Nonsinusoidal load current effect on the electrical and thermal operating parameters of oil filled power distribution transformers

Most of the in-service power distribution transformers from the industrial electric facilities were designed to operate under pure sinusoidal state conditions. Nevertheless, the majority of the loads from modern electric installations are intrinsically nonlinear and consequently, they generate nonsinusoidal currents. These high-order harmonics (present into the current waveform spectrum) have a negative impact on the transformers thermal and electrical operating parameters and, ultimately, limit the machine loading capability. The paper quantitatively investigates the transformer behavior under distorted current conditions by developing a procedure for evaluating different relevant operating quantities (maximum permissible current, the hottest spot temperature, aging acceleration factor and the machine remaining lifetime). The computation relays on the international standard recommendation and mainly uses the load current harmonic spectrum and the transformer rated data. To reveal the proposed procedure performance, a distribution transformer of 400 kVA from an industrial plasterboard facility is investigated.