Transformer Fault Analysis

[etgbccc2003]

We have a transformer fault on a 33kV/6kV secondary linkbox of a 15MVA transformer.

The event recorder at the differential protection (using GE multilin 745) shows that the secondary CTs shows about 4000A (not the same value for all 3 pahses but about 4000A while the primary CTs shows about 1750A (not the same value for all 3 pahses but about 1750A).

If we use these values to estimate, a total outzone fault should have caused a secondary CTs to measure about 9625A (1750 X 5.5). Since this transformer measures 4000A at the secondary side, there seems to us that there is also an inzone fault besides outzone fault at all 3 phases. In addition, this transformer have a 8.2% impedence and by calculation, a fault could have caused a 17000A fault current. This suggest that the fault is a higher impedence fault than true fault.

Finally , this linkbox is certainly ingressed by moisture as we can see a lot of rust inside the linkbox. The windings of the transformer wasn't damaged too.

I'd like to ask if the above analysis is correct? Also, what could be the likelihood of the cause of this kind of failure (both inzone and outzone fault occurs at the same time? Could it have been moisture? If there's high humidity in the linkbox, could it have caused a flashover allover?

I find it difficult to explain the coincidence of fault happening before and after the secondary CTs and at all 3 phases.


Thanks.

 

[stevenal]

Ralph,

Yes, you are missing something; CT polarity change from primary to secondary for load current and through faults. By your reasoning above, and substituting a wye-wye for simplicity, primary and secondary would normally be in phase. Due to CT polarity convention they are normally 180 degrees out of phase.

After finally figuring out the rotation is ACB, I agree with etb's declaration that this is a 120 degree phase shift from normal. This is assuming Cts are connected wye and relay reports uncompensated current.

My imagination was lacking the other day. David is right, a source on the secondary side could shift the current up to 180 degrees.

 

[RalphChristie]

Stevenal:

Point taken.
Thanks.






Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon

Regards
Ralph