In the master's thesis, the cooling of the transformer with a water cooling system was studied. Properly dimensioned cooling system is a guarantee for the long and smooth operation of the transformer. We presented the problem of the dimensioning of the cooling system, described the heat generation in the transformer, and the methods of heat transfer of these losses into the environment. With Trnsys software we set up a numerical experiment, where we modeled the operation of the cooling system with the control function that is needed for the successful transfer of heat losses from the transformer. Numerical model was validated with the results of heat run test measurements on the selected power transformer. It turns out that the numerical model predicts the operation of the cooling system well. In addition the operation of the transformer in real dynamic conditions was simulated and cooling system analyzed. With the use of the results we provided suggestions for further improvements and upgrading of numerical experiment.