Derating the three-phase power distribution transformers under nonsinusoidal operating conditions: A case study

The paper quantitatively investigates the problem of derating the in-service distribution power transformers that supply heavy nonlinear balanced loads. The adopted computation method basis on the international standards and only requires the basic power quality parameters measured at the secondary winding of the transformer and its certified test report. Hence, by computing the major transformer harmonic factors, an on-site derating report is generated, which provides the additional power losses generated by the harmonic currents and the corresponding transformer maximum permissible current (reduction in apparent power rating). The paper also critically analyzes other solutions that could avoid or minimize the harmonics detrimental effects on the transformers steady-state operation. The study is carried out on a 1600 kVA oil-filled type distribution transformer that serves both linear and nonlinear industrial loads.