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Transformer Grounding

08/24/2012 9:47 AM

What were the possible causes of having a voltage readings between three lines (phase ABC) and the grounding system even the three lines were disconnected from the circuit.The secondary of the 13.2KV/220V distribution transformer is a delta-connected/ corner grounded and bonded to the primary windings.The line to line voltages were Zero in this condition but the line to ground voltages at load side were 180V, 150V and 80V for line A,B & C respectively. ( the trasformer was energized but the main breaker was in OFF position.) Hoping for your comments.

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#1

Re: Transformer Grounding

08/24/2012 11:04 AM

You've got a short somewhere. You'll have to have it lengthened.

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#9
In reply to #1

Re: Transformer Grounding

08/27/2012 6:41 AM

Lyn, maybe there is no possibilty of short because our main breaker that was in OFF position is located right after the transformer secondary. Thanks for your comment.

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#2

Re: Transformer Grounding

08/24/2012 11:08 AM

delta-connected/ corner grounded and bonded to the primary windings...
Can you plz explain the above terms.

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#5
In reply to #2

Re: Transformer Grounding

08/25/2012 6:08 AM

Our utility company have a standard of grounding the neutral of star connected primary to earth while the secondary delta connected has no neutral but one of its corner between two windings is grounded to earth. Then from this transformer a main breaker is installed to to protect the secondary windings. But we notice that even this main breaker was in OFF position a line to ground voltage registered in the three lines to the ground. Is this normal to this type of system?

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#3

Re: Transformer Grounding

08/24/2012 12:07 PM

delta-connected/ corner grounded and bonded to the primary windings

Have you a diagram? Sounds like a good setup for a ground fault.

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#8
In reply to #3

Re: Transformer Grounding

08/27/2012 6:37 AM

Sir, the diagram of WAREAGLE below is the set-up of our system. Hope it is in order!

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#4

Re: Transformer Grounding

08/24/2012 6:21 PM

This type of setup can be found in many industrial locations. The delta secondary does not have a neutral connection. If one of the phases faults to ground, there is no indication of the fault because there is no connection to ground. To solve this, one of the phases is intentionally grounded so that if any of the other phases fail there is a path of return. There are methods to indicate the fault such as lights or some sound method. The reading appear to be phantom reading. Are you using a digital voltmeter? You can see from the dwg that the voltage reading from C to gnd has to be zero. To check, connect a load from phase to ground and then check the the voltage. Please do not just take this information and drop out of site. Return and tell us what you found.

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#7
In reply to #4

Re: Transformer Grounding

08/25/2012 6:26 AM

Sir your are correct there was no voltage reading between line C and ground. If there will be readings FOR THE OTHER 2 LINES TO GROUND, any solution to the problem? I alraedy ask the assistance of the utility technicians to check the connection of their distribution transformers and to find out why their installed lightning arresters often busted even during summers.

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#6

Re: Transformer Grounding

08/25/2012 6:19 AM

Thanks for a bright idea. Sir we were alarmed to this condition because everytime we did a PMS to the motors our electricians cannot touch the motor without using an insulated gloves though our main breaker from the secondary windings was in the OFF position. I will try to use an incandecent bulb to connect it to the line to ground if it will light as you suggested.

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