Comparing transformer derating computed using the harmonic loss factor FHL and K-factor

Abstract

The transformer losses will increase while it is supplying nonlinear load current. The derating measure for normal transformers is always taken to limit the temperature of windings or top-oil. In this paper, the formulations how to calculate the maximum permissible rms non-sinusoidal load current (Imax) using the harmonic loss factors FHL and K-factor were introduced. The difference of them was also reasoned and a case study illustrated the process of reasoning. The applications of these two factors were discussed too. Imax computed using FHL for normal transformer is more economical because Imax computed using K-factor is more conservative. Based on the actual monitoring data of harmonic component, K-factor is suitable for selecting right transformers in design and FHL for establishing transformers operation capability, therefore there is a harmonic loss margin when the transformers supply non-sinusoidal load.