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 Post subject: Ground Fault Protection for Primary of a Transformer
PostPosted: Mon Nov 26, 2012 10:31 am 
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Joined: Mon Oct 10, 2011 4:07 pm
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Location: Phoenix
I wanted to reach out to this forum for some opinions on this subject. System is a 4160V primary 480V Secondary 575kVA XFMR. Electro-mechanical relays I've set the GFPU settings somewhere between 25-30 % of the phase pickup settings as well as the time dial to roughly the same. Any opinion or guidance is appreciated. My question is on the intananeous setting and on ground fault protection all together.

Do the ground fault settings have to account for the transformer inrush or does this only come into play on the phase to phase?

The system is an ungrounded (open Delta system), is ground fault protection even efficent on an ungrounded system?

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PostPosted: Mon Nov 26, 2012 12:37 pm 

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25 - 30% sounds like a pretty common rule of thumb that I have heard before. However, I'm not sure GF protection is necessary on an ungrounded system. My way of thinking is Ungrounded = no ground faults.

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PostPosted: Tue Nov 27, 2012 7:58 pm 
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You are required by Code in the U.S. at least to implement ground fault alarming. This is done by detecting voltage imbalances (e.g. phase lights), rather than by current imbalances.

Though the transients generated by an "ungrounded" (capacitively grounded) system are particularly nasty for non-self restoring insulation, to say nothing of an L-G-L fault (burndown), ground fault relays won't help you one bit to do anything about it.

A much better solution is a high resistance grounding system. In this case you still have very low ground currents (less than your normal phase currents), the ability to trip or alarm on them (your choice), potentially no harm to personnel in a ground fault incident (if you do trip and keep it sufficiently low), and you can have coordination on ground faults making fault localization much easier. The cost for a 480 V system is not that much compared to 4160 V or higher where the resistor itself can get fairly expensive. How much? Usually around $10K for a 4160 or 7200 volt system with a 25 A resistor, or up to around $40K for a 400 A resistor on a 22.9 kV system are relatively recent numbers I have.

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PostPosted: Mon Dec 03, 2012 6:56 pm 

Joined: Sat Apr 18, 2009 6:31 am
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Location: Kuala Lumpur, Malaysia

Apart from system grounding method, the transformer inrush current can produce spill zero sequence current hence disturb the ground fault relay.

We normally set the [SIZE=2][font=TimesNewRoman]GFR curve so that the tripping time is in between[/font][/size]

0.6s and 1s, or even longer depends on transformer rating in order to ride through the inrush.

575kVA xfmr typically has time constant less than 0.4 sec.

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