overloading a tranny

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The Red Line is dangerous and so are the escalators. And the people who run this outfit don't seem to care that they were written up the papers several times.



Here are two derating rules

"For operation at higher ambients, transformer
loading should be reduced 0.6% of rated KVA for each degree above
30?C average ambient to a maximum of 50?C (122?F). "

and from this thread,
"Load can be increased 1% per degree below 30 degrees C.
For every degree above 30 degrees C the load is decreased 1.5 %. "

I think it's possible to get the conductor to ambient thermal resistance or the conductor temp or some ratio relating these from just these numbers. Then using the 10 C insulation rule may allow pretty accurate lifetime predictions. This should also work for Romex lifetime predictions.

BTW, does anyone know the shelf life of transformers or of Romex?
 
Good stuff jraef.

I read the entire thread but it was never mentioned what kind of transformer and the ambient conditions.

If we're talking about an oil-filled transformer, take a look at ANSI std. C57.91. I've found that document very useful in both understand and applying overload conditions.
 
Temperature Indicators are typically calibrated from 0 C to 160 C. The temperature indicator a white or yellow temperature pointer, and a red maximum indicating pointer that can be reset with an external magnet held by a small chain. The Two-Switch Temperature Indicator used in Class FOA and larger transformers have two micro switches: The first switch closes at about 60 C to start the fans, and the second switch closes at about 90 C to operate an alarm. Each switch opens at approximately 7.5 C below the closing temperature. Naturally, the temperature of the oil will fluctuate, depending on ambient temperature and load. When the temperature rises high enough to activate an alarm, it means the transformer is severely overloaded, the ambient temperature is high, or the fans are inoperative, or a combination of all three factors.

Hottest Spot Temperature Indicator - This indicator is strictly a winding temperature indicator, calibrated in degrees Celsius. A small CT around one winding, which in turn heats a bimetallic strip immersed in oil, operates it. Unlike the percent thermal overload relay, this indicator reads only winding temperature, not top oil temperature. Temperature indicators are typically equipped with auxiliary switches for fans, alarms, and tripping of the transformers circuit breaker. They also have a red maximum temperature needle, which can be reset by a magnet on some models or a push-button on others.

Winding Temperature - Indicators are designed to have the detecting bulb embedded in the center of the winding and are used in both dry-type and liquid-filled transformers. They differ from the percent thermal load and hottest spot indicators in that they read temperature directly. These indicators are calibrated in degrees Celsius and, depending on the manufacturer, read from 0 to over 200. They have a maximum reading pointer, are magnetically re-settable, and are equipped with switches to initiate specific functions (fans, alarms and tripping).
 
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The program below seems to indicate that transformer service life is extremely sensitive to kvA value.
Assuming you use the values that I entered below, with this spreadsheet I fussed with "desired run time in years at new kvA value" until I used up almost all the design life and I found that running it for 24 minutes at the higher value uses up 97% of the design lifetime.

"
Loading should be reduced 0.6% of rated KVA for each degree above
30?C average ambient to a maximum of 50?C (122?F).
so at 50 we have 12% decrease in kva
^From this derating rule Tconductor max = 197 C.

20 >enter design lifetime in years
197 >enter Tconductor at rated kvA in degrees C
30 >enter Tambient in degrees C
300 >enter rated kvA at Tambient


30 >enter new Tambient in degrees C
678 >enter new kvA value
0.000045 >enter desired run time in years at new kvA value
0.01643625 = calc'd run time in days at the new kvA value
0.39447 = calc'd run time in hours at the new kvA value
97 =percent of Design lifetime used
"
 
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