New indices to evaluate the impact of harmonic currents on power transformers

This paper presents a new concept to quantify the impact of harmonic currents on power transformers. The basic idea is to convert the impact of harmonic currents into the impact of an equivalent additional fundamental frequency current. Two equivalent loading indices are developed accordingly with the support of well-established industry standards. Several analytical examples are presented to demonstrate the merits of the proposed approach. Based on field data, the method has been applied to assess the impact of nonlinear residential loads on distribution transformers. It was found that the impact of residential load harmonics on transformers is approximately proportional to the square of current THD. Other applications of the proposed method include estimating transformer overloading level due to harmonics and determining the rating of a regular transformer that can operate properly for a given harmonic distortion condition.