Model test for harmonic characteristics of convertor transformer under Dc bias

According to equivalent principle, the paper establishes a model transformer in order to investigate the harmonic characteristics of the convertor transformer under DC bias. An almost linear relationship is observed between the values of 2nd harmonic and DC current injected to the winding in the experiment. And the linear value is changed with the AC excitation status. The value reaches maximum when over AC excitation supplies and minimum when under AC excitation supplies. Based on fact that the same excitation current amplitude arose from the same magnetic flux magnitude, the paper purposes a method to denote the effect of dc current on DC magnetic flux by AC excitation voltage. DC magnetic flux is higher when under AC excitation supplied but lower with over AC excitation according to the method, which is agreed with the theoretical analysis.