How to Conduct the DC Bias Test for Single-phase Power Transformer

Abstract: In Six single-phase transformers constitute two three-phase transformers. The waveform of exciting current and primary winding current of such three-phase transformer under different DC bias and loads are measured. The harmonic distribution of exciting current and primary winding current is obtained by FFT. The relation between primary winding current peak and harmonic magnitude and DC current is analyzed. Test results show that the DC bias of power transformer will cause no-load current distortion, and no-load current peak value and harmonic magnitudes will rise according to the increasing current. The lower the harmonic order is, the faster its amplitude increases. If the power is greater, odd harmonic increases slower according to the current variation while the load has no effect on rising speed of even harmonic.   The relation between non-linear hysteresis loop and exciting current waveform is used to do simulation calculation for test model. Meanwhile, the hysteresis loop is obtained with and without DC bias. The model is established based on the theory of Jiles-Atherton. The simulation result is consistent with test result, which proves that non-linear model of power transformer is effective and correct.  

Key words: Single-phase transformer, DC bias, exciting current, primary winding current, distortion, harmonic distribution