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Technique used in temperature rise test of oil-immersed transformers to determine the thermal constant by an experimental estimation

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Update time : 2024-09-27 08:41:19
List of symbols: 

Temperature in °C

Oil temperature, varying with time (this may be the temperature of the oil at the top, or average oil temperature

External cooling medium temperature (ambient air or water) assumed to be constant

Oil temperature rise above
Ultimate values in steady state

Remaining deviation from steady-state value
Time constant for exponential variation of bulk oil temperature rise

Time interval between readings

Three successive temperature readings with time interval h between them. 

In principle, the test should continue until the steady-state temperature rise (of the oil) is ascertained.  
It is considered that: 

– the ambient temperature is kept as constant as possible; 

– the oil temperature will approach an ultimate value along an exponential function with a time constant of To; 

– the equation (2D.2) is a good approximation of the temperature curve (see Figure 2D.1). 

Given three successive readings, the exponential relation of equation (2D.2), is a good approximation of the temperature curve, then the increments will have relation.

Successive estimates are to be made and they should converge. In order to avoid large random numerical errors the time interval h should be approximately To should be not less than 0.95. 

A more accurate value of steady-rate temperature rise is obtained by a least square method of extrapolation of all measured points above approximately 60 % (estimated by the three point method). 
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