Methods of determining temperature of transformers and of cooling medium

The paper discusses methods of determining the effective temperature of the cooling medium and the actual temperature of heated transformers. Particular attention is called to the errors resulting from variations in the temperature of air during a heat run on a self-cooled oil-immersed transformer because of the slowness of the large mass of material in following changes of temperature in the surrounding medium. It is pointed out that the effective temperature of the cooling medium is practically the average temperature of a duplicate transformer, subjected to the same cooling conditions and without load. This temperature should be used as the base for calculating rises. Methods of measuring hot temperatures by rise in resistance are discussed. Attention is called to the advantages obtained by comparing resistance of the loaded transformer with that of a duplicate idle transformer. The cooling of windings after load is removed, but before resistance readings are obtained, causes an error in the determination of the temperature rise. This drop in temperature can be avoided by measuring the resistance of the windings without cutting off the load. A method of doing this is described. Actual test results are given to show the advantages gained by using a duplicate idle transformer as a base for determining temperature rises. Instructions for using such an idle unit are also given. The paper ends with recommendations for additions to the rules to the effect that duplicate idle transformers be used whenever possible as a base for determining temperature rises.