Research on DC bias analysis for transformer based on vibration Hilbert Huang transform and ground-state energy ratio method

Abstract

With the development and construction of (high voltage direct current) HVDC system, more and more DC bias problems caused by monopole operation of HVDC are appeared in grid which leads to efficiency droop or potential risk of transformer. This paper proposes a new non-electrical method, Ground-state energy ratio (GER), based on Hilbert-Huangtransform (HHT) characteristics of vibration signals which can reflect the DC bias accurately. Firstly, the transformer DC bias experiment platform is set up and the transformer vibration signals are measured. Meanwhile, the displacement signals are obtained by the quadratic integral. Secondly, the time-frequency spectra of the DC bias vibration signals are obtained by HHT analysis. According to the energy change of each mode in HHT, the GER method is proposed to quantify the DC bias of signal at each point. Lastly, Comparing the GER values of different parts of transformer, such as core, shell and winding, the vibration signals that can accurately reflect the degree of DC bias are confirmed. Results show that the HHT-GER values of vibration signals which on axial vibration of the iron core column and at the top of the transformer shell increase with the increase of DC bias. Therefore, the HHT-GER is an effective method to analyze the DC bias condition of transformer and diagnose the abnormal vibration of transformer core.