TRANSFORMER DESIGN AND APPLICATION CONSIDERATIONS FOR NONSINUSOIDAL LOAD CURRENTS

The use of adjustable-speed drives requires transformers capable of wi thstanding high levels of harmonic currents under normal operating con ditions. Experience has been that overheating problems are much more c ommon with dry-type transformers than with liquid-filled transformers, Transformer insulation life is determined by the hot spot temperature but confirmation of hot spot temperature rise is one performance char acteristic which is ignored in industry standards. This is especially important for transformers rated for nonsinusoidal load currents, not spot allowances used in IEEE standards for ventilated dry-type transfo rmers were developed in 1944 and recent data indicates that revisions are required, The design of transformers for nonsinusoidal load curren ts should include an analysis of the eddy loss distribution in the win dings and calculation of the hot spot temperature rise, Calculations a nd thermal tests giving only average winding temperature rises are not sufficient, Thermal tests with nonsinusoidal currents and measurement s of hot spot temperature rises are extremely difficult on large trans formers, The combination of testing and analysis may be the only econo mically practical approach. Analysis indicates that the dry type trans former hot spot temperature is very sensitive to the eddy loss magnitu de and distribution, The Underwriters Laboratories Inc, (UL) K-factor rated dry type transformer and the recommended practices given in ANSI /IEEE C57.110 are reviewed, When purchasing transformers subject to no nsinusoidal load currents, considerations should be given to the manuf acturer's development program and capability to calculate the eddy los s distribution and hot spot temperatures.