Partial Discharge Electromagnetic Wave Propagation Characteristics Of Valve-side Bushing Of Converter Transformer

The electromagnetic wave signal excited by partial discharge contains rich insulation state information.At present,non-contact partial discharge detection methods represented by RF and UHF methods have been widely used.The valve-side bushing of the converter transformer,as an essential part of the converter transformer,is not only subject to DC and AC power as well as the core of the bushing itself,but also to the influence of the insulation layer on the outside of the pole plate,which is the most complex part of the converter transformer electric field.The main insulation of converter transformer valve side bushing is generally made of multi-layer metal winding and filled with epoxy impregnated paper.The propagation path and attenuation law of electromagnetic wave inside the valve side bushing is not clear.This seriously restricts the development of non-contact online monitoring technology for valve side bushing.In order to solve this problem,this paper constructs the actual ±800kV converter transformer valve side bushing finite difference time domain three-dimensional model.The propagation path and attenuation pattern of electromagnetic wave excited by partial discharge inside and outside the valve side bushing are also simulated and analyzed.The main work of this paper and the main conclusions obtained are as follows:(1)The electromagnetic wave is influenced by the excitation source and frequency as it propagates through the valve-side bushing.When the excitation source pulse width is increased,the frequency band of electromagnetic wave is more distributed in the lower frequency band.The excitation source pulse amplitude affects only the radiation field intensity and varies proportionally with it.Dispersion materials with changing dielectric constants have a more concentrated distribution of electromagnetic waves.And when the frequency increases,the amplitude of the electromagnetic wave propagated by the insulating medium of the dispersion material will decrease.(2)The spatial and temporal distribution characteristics of the electromagnetic wave inside and outside the valve-side bushing are characterized by the decaying trend of the electric field intensity of the electromagnetic wave with increasing time and distance.The specific attenuation law is non-linearly related to the distance.The attenuation range of electromagnetic waves after the capacitor core is 14.11～47.47 dB/m.The attenuation range of electromagnetic waves in the external space is 2.74～13.07 dB/m.(3)The propagation path of electromagnetic waves in the valve side bushing is the epoxy-impregnated paper between the capacitor screens and the SF6-filled area on the outside.The electromagnetic waves propagate independently between the capacitor screens and superimpose on each other after passing out of the capacitor core.The superposition of electromagnetic waves after the capacitor core is mainly concentrated in the high frequency band.There are two paths for electromagnetic wave propagation from the valve side bushing to the external space,and the propagation of electromagnetic wave is mainly by virtue of path one,i.e.,through the flange to the external space after the simulation calculation.It was also verified in combination with laboratory tests,in which the first wave of the UHF sensor at the flange in the external space of the dry casing appeared early.Therefore,the UHF external sensor can be considered to be installed at the flange.(4)When partial discharge occurs inside the bushing,the electric field strength of electromagnetic waves inside the valve-side bushing decays in the time domain as the distance increases.After the capacitor core,the frequency band of electromagnetic wave becomes narrower,and the electromagnetic wave in the low frequency band 0.5～1 GHz decays quickly,and the electromagnetic wave in the high frequency band 3-3.5GHz also decays quickly.The smallest attenuation is the electromagnetic wave of 1.5～2GHz.In the external space with the increase of distance,electromagnetic wave either along the axial or radial will produce attenuation.The frequency band distribution of electromagnetic wave tends to higher frequency band.The frequency band range of electromagnetic waves in the external space of the valve side bushing is 1.5～2.5GHz,which can provide the theoretical basis for the selection of UHF external sensors.