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Electrical Connection: Effects of Single-Line-to-Ground Faults

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Article below this video of a serious ground fault:

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Effects of Single-Line-to-Ground Faults

Rich Bingham

If you're confronted with a voltage sag on a three-phase system that you suspect was caused by a line-to-ground fault, here's a handy table of voltage reductions.

In an IEEE paper on "The Impact of Voltage Sags of Industrial Plant Loads" by Charles Smith, Jeff Lamoree, Paul Vinett, Tom Duffy, and Mike Klein, the following table shows what the transformer secondary voltages would be with SLTG faults. The table shows the tradeoffs on the impact to the phase voltages that occur, based on the wiring configuration of the transformer.

(Thanks to David Lubkeman for spotting and correcting an error.)

 

TRANSFORMER SECONDARY VOLTAGES (pu)
WITH Single-Line-to-Ground FAULTS
TRANSFORMER CONNECTION
(primary - secondary)		 Phase-to-Phase Phase-to-Neutral
  Va Vb Vc Van Vbn Vcn
Grounded wye - grounded wye 0.58 1.00 0.58 0.0 1.0 1.0
Grounded wye - ungrounded wye 0.58 1.00 0.58 0.0 1.0 1.0
Ungrounded wye - ungrounded wye 0.58 1.00 0.58 0.33 0.88 0.88
Ungrounded wye - grounded wye 0.58 1.00 0.58 0.33 0.88 0.88
Delta - delta 0.58 1.00 0.58 ------ ------ ------
Ungrounded wye - delta 0.33 0.88 0.88 ------ ------ ------
Grounded wye - delta 1.00 0.5 0.5 ------ ------ ------
Delta - grounded wye 0.88 0.88 0.33 0.58 1.00 0.58
Delta - ungrounded wye 0.88 0.88 0.33 0.58 1.00 0.58

 

For wye-wye and delta-delta connections two phase-to-phase voltages will drop to 58% of nominal, while the other phase-to-phase is unaffected. However, for delta-wye and wye-delta connections, one phase-to-phase voltage will be as low as 33% of nominal, while the other two voltages will be 88% of nominal. It is the circulating fault current in the delta secondary windings that results in a voltage on each winding.

 

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