Line to Line fault upstream of Delta / Wye(Grounded) transformers

[kzoo]

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.

 

[dpc]

Not sure why a line-to-line fault **upstream** of a transformer would have much impact on it at all. You would loss voltage on those phases during the fault, which might be a problem for some loads, but shouldn't really bother the transformer too much.

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.

 

[kzoo]

dpc,

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.

 

[rbulsara]

What type of damage?

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?

 

[kzoo]

12.7kV/480V
~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...

 

[kzoo]

I don't see where you can edit a post so I will post twice...

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.

 

[dpc]

I suppose reclosing could result in a transient overvoltage at the transformer, but normally transformers have surge arresters on the primary terminals that would protect them.

What type of damage occurred?

 

[kzoo]

They didn't have surge arresters on the primaries, the transformers had to be replaced... 2 were catastrophic.

 

[tinfoil]

You might want to look into what initiated the fault.

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.

 

[dpc]

If there were no surge arresters, it's possible that lightning caused the failures, along with the upstream faults.

You should make sure the new ones get surge arresters installed on the primary.

 

[jghrist]

It would be unusual for lightning to cause a L-L fault because L-L BIL will usually be higher than L-G BIL. Do you know the cause of the fault? Voltage on the unfaulted phase can reach 1.7 times normal during a L-L-G fault. You could get higher voltages during a arcing faults that restrike when there is significant capacitance (either line capacitance or L-G connected capacitor banks).

I assume since you said the breaker tripped that there was no single phasing of the tranformers. Single phasing could cause ferroresonant overvoltages.

 

[rbulsara]

Key is to look for the cause of the overvoltages and regardless of that implement protection against overvoltages at the transformers' primaries. As dpc said it would include surge arresters (a better term than lighting arresters).

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.

 

[kzoo]

Sorry people, I went back a looked at my notes and the transformers were wye-wye grounded not delta-wye. I'm fairly sure that it did not involve ground. The transmission lines also supply houses and no issues with single phase transformers were reported.

 

[rbulsara]

I don't think wye-wye or delta-wye has anything to do with the incidents. I still stand by earlier assessment, from the information available.