Research on the Harmonic Surge of Secondary Voltage of Three-Phase Three-Element Combined Transformer Based on Excitation Curve

The secondary voltage of the three-phase threeelement combined transformer will experience a surge in harmonics under the Yyn0 connection, and the main harmonic is the third harmonic, which greatly affects the measurement accuracy of the combined transformer. However, there is little research on the mechanism and quantitative analysis of this phenomenon. In order to quantitatively analyze the correlation between the excitation curve of the transformer core and the secondary voltage harmonic content under the specific connection of Yyn0, this paper proposes a fitting method for the excitation curve of the three-phase three-element combined transformer, that is, the excitation curve is fitted using the hyperbolic tangent function and the characteristic coefficient. A simulation model of the three-phase three-element combined transformer under the Yyn0 connection is built, and the secondary voltage harmonic content of the combined transformer under different excitation curves is measured. Different excitation curves are fitted by the method proposed in this paper, and the relationship between the characteristic coefficient in the fitting function and the secondary voltage harmonic content is obtained. Finally, the calculation of the secondary voltage harmonic content is realized by the characteristic coefficient. Compared with the results of the test platform, the relative error is less than 4 %.