Transformer Rating 3

[KnicksJets]

Hello all
If my transformer has the following rating:
337200 KVA with ONAN cooling (55 deg C rise)
What does that mean?
I am guessing with only natural oil circulation/cooling (no fan/pumps), the transformer is rated for 337MVA output, right? Then what is 55 deg C rise? Does that mean after the winding temperature goes above 55 deg C, the MVA output from the transformer should be reduced to 337MVA? Just cause I loose cooling in my transformer (unless winding temperature has gone above 55 deg c + the ambient) I don't think I have to reduce the load to 337MVA.

What if the deg C rise is not given? One of the transformer says:
Rated Power CLASS OA 364000 KVA, but no rise is given for any stage.
Later the nameplate says:
Temperature rise winding: Max 65 deg C, so I guess I'd use 65 deg C here.

 

[magoo2]

ONAN is a self cooled rating (no fans, no pumps). (Oil Natural Air Natural).

55 C is the average winding rise at the nameplate rating. Older transformers (prior to 1960 or so) all used to be 55 C rated.

Then the 65 C insulation system was developed. For the same MVA, a 65 C rise design is smaller than a 55 C design.

In the transition, a 55 C/65 C rating was used. This gave the output for the same transformer at a ration of 1:1.12. In other words, you get 12% more MVA when you allow the average winding temperature rise to reach 65 C compared to staying at 55 C.

 

[KnicksJets]

Thanks for the response, but it basically elaborated on what I was saying.
My question is:
If this is the nameplate rating:
337200 KVA with ONAN cooling (55 deg C rise)

Does that mean, my transformer is rated for 337MVA only, if no forced cooling is provided? Does that mean I have to lower the MVA output (via Generator) to 337 MVA (normal is 500 MVA) right away, or do I wait until the winding temperature is 55 deg C over the ambient? Bottom line is how does this rating help in terms of what I need to do when my cooling (fan/pumps) is lost.

 

[magoo2]

The ONAN rating is the capability of the transformer without any additional cooling. If that's the situation, then you're limited to 337 MVA.

 

[dpc]

Yes it means the maximum transformer rating (per the nameplate)with no cooling is 337 MVA. If the transformer is equipped with fans and pumps, it should have multiple ratings above 337 MVA based on the additional cooling.

It would help if you provided the complete rating data from the nameplate.

But basically, with no fans or pumps running the continuous load should not exceed 337 MVA at the rated ambient temperature.

The true limitation is the actual temperature of the windings. Assuming the transformer is rated for an average ambient of 30 deg C, the average winding temperature , based on the ONAN rating should not exceed 30 + 55 or 85 deg C. In addition, there will be a hot spot allowance, probably 10 deg C above this, so that the maximum winding temperature does not exceed 95 deg C. Because of this, at colder ambient, it may be possible to exceed the 337 MVA rating provided you have some means of monitoring the winding temp.

In addition, ANSI C57 gives recommendations on the probably loss of transformer life under various overload scenarios.

 

[Zogzog]

Class “A” insulation - 8C above max temp rating= ½ life expectancy
?Class “B” insulation - 12C above max temp rating= ½ life expectancy

I think thats the C57 reference you were talking about.

 

[stevenal]

I agree with DPC, except I see no reason to drop the loading immediately upon loss of cooling. The heating takes time.
To avoid loss of transformer life, if the pumps and fans fail, repair the problem or reduce loading prior to the hot spot winding temperature gages reaching 95.

 

[KnicksJets]

Thanks DPC, Magoo and Stevenal for the input.