DC Magnetic Bias Diagnostic Method Of Large Power Transformers Based On Vibration Noise

Geomagnetic induced current (GIC) and Ground electrode current of HVDC can invade power transformers through grounded neutral point, cause transformers'DC magnetic bias, lead to half-wave saturation of iron core and distortion of exciting current, produce a large number of harmonics, result in the increase of temperature rise, vibration and reactive loss. Serious magnetic bias saturation can increase leakage flux which give rise to loss increase and local overheating of metal structure, and then damage insulation, harm transformer or reduce its working life. Therefore, the monitoring of transformers' DC magnetic bias and the diagnosis of its reason are of significance to the safe operation of transformers and power systems.In this paper, via the research on mechanism of transformers'vibration noise and DC magnetic bias, combining the analysis of a large number of actual events, find that the main reason of transformers'abnormal vibration noise in recent years is DC magnetic bias. Put forward to using wavelet packet to analyze vibration signals under DC magnetic bias, creat eigenvector based on "energy-frequency band" to determine the influence degree of DC magnetic bias to transformers. Through theoretical derivation, simulation analysis and experimental research, exciting current harmonic characteristics, temperature rise mechanism and power loss reason are analyzed. Determine to use exciting current harmonics, eeactive power loss, vibration and neutral current as the characteristic parameters for the diagnosis of DC magnetic bias, and the methods of calculation is given. On that basis, a diagnosis method based on exciting current even-order harmonic is given and can diagnose the occurrence of trasformers'DC magnetic bias effectively. Throug analyzing the traits of GIC and HVDC and statistical analysis of observed data, the method to discriminate the two kinds of DC current which caused DC magnetic bias are given.