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

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 (I_max) using the harmonic loss factors F_HL 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. I_max computed using F_HL for normal transformer is more economical because I_max 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 F_HL for establishing transformers operation capability, therefore there is a harmonic loss margin when the transformers supply non-sinusoidal load.