The paper analyzes the intentional reduction of the maximum load capacity (derating) for the in-service dry-type three-phase power distribution transformers that operate under nonsinusoidal steady state condition. A real-time, less intrusive and efficient derating procedure was developed and implemented in accordance with the international standards recommendations. Due to a flexible developed software package, the proposed computation method only requires the nameplate data of the investigated transformer and the harmonic spectrum of the load current measured at transformer secondary (low-voltage) windings. Thus, the additional harmonic losses within the transformer and their corresponding maximal permissible nonsinusoidal current are continuously computed according to the characteristics of load changes. In order to validate the results, the procedure was tested on a distribution transformer that supplies heavy nonlinear loads of an industrial facility.

Keywords: Transformer derating, Nonlinear loads, Harmonics, Power quality