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 (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.