BENEFITS OF APPLYING NATURAL COOLING OF POWER TRANSFORMERS WITH HIGHER RATED POWER

Natural cooling „ONAN“ represents the simplest method of cooling power transformers. Losses created in the windings and the magnetic core assembly are transmitted by the principle of convection and radiation to the surrounding fluid–air both from the surfaces of the radiators and from the surface of the transformer tank. In general, power transformers with higher rated power have higher load losses and no–load losses, meaning more intense generation of heat during operation, which necessitates the need for increased heat removal, i.e. cooling of the transformer. One of the ways to increase heat removal is to increase the heat transfer coefficient between the outer surface of the radiator and the air, i.e. the application of ONAN/ONAF cooling, where the forced flow of the ambient fluid–air with the help of a fan achieves greater heat transfer power. Specific guidelines for choosing between ONAN or ONAN/ONAF strictly do not exist, and in the territory of the Republic of Serbia it is an established practice, and an unwritten rule, that almost all power transformers with a rated power of 16MVA to 63MVA are cooled by the ONAN/ONAF cooling system, that is, all power transformers with a rated power of up to 16MVA by the ONAN cooling system. As the advantages of applying the ONAN cooling system in relation to ONAN/ONAF are numerous, and the trends of energy efficiency/reducing the power losses of power transformers are increasingly prevalent, the idea for creating this work arose in order to consider the justification, that is, the advantages of applying a natural way of cooling power transformers of higher power. The focus of this work will be on power transformer units with rated power 16MVA to 63MVA, voltage level 110kV.