Derating of power distribution transformers serving nonlinear industrial loads

The actual energy efficiency requirements generated a large usage of the power electronic converters for driving asynchronous motors in numerous industrial facilities. The associated power quality issues of these adjustable speed drives, especially harmonic currents, adversely affect the power distribution transformers by increasing their losses and decreasing their life time expectancy. Accordingly, the intentional reduction of the transformers apparent power rating (derating) becomes mandatory in order to preserve their rated losses and the equivalent admissible thermal stress. The paper investigates both theoretically and experimentally the transformers harmonic behavior by evaluating the additional losses and the corresponding maximal permissible supplied power. The transformer derating factors, according to different international standards, are also computed and critically analyzed in terms of their suitable usage. The study is illustrated on 160 kVA oil-type three-phase power distribution transformer that mainly supplies induction pump motors controlled by frequency converters. The transformer operating parameters are acquired at its low-voltage secondary windings by a portable power quality analyzer.