Experimental Study On Propagation Characteristic Of Partial Discharge Signal In Stator Winding Of Hydro Generator

The structure of stator winding in large-scale hydro-generator is very complex,the propagation of partial discharge signals from the discharge source to the detection point,will occur in varying degrees of attenuation and deformation,so the study of the propagation characteristics of the PD signal in the stator winding is the premise of accurately measurement of the PD signal.Based on the summary of the structure of stator windings and the existing research status,this paper proposes to use the wide-band test signal to strictly simulate the actual PD signal and tests in the stator bar and the whole winding.First,the propagation law of the PD signal along the single bar was measured in the laboratory,where the multi-layer aluminum foil was used to simulate the generator stator core.It was found that the signal attenuation above 1MHz was serious,which caused the stator winding to show low-pass characteristics.In addition,there are multiple resonant frequencies in the band 20 to 160 MHz,which have an important effect on the band selection of the PD system.Inappropriate detection of the frequency band may detect abnormal high discharges and produce false positives.Secondly,according to the controversy over the electromagnetic coupling model in the existing literature,the electromagnetic coupling test between the upper and lower wire rods and the end wire rods is designed.It is found that the two structures have high frequency signal coupling,especially the end coupling.Experiments conducted in actual generators also show high frequency coupling phenomena and resonance phenomena.These phenomena not only cause the PD signal to form the fast reach component at the detection point,but also cause the signal attenuation law to be more complicated,and also lead to the inter-phase cross-coupling in the on-line measurement,which confuses the discharge and increases the difficulty of the discharge source.In this paper,the experimental results show that there are slow mode which propagates along the bar and fast mode which cross-coupled in the stator windings,and the frequency characteristics of the two propagation modes are studied.It provides a basis for rational selection of detection points and detection bands,and is also a inspiration of noise suppression,and can further excavate the location of the discharge source and discharge calibration method.