Temperature rise of small oil-filled distribution transformers supplying nonsinusoidal load currents

Temperature rise tests on standard single-phase, oil-filled distribution transformers are compared for sinusoidal and nonsinusoidal current conditions. In addition to a stock 50 kVA unit, two 25 kVA transformers were specifically constructed with embedded thermocouples that permitted the true hot-spot temperatures to be measured under load. Test results show that when subjected to full-load currents having low-order harmonic distortion values up to 40 percent THD, the average winding temperature rise is 2 to 7°C hotter than for full-load sinusoidal currents. The greatest difference recorded between the average winding temperature and the hot-spot temperature was 7°C, significantly less than the 15°C allowance given in the standards. This study indicates that the present standard for sizing small distribution transformers supplying nonsinusoidal currents appears to provide good estimations of load capability