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Thread: Delta Wye Transformer Phase to Phase Fault

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    Delta Wye Transformer Phase to Phase Fault

    Hey guys, this is my first post.

    Question: I have a 1500 kVA delta (HV)-wye (LV) transformer and the neutral is high resistance grounded. The local utility is on the primary side of the transformer. Recently, during a rain/lightning event, the utility had a phase to phase fault on their distribution line. The fault lasted half a second. During this event, a breaker that was downstream of the transformer tripped on short time protection.

    During a phase to phase fault on the primary side of this transformer, would I have seen any/enough fault current on the secondary side of this transformer to trip any of my breakers? Would lightning have caused this trip?

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    What type of load was the breaker feeding?

    During a primary side fault, you would not expect to see an fault current on the transformer secondary. However, your loads will experience a loss of voltage associated with the primary side fault. Three phase motors will continue to rotate during this condition and will likely draw more current possible resulting in your breaker trip.
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    Awesome. This makes a ton of sense. Appreciate it.

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    Quote Originally Posted by jason.durby View Post
    Hey guys, this is my first post.

    Question: I have a 1500 kVA delta (HV)-wye (LV) transformer and the neutral is high resistance grounded. The local utility is on the primary side of the transformer. Recently, during a rain/lightning event, the utility had a phase to phase fault on their distribution line. The fault lasted half a second. During this event, a breaker that was downstream of the transformer tripped on short time protection.

    During a phase to phase fault on the primary side of this transformer, would I have seen any/enough fault current on the secondary side of this transformer to trip any of my breakers? Would lightning have caused this trip?
    Could it be tripping on unbalanced phase voltages like phase loss?

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    Quote Originally Posted by jim dungar View Post
    What type of load was the breaker feeding?

    During a primary side fault, you would not expect to see an fault current on the transformer secondary. However, your loads will experience a loss of voltage associated with the primary side fault. Three phase motors will continue to rotate during this condition and will likely draw more current possible resulting in your breaker trip.
    High inertia loads also will act sort of like a generator if there is a loss in the normal supply. May not supply the lost leg all that efficiently, but may increase current drawn from working lines also, kind of turns it into a phase converter during such an event.
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