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Meggar a transformer

  1. Jul 15, 2013 #1
    Could you kindly show me as to how to meggar a transformer step by step with relevant pictures ...?? Very urgent..kindly reply.
     
  2. jcsd
  3. Jul 15, 2013 #2
    Do you have a megger?
     
  4. Jul 15, 2013 #3
    This is a potentially hazardous activity - you should get a qualified person to do or teach you.
     
  5. Jul 15, 2013 #4

    berkeman

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    What's megger? Is it like a Hi-Pot test?
     
  6. Jul 15, 2013 #5
    HiPOt is a High voltage (AC or DC) test that is (should be ) use as a Proof Test - i.e. Pass Fail - typically if the unit under test (UUT) is able to withstand the voltage for 1 minute it has passed (For cables or MV equipment the process may take much longer as the voltage is slowly brought up to maximum).
    A megger is a Meg-Ohm test, usually at elevated DC voltage, but not as high as a HiPot.
    The benefit of the Megger is that it should provided a Meg-Ohm reading, which can be compared to past or future readings - as well as the manufacturers suggestions. There are still many factors affecting the reading however (Temperature, moisture) - so experience is needed in evaluating the results. A larger 600V Class Motor or transformer - may be meggered at 1000 to 5000V - since the winding do have significant capacitance to ground, it is possible to leave a stored charge on the motor - creating the hazard. Most meggers do discharge the UUT after the test, but this will not protect you if you pull the leads off during the test - or the leads just fall off and then you try to re-apply them with the stored charge.

    Also - in the electronics world ( think PCBs) I have often seen the HiPot and Megger used interchangeably - drives me crazy, they are usually doing a hiPot ( proof) test.

    BTW -- DC HipPOt of MV cables is particularly hazardous ( and fallen out of favor for AC, due to damage it may cause to the cable) but if someone is properly trained to do DC HiPot they should be OK with the megger.
     
  7. Jul 16, 2013 #6
    If you don't know how to megger test a certain piece of equipment you shouldn't do it yourself. The typical way to do it is:
    1. check if the the device is running, in what mode, can you safely switch it off, do other people need the device on standby etc.
    2. switch off the power supply to the device
    3. prove there is no voltage with a voltmeter
    4. test the device with a 500 or 1000 volts between each of the phases and the ground
    5. if it's a motor or a device with windings you can megger test between winding and winding

    if it's any high voltage equipment you SHOULD NOT do it if you are not qualified for that kind of a job. also, be careful wile testing electronic equipment since 500-1000 volts can burn the electronic elements. in either case, don't do it if you never did it before. ask someone to help you out.
     
  8. Jul 16, 2013 #7

    jim hardy

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  9. Jul 17, 2013 #8
    Hello Jim - I agree with that, I believe the OP was about Meggering the Transformers specifically - how to connect, what to do with the other windings, etc.
     
  10. Jul 17, 2013 #9

    jim hardy

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    ahhh thanks I was confused - thought perhaps he was just afraid to get started.....

    he needs to watch somebody who's accustomed to doing it and ask about the little details .

    It becomes second nature to be safety- aware but he'll need a mentor for first few times through.

    Old timers do little things like not grasp a wire before they've brushed it with the back of their hand. If you grasp it and it's 'hot' your finger and arm muscles will tighten up and you can't let go... so they form the habit of brushing a hand across it in a position that tightening muscles will pull the hand away, making it an 'inherently safe' gesture.
    These things are generally not taught in engineering curricula.


    My advice to OP - find the oldest electrician in your facility and ask him for a lesson in how to use a megger. He may be a crusty and grumpy old guy, but will appreciate an honest question if it is asked with humility. Tell him you're a 'tenderfoot' .

    old jim
     
  11. Jul 18, 2013 #10
    There are old field engineers, and bold field engineers - but there are no old bold filed engineers!
     
  12. Jul 18, 2013 #11

    Baluncore

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    What a field engineer should show you is how to isolate and lock-out the power, then verify it is safe. All lines from the transformer must then be labelled and disconnected.

    With an ohm-meter verify that the different windings are continuous as expected and identify the isolated groups of terminals to each winding. All connections to the primary will then be joined, likewise all secondaries will be joined to their own other terminals. You will then have a number of terminals, one for each winding, plus one for the transformer's core or chassis. There may also be an internal shield or screen with only one terminal.

    A Megger will then be used to test the breakdown voltage from each winding terminal to every other winding terminal plus the chassis and screen. If leakage current is less than the specification it will pass. Once all combinations have passed it can all be joined back as it was. Check it is all correctly wired before beginning the re-commissioning process.

    You will never work on HV equipment alone. You and your partner will both be trained in electrical isolation procedures, recovery and resuscitation.
    With time you may find and begin to take short-cuts to get the job done quickly at the expense of safety. Then, when you are overconfident and tired one day, you will die.

    Humans have evolved to respect animals such as crocodiles, tigers, snakes, sharks and mosquitoes but not yet HV. If you now start to think of all HV circuits as having a malevolent personality, with only one aim, namely to kill you, then you will show HV the respect you would a wild animal and so live a long and productive life.
     
    Last edited: Jul 19, 2013
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