Disconnector problems!

[honglien]

Hi!
Our factory is supplied by using a transformer_1000kVA-35/0.4kV, through an overhead line. At the connection point of the cable, supplied the transformer, we use an arrester to protect all equipments behide. Before the transformer, we use a disconnector with fuse.
One day, after a shortage, the utility company closed again the circuit breaker at the substation, support our factory, and the disconnector was broken.
I don't know why. Someone guessed the broken disconnector caused by the transient surge when switching CB.
Do you think so? Could you tell me your opinion?
Thanks and best regards!

 

[pisimatza]

I think you made a small error: not the disconnector was broken,but the fuse has blown?

 

[RRaghunath]

honglien,
I think pisimatza is right.
It could be fuse, most likely. The HV fuses can blow during transformer energisation.
This happens due to wrong selection of fuse rating or due to ageing.
Fuse is part of 35kV Disconnector assembly, any way.

 

[honglien]

Thank Pis & Rag!
The fuse blown and the support insulator of disconnector was broken. Ofcourse, the fuse is part of 35kV DIS assembly.
The equipments are quite new equiped secondly after the first similar problem 2 months ago. And the specifications for the purpose are okey, I've checked.

 

[jraef]

More likely that whatever fault caused the utility breaker to clear, also cause a lot of mechanical stress on the disconnect and damaged that insulator. I would plan a shutdown to replace them all. I would also do a thorough examination of the transformer, especially the primary winding insulation.

 

[burnt2x]

honglien,
I agree with raghun. Fuse rating and ageing could be the cause. Or maybe your surge arrester is bad. If all else are okay, check transformer.

 

[honglien]

Thanks for all and sorry for not deccribing the problems clearly.
the more clearly problem shown in the file attached.
Thanks!

 

[jghrist]

Most likely cause:
1. Fuse blew because it was not coordinated properly with low side breaker relay.
2. Disconnect switch insulator broke because it was already damaged and the short circuit forces finished the job.

 

[honglien]

Thank jghrist!
1. I agree with you for the first reason. But as I mentioned, I wonder why transformer was not damaged?
2. The second is not very satisfactory because the was not any problem with the DIS' insulator. We had the Disconnector checked before installing and every parameters are ok. I eliminate the supposition that the disconnector was damaged because during the short time operating (2 months), there were no switchings.

Do you think the transient surge from the Overhead Line (when closing the CB at the 110/35kV Substation, supplied our transformer) broke the insulator?

 

[burnt2x]

"Most likely cause:
1. Fuse blew because it was not coordinated properly with low side breaker relay.
2. Disconnect switch insulator broke because it was already damaged and the short circuit forces finished the job."

What jghrist said in 1. is that the fuse time-current curve is lower than the TCC of the low voltage side relay! On your question why the trafo was not damaged; trafo damage curve was not reached therefore the trafo withstood the fault current.
After faults, you have to suspect hidden damages to equipment that the fault passed through. You can only say "there was no/ any problem with the disconnect switch" if you have tested and verified it.