Is it possible to calculate the transformer hot spot temperature with only the information given below?
Top oil temperature
Ambient temperature
Load current
Rated current
Transformer voltage and KVA rating
Results of DGA and Oil analysis that gives the values of water content,interfacial tension, acidity, furans etc.
Thanks
I think you ask a problem about large distribution or high voltage grid transformers, which power, which voltage? The manufacturers themselves are the persons who have most experience in it. Using no-load and short circuit tests. Finite element methods do give less accuracy compared to well made practical experiments. As you are from Autralia, probably the problem is that the transformers have to be kept "cool" in high abient temperature and solar radiation. Is the color white and is it well screened from the sun? There is very little "headroom" in hot spot temperature compared to ambient. White paint is "black" for infrared (high emissivity), so it can still radiate infrared and absorbes less sun. A good option for a grid is to use much more distributed solar energy: It delivers power when the transformers have most difficulties to cool in the sun. In the evening still the aircos are on, but no solar radiation.
It might be interesting to look at the temperature rise in hot countries even without being connected to the grid: ambient air, building, solar radiation. Algeria and other Magreb countries have similar problems.
Could one create thermal inertia with water or cool by evaporation?
Thanks for your reply. I am thinking about temperature raise in both power and distribution transformers. When the load on the transformer remains (almost) constant, what will be the effect of ambient temperature on the transformer oil in out door transformer. All of them are ONAF type. What I wanted to find is the difference between transformer oil temperature and ambient temperature. I can get the transformer top oil temperature from the oil temperature indicator, however I wanted to know the hotspot temperature and find out if ambient temperature has any effect on oil temperature.
If the same cooling is used, there is a kind of thermal resistance for the average transformer temperature, but the hot spot may have 20% more temperature rise in steady state and a shorter time constant. So the transformer temperature follows the ambient temperature with a time constant determined by the heat capacitance and thermal resistance. The thermal capacity can be evaluated by the specific heat capacity of the used materials some 0.4 Joule/Kg/K. The thermal resistance in loss/temperature change. Thermal heat capacity * thermal resistance is thermal time constant. I guess the time constant between 1 and 2 hours, so it may follow the day temperature. In practice, there might be too much temperature change (and change in oil volume), and then it is better to turn on or of the fans, but then it stabilises to a high temperature... More PV in the grid alleviates the load of the transformer during the day and at the end of the day the maximal temperature is expected. A direct sun on the transformer is 1000W/m^2 if the surface is dark and perpendicular to the sun. It is often neglected, but can be relevant.
There is IEEE standard for it. You can refer to that which has some examples.
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