Difference between Air cooling and Water cooling Transformer

Difference between Air cooling and Water cooling Transformer

First of all, the cooler the better since life expectancy of power transformers is mainly related with (high) temperatures of paper and oil inside. This is one of the reasons why water cooling is preferred, because

Maximum air temperature (mainly 40 or 50 °C or in some places much higher) is well above maximum cooling water temperature (mainly 20 or 30 °C or some places even below that) and
Temperature difference between oil and air in radiators is larger than temperature difference between oil and water in heat exchangers.

In some cases you my receive a much smaller and cheaper transformer if you order a transformer with cooling medium water instead of cooling medium air. (On board of a ship why is diesel engines cooled with water while cars got air cooling?) Also size and costs of the transformer room is much smaller/lower with water cooling if you compare with air cooling.

Due to these reasons large transformers got water cooling wherever possible (look at above pictures), and many of these transformers are used for heating of rooms and warm water. It's much cheaper to use relatively small heat exchangers for water (ODWF or OFWF cooling) than using large amounts of huge radiators where transformer oil.

Is either pumped through (ODAF or OFAF)
Or is moving in natural way due to temperature differences (ONAN or ONAF).

So, air cooling is something for deserts and small and medium transformers up to about 40 MVA. (Under special circumstances there is also air cooling of dry transformers without oil, e.g. cast resin transformers on board a ship or inside of mines…)

Also fans of radiators produce noise which has to be avoided in many places like urban areas. Of course these fans are switched off as soon as possible (at low load conditions of transformer) but average power consumption of water cooling might be much lower under most circumstances. Auxiliary power is added to power losses (on-load and off-load losses) which

Both depend on temperatures and
Are a mayor economic factor as well as
Room and warm water heating.

Anyhow highest temperatures inside a power transformer should never exceed 95 °C but you can hardly measure at that location (hot spot). That's why most measurements are restricted to oil temperature (top oil) and others which are (much) cooler. We assume that hottest location inside of power transformer is conductor at upper part of windings and it's very close to paper insulation which must be protected from aging (de-polymerization).

Control of power transformer consists of 2 major parts:

Protection system and
Heat regulators.

The original question is more or less concerning heat regulators by using 2 different kinds of cooling system. The main items of the answer is that

Water cooling may include heating system (of rooms and warm water)
While air cooling might include switching off fans due to avoid noise (in urban areas).

One of the main (economic) aspects is that water cooling

Is much cheaper (than air cooling) and
Avoids fuel costs for heating of rooms and warm water.

Actually there is not too much electronics need to handle transformer cooling, and there is not too much difference between water cooling and air cooling of power transformers. The main differences are concerning

Switching (on and off) fans if there is any fans and
Switching between cooling and heating of water cooling system if there is heating of rooms and warm water with the transformer.

Both items may be handled with ordinary thermocouples and/or thermostats. There is old fashioned technology practiced as well as sophisticated high tech solutions for the same aspects but temperature of cooling air might be significantly higher than temperature of cooling water.