Power Transformers

[Pup7]

We have two 6KV XFR that feed two individual 380V MCCs with an ATS that closes the Tie brkr between them whenever one of the XFR feeders fail. Last night one of the XFR feeders tripped on over current and we found the XFR that tripped is pulling five times (300amps) on the primary side then the other XFR, while the Tie brkr is open. When we open the XFR feeder and close the tie putting all the load on the other XFR, the amperage on that XFR goes from 65 amps up to 160 amps. which indicates normal load. We can close the brkr on the XFR that tripped without any problem, but as soon as we close the secondary side brkr it will go into this high current condition on the primary, are we looking at a possible secondary winding fault problem, that only the primary side protection is picking up?. Any help will be appreciated at this time, as we are in the process of further investigation at the present time.

 

[rbulsara]

Check the secondary side breaker of the problem transformer for some sort of a fault/ insulation failure. Winding fault would be picked up without closing the breaker.

I presume that when you close the transformer secondary of the second transformer, the tie breaker is open.



Rafiq Bulsara

http://www.srengineersct.com

 

[rbulsara]

Was this ever working OK? or is this a new installation?

Rafiq Bulsara

http://www.srengineersct.com

 

[Pup7]

It is a new installation but has been on line and working per designed for about 5 months now. This problem just started last night and yes your assumption is correct the tie brkr is definitely open when we close the secondary side brkr

 

[waross]

It sounds like a bad transformer. A loose winding that is moved by the magnetic forces when under load may have finally damaged the insulation, but only shorts out when the load current causes it to move.


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

 

[rbulsara]

Is the 300A current in all 3 phases or only one or two? Are there any recordings? Is the 300A a steady overload that lasts for a few seconds or a momentary spike?


Rafiq Bulsara

http://www.srengineersct.com

 

[jghrist]

Is it possible that the CT ratio is incorrect but has not caused a trip before because the loading was lower?

 

[rbulsara]

jghrist:
I encountered that type of incident once, not too long ago. There, in fact, one the CTs was connected backwards, but on low loads it was not evident.



Rafiq Bulsara

http://www.srengineersct.com

 

[Pup7]

The 300 amps is on all phases and is steady, the only variance is load related on the MCC. It can start at 285 amps when the secondary brkr is closed and progress form there until it reaches the over current trip setting.
The CT ratio is identical on both XFR’s and both have been on line since the beginning with a comparable load, which is why this has thrown us for a cruve since it just appeared a couple nights ago. The other XFR is carring the load for both MCC’s without any problems @ +/- 150 amp primay load
Any way to verify a bad XFR (the damaged insulation) ???? as we are in a remote location, thanks for the help

 

[mgtrp]

A balanced three-phase fault of relatively high impedance that's only present under load? I think that I'd be looking very closely at the protection relay for some kind of issue there, it sounds a bit too neat to be a problem with the transformer insulation.

 

[rbulsara]

I tend to agree with mgtrp at this time. It is tough to diagnose an issue without being at the site.

Although checking the transformer itself should be easy, as you are able to isolate it without affecting the load.

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

Can you get an independent meter in the circuit somewhere? Just one phase to verify that your metering is ballpark correct.

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