Transformer Overheating 2

[AaronLentswe]

I have two 10MVA ONAN rated transformers operating in parallel. These serve as standby supply for a Soda Ash mine, who has own generation plant of 15 MVA.

When the mine generation is out, the total load is about 8MW on each transformer.

At this load one transformer's winding gauge shoots to 105 degrees Celcius whilst the other remains 85 degrees Celcius.
The oil gauges of the two transformers are comparable, reading close to 65 degrees Celcius.

What could be the cause?

 

[Zogzog]

Clogged cooling fins, harmonics, oil dielectric breakdown, etc. Have you run any tests?

 

[edison123]

Gauge fault ? Try interchanging the gauges and see if the high temp goes with the gauge or stays with the trafo.

* If a little knowledge is dangerous, then I am the safest person in the world *

 

[DaveScott]

My first thought was: have the transformers the same impedance voltage? For this size, I could guess about 10%?If not, there will be circulating currents. But I think this is probably not the case, since the load looks the same.

The next thought was: the winding temperature instruments are calibrated to match the characteristics of the transformer. The high reading on one of them could be simply that it is not reading correctly.

 

[unclebob]

Hummm. What about the oil temperature?

 

[waross]

If one transformer has more voltage applied, if one transformer has the taps set at a lower voltage, if one transformer has a slightly lower ratio, that transformer will probably run hotter.
If the impedance of one transformer is less than the other, that transformer will take a greater share of the load.
respectfully

 

[jghrist]

If the oil temperature and current is the same for both transformers, and the winding temperature is different, then the calibration of the winding temperature indicator is different. An analog winding temperature indicator uses the current from a CT to heat the oil in a well to simulate the increase of winding temperature over oil temperature. It doesn't directly measure the winding temperature. I'd go along with DaveScott and check the indicator.

 

[electricpete]

My vote is to divide and conquer.

First step is to check the current on CT secondary to see if you have a current difference at that point.

If different: next step is to check CT taps for equivalency. Then investigate for actual primary current difference.

If same, investigate temperature indicator and heater circuit.

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[a430]

Hello everybody,
Iam with electricpete,check for any loose connection on the CT circuit.