Transformer Plate Question

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sundowner

sundowner

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West Wisconsin
Could someone give me a good explanatioon of what the "rise " means on a transformer nameplate. I see 150 C designations but am not quite sure what I'm looking at. Is it impedence per degree temp rise, while still in voltage tolerance ???

Thanks in advance

Steve
 
IEEE C57.12.00:

4.1.2.1 Cooling air temperature limit
When air-cooled, the temperature of the cooling air (ambient temperature) shall not exceed 40 ˚C, and the average temperature of the cooling air for any 24 h period shall not exceed 30 ˚C.

4.1.2.2 Liquid temperature limit
The top liquid temperature of the transformer (when operating) shall not be lower than ?20 ˚C. Liquid temperatures below ?20 ˚C are not considered as usual service conditions.

4.1.2.3 Cooling water temperature limit
When water-cooled, the temperature of the cooling water (ambient temperature) shall not exceed 30 ˚C, and the average temperature of the cooling water for any 24 h period shall not exceed 25 ˚C. Minimum water
temperature shall not be lower than 1 ˚C, unless the cooling water includes antifreeze suitable for ?20 ˚C operation.

5.11.1.1 Winding temperature rises
The average winding temperature rise above ambient temperature shall not exceed 65 ˚C at rated kVA when tested in accordance with C57.12.90-1999 using the particular combination of connections and taps that give the highest average winding temperature rise. This will generally involve those connections and taps resulting in the highest losses.

The maximum (hottest-spot) winding temperature rise above ambient temperature shall not exceed 80 ˚C at rated kVA for the particular combination of connections and taps that give the highest maximum(hottestspot) winding temperature rise. This will generally involve those connections and taps resulting in the highest losses. The maximum (hottest-spot) winding temperature rise above ambient shall be determined by one of the following conditions:

a) Direct measurement during a thermal test in accordance with IEEE Std C57.12.90-1999. A sufficient number of direct reading sensors should be used at expected locations of the maximum temperature rise as indicated by prior testing or loss and heat transfer calculations.

b) Direct measurement on an exact duplicate transformer design per a).

c) Calculations of the temperatures throughout each active winding and all leads. The calculation method shall be based on fundamental loss and heat transfer principles and substantiated by tests on production or prototype transformers or windings. The maximum (hottest-spot) winding temperature rise above ambient temperature shall be included in the test report with the other temperature rise data. A note shall indicate which of the above methods was used to determine the value.
 
Transformer insulation life is related to its heating. Dry type and liquid filled transformers have slightly different design temperatures.The temperatures to be aware of are:

Insulation rating: 220C is the design life limit for most dry type transformers which is based on class H insulation. The insulation used in liquid filled is usually only rated for about 125C.

Ambient temperature around the transformer core and coil assembly: The design level is 40C for dry types (liquid filled are based on 30C ambients)

Hot spot allowance: 30C - this is a theoretical fudge factor because we can not predict where to measure the actual hot spot inside the assembly.

So the math for the typical allowable dry type temperature rise is: 220C-40C-30C = 150C of allowable temperature increase due to the loading of the transformer.

There are other temperature rises available (i.e. 115 and 80C) which can be due to the insulation used (i.e. class F or B) or the desire for overload capability (i.e. 55/65 in liquid filled).
 
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