Investigation of vibration impacts on HVAC transformer from HVDC system under monopole operation

When the high voltage direct current (HVDC) system operates in monopole ground-return mode, DC current flows through the neutral of AC power transformer with the neutral grounded which exacerbates the vibration of the transformer. During the field test of Yibin-Jinhua HVDC system in China, the vibration detection system is deployed to detect the vibration signal of the 500 kV transformer in Luzhou substation, which is close to the grounding substation. The spectrum of the transformer vibration signal is analyzed by means of the wavelet transform, through which the vibration acoustic fingerprint and three typical characteristic parameters, i.e., the distribution percentage of spectral energy, the energy ratio of odd and even harmonics, and the waveform distortion ratio, are extracted respectively. When the HVDC system current increases from 0 A to 5 kA, the impact of DC bias on the operation condition of the 500 kV transformer is investigated through the analysis of the variation of vibration acoustic fingerprint and three characteristic parameters. It is indicated that the energy radio of odd and even harmonics of the 500 kV transformer vibration signal decreases significantly with the increase of the current. In addition, the distribution percentage of spectral energy and the vibration acoustic fingerprint change significantly. This paper demonstrates that the effect of DC bias on the operating condition of the transformer can be accurately assessed by the proposed indices.