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.

I. Introduction

Transformers deliver power energy to loads with different voltage levels. The original loads gradually had replaced with nonlinear loads that inject harmonic currents^[1]. Under such conditions we call for transformers derating or replaced with more tolerance to harmonic thermal effect. Recommendations for the matching of a given load with the right transformer, or for computation of the needed derating at a given load, are provided in [2]–[6]. The operating capability of transformers to supply non-sinusoidal load currents is determining by the use of a harmonic loss factor, or . factor. is defined as follows:

$$\eqalignno{F_{HL}=&\sum_{h=1}^{N}(I_{h}^{2}h^{2})/I^{2}=\sum_{h=1}^{N}(({I_h\over I_{1}})^{2}h^{2})/\sum_{h=1}^{N}({I_h\over I_{1}})^{2} &\hbox{(1)}\cr &\ \ {\bf Or}\quad =\sum_{h=1}^{N}(({I_h\over I})^{2}h^{2})/\sum_{h=1}^{N}({I_h\over I})^{2}}$$ where is rms current at harmonic “”, is the highest significant harmonic number, and .