agree Agree x 2
  6. westward10
    Offline

    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    Most large sites/factories have them and in some cases factories can use synchronous motors which have the same effect.
     
    • Like Like x 1
    • Agree Agree x 1
  7. Tiny Spark
    Offline

    Tiny Spark Electrician's Arms

    Location:
    Telford
    Basicly PFC is correction to increase the efficiency of the system.

    I work in a large manufacturing plant so we have some very large and current consuming kit.

    All about the cost saving really lol
     
    • Like Like x 1
  8. LankyWill
    Offline

    LankyWill Electrician's Arms

    Location:
    Northants
    I remember now, capacitance reactance is used to correct the sine wave lead or lag. This occurs in installations with lots of motors and lighting. The cap is inserted in parallel with the motor to help combat its inductive nature.
     
  9. Lucien Nunes
    Offline

    Lucien Nunes Mercury Arc Rectifier Trusted Advisor

    Location:
    London
    Parallel capacitance can only correct lagging pf, but a typical industrial load is always significantly lagging. Unless, as per Westward, there are large synchronous (not induction) motors that can be run overexcited to give leading pf, or installed without a load to act as variable capacitors (controlled by the excitation). They are still referred to as 'synchronous condensers'. There's a pic of a nice 125MVAR unit here: Synchronous condenser - Wikipedia - https://en.wikipedia.org/wiki/Synchronous_condenser#/media/File:Templestowe_Synchronous_Condenser_1.jpg[​IMG]
     
    • Like Like x 2
    • Informative Informative x 2
  10. Andy-1960
    Offline

    Andy-1960 Electrician's Arms

    Location:
    Wolverhampton
    The way I do it is is measure the current in each phase feeding each capacitor. A 50 kvar capacitor connected to a 415v supply should draw approximately 70 Amps per phase.
    You will find the capacitors will degrade depending on the age of the units, (generally they have a life of about 10 years) and depending on the power factor of the system you will then have to determine if they need replacing. Based on the current reading you are getting you will be able to determine the percentage of life left in each stage.
    Usually the relay on the front of the panel will tell you the power factor of the system.
    Be careful whe taking the readings as unfortunately it does mean doing live testing.
     
    • Like Like x 2
    • Agree Agree x 2
    • Informative Informative x 1
  11. Tiny Spark
    Offline

    Tiny Spark Electrician's Arms

    Location:
    Telford
    Thanks for that reply Andy.

    I had read something about this method but was not overly keen on the idea as the bus bars that feed the caps are fully exposed down each side of the cabinet and its not a big cab to start with!!

    May have to look at a remote testing method.
     
    • Like Like x 1
  12. mache
    Offline

    mache DIY

    Or get ABB in to survey and report???
     
    • Agree Agree x 1
  13. Rob
    Offline

    Rob Trusted Advisor

    Location:
    North west
    Could do that as an initial start point, but wouldn't be economical to do as a ppm.
     
    • Agree Agree x 1
  14. Tiny Spark
    Offline

    Tiny Spark Electrician's Arms

    Location:
    Telford
    Thats what they have done in the past, trouble is they only do it once in a blue moon due to the cost so want us to do it in house on a more frequent basis.
     
  15. Cadgey123
    Offline

    Cadgey123 Regular EF Member

    Location:
    London

    Always going to be a bit dodge testing with the power on.
    You can get wireless loop ammeters now which you could stick on each phase of the capacitors on load to see what they are pulling.
    As Andy said they should pull around 70 amps give or take a couple of amps on each phase.
    If you have any imbalances at all I would change the cap.

    Ps always be safe when doing these kind of tests.
     
    • Like Like x 1
  16. Pete999
    Offline

    Pete999 Trusted Advisor

    Top Poster Of Month

    Location:
    Northampton
    Private First Class US Army:p as in PFC LankyWill or PFC Pete999 Sorry couldn't resist:rolleyes:
     
    • Like Like x 1
    • Funny Funny x 1
  17. Tiny Spark
    Offline

    Tiny Spark Electrician's Arms

    Location:
    Telford
    7DE9D226-4DDF-4FED-8AC1-0A01E4A3EC2D-843-00000084E2D48DC4.jpg
     
    • Like Like x 1
  18. Tiny Spark
    Offline

    Tiny Spark Electrician's Arms

    Location:
    Telford
    Well heres a pic of what ime working with, may be of interest to those who have not seen inside one before.

    On a technical note though, I put a few clamps on dead then energised the system to see what sort of Amps I was getting on some of the stages.

    I got one at 42A which is obv low and one at 63A.

    What sort of tolerances would be exceptable? 10%??

    Had a chat with the boss and I think we will go down the road of getting some VDE gloves, masks etc and just testing them live with the clamp meter as mentioned above.

    Thanks for the help guys, Ive been off a while but glad to see everyone is still ticking over on here even if its dead on tapatalk!!

    IMG_2840.JPG

    IMG_2841.JPG

    IMG_2846.JPG
     
  19. UKMeterman
    Offline

    UKMeterman Electrician's Arms

    One way to keep an eye on the unit would be to install a modbus based power quality anyliser, these are about £200 for the main unit and then the CTs on top. Then you could monitor the current in all three phases remotely when it is in auto or manual mode. It may be possible with a suitable computer to trigger an alarm when things get seriously out of spec.

    As for live testing readup on arc flash....
     
  20. Tiny Spark
    Offline

    Tiny Spark Electrician's Arms

    Location:
    Telford
    I need to measure each stage individually though not just the main 3 phases.

    So potentulially up to 36 seperate phases per unit.

    As for Arc flash yeah nasty stuff.
     
Loading...
Similar Threads - Checking capacitors Forum Date
Checking other circuits after fitting outdoor socket? Electrical Wiring, Theories and Regulations Jun 28, 2017

Share This Page

  • Electricians Directory Post a Domestic Job Post a Commercial Job