Consequence of an unbalanced supplying condition on a distribution transformer

Most of the time transformers work under balanced supplying conditions. However, after some fault conditions or anomalous switching operations, zero sequence component in excitation may be expectable. Transformers behavior under zero sequence excitation may be very different from usual behavior and might become dangerous. Some simulations are performed with the Alternative Transients Program (ATP), representing a three-legged core type three-phase transformer with a model available in the ATP library. Results show overvoltages and high saturated currents on the unloaded transformer. The proper use of the three-legged core type transformer model, shows evidence of the influence of the characteristics of zero sequence transformer on the development of such overvoltages and high transformer currents. Besides, regardless of load conditions, important homopolar currents are observed flowing to ground from the transformer high voltage neutral. The resultant homopolar fluxes could cause heating and damage in a three-legged core type transformer