Thursday, August 9, 2012

Zig-Zag grounding transformer

Figure below shows a zig-zag grounding transformer connected to an ungrounded three-phase system. The two windings marked as “a1” and “a2” are on the same core
leg; likewise, “b1” and “b2” are on another leg, and “c1” and “c2” are on the third core leg. All windings have the same number of turns, but each pair of windings on a leg
is wound in opposite directions. The ground-fault current distribution for the fault on phase “A” is shown.
The short time rating of the zig-zag grounding transformer is equal to line-to-line voltage multiplied by the rated neutral current. The transformer is designed to carry
its rated current for a short time only, such as 10 s or 1 min. The continuous current shall be at least 10 or 21% of the short-time 10 s or 60 s current rating, respectively.

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