Line to Line fault upstream of Delta / Wye(Grounded) transformers
Line to Line fault upstream of Delta / Wye(Grounded) transformers
(OP)
I am looking into an issue where several Delta / Wye(Grounded) transformers where damaged due to a line to line fault upstream from the transformers. The transformers are step downs for businesses. Can someone explain why the primary fault protection fuses would not protect the transformers from damage?
Thanks in advance.
Thanks in advance.






RE: Line to Line fault upstream of Delta / Wye(Grounded) transformers
The transformer fuses wouldn't know anything about an upstream fault.
But primary fuses provide only marginal transformer protection.
Did you mean to say **downstream** of the transformer? This would be a through fault and could definitely cause problems for the transformer if not cleared quickly enough. Primary fuses may not provide adequate protection against through faults.
RE: Line to Line fault upstream of Delta / Wye(Grounded) transformers
Thanks for the reply....
The fault was upstream about 1/2 mile. The transformers were at the end of the line so to speak as they were step down transformers that serviced buildings.
RE: Line to Line fault upstream of Delta / Wye(Grounded) transformers
Multiple transformer failure is more of a sign of overvoltage conditions, think lighting induced overvoltages. I agree with dpc too.
Check lighting protection on failed transformers.
Any other thing common to those transformers? location , type, cable connection?
What makes you think the damage is directly related to line to line faults.
Describe the system in more details, voltage, kva ratings, type of primary distribution-overhead?
RE: Line to Line fault upstream of Delta / Wye(Grounded) transformers
~200KVA
They were all within .25 miles of each other. A reported phase to phase fault occured the night that the transformers were damaged. I checked strikenet and no apparent lighting in the area. The clearing breakers upstream from the fault tripped and then attempted a reset. I'm curious about the breaker reset. I talked to another EE that is familiar with this kind of thing and he said that sometimes the transient voltage as a result of the breakers resetting can approach 5-7 times the nominal. If this occurs the winding insulation may get damaged resulting in the shorting of phases or arcing to case ground etc...
RE: Line to Line fault upstream of Delta / Wye(Grounded) transformers
Edit: The transformers are all pad mount, Delta/Wye(Grounded). The service from the utility overhead until the utility pole and then is routed underground to the transformers.
RE: Line to Line fault upstream of Delta / Wye(Grounded) transformers
What type of damage occurred?
RE: Line to Line fault upstream of Delta / Wye(Grounded) transformers
RE: Line to Line fault upstream of Delta / Wye(Grounded) transformers
Lighting can be the cause of L-L faults in certain cases, particularly in three-wire (delta) networks.
It is possible that the same lightning that caused your L-L upstream fault ALSO zapped your xmfrs.
Lightning (or any other transient) propagating down a line will 'pile up' (for lack of a better term) at points of impedance change, such as a transformer winding.
RE: Line to Line fault upstream of Delta / Wye(Grounded) transformers
You should make sure the new ones get surge arresters installed on the primary.
RE: Line to Line fault upstream of Delta / Wye(Grounded) transformers
I assume since you said the breaker tripped that there was no single phasing of the tranformers. Single phasing could cause ferroresonant overvoltages.
RE: Line to Line fault upstream of Delta / Wye(Grounded) transformers
Is the primary source grounded or ungrounded? Impedance grounded? Ground faults on ungrounded or impedance grounded system can also cause overvoltages. How sure are you that it was just a line-line fault not involving ground?
Large capacitor switching can also cause voltage transients.
RE: Line to Line fault upstream of Delta / Wye(Grounded) transformers
RE: Line to Line fault upstream of Delta / Wye(Grounded) transformers