Phase to Phase fault on 800kVA TX

[Padlock01]

Hi Guys
Our site guys tested a phase to phase fault on the MV side of a 800kVA Dyn11 transformer after it blew fuses out 2 phases on the incoming 22kV Line. The 22kV line is shared with 14 other customers. The testing of the LV side 400V was clear. There was no lightning strike.
Feeder cable from the switchgear was tested and found ok. The transformer was repaired some two months prior but has been in service for 6 weeks operating ok. Any thoughts on why it failed.

 

[RRaghunath]

Guess it is 22kV/400V transformer.
Blowing of fuses at 22kV suggests that there is problem on 22kV side of transformer (you have already ruled out problem on 400V side).
Could you isolate on 22kV side and check the insulation resistance!

Rompicherla Raghunath

 

[Padlock01]

HI Raghunath
The 22kV Transformer Windings tested A-C 41 Mega Ohms C-B 41 Mega Ohms A- B "0" Ohms I have a suspicion that phasing was not done on the new 400V MCC between the 2 Incomers The Incomers and Bus section should have been interlock Although both transformers are Dyn11 4 % Z it is designed to run with Bus section open and only close the bus section when one transformer is down.

 

[waross]

If you are testing phase to phase on the primary with a megger, A-B is probably good. There is an open in either A-C or C-B.
A winding may be open or a winding end lead may have broken or burned off.
Any power transformer winding will read zero ohms when the continuity is measured with a megger. The resistance of a power transformer winding is often lower than the lowest reading available on a meg-Ohm scale.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jghrist]

I have a suspicion that phasing was not done on the new 400V MCC between the 2 Incomers The Incomers and Bus section should have been interlock

Did the fuse blow when the secondary bus sections were tied together?

 

[Padlock01]

Hi Waross yes we found C-B had burnt open at the manufactures repair workshop.

Hi Jghrist both fuses blew on the 22kV OHL pole. As I don't work on this remote small coal Mine, it was a case of mums the word when asking the electrical contractor what happened that day.