Attenuation of very fast transients in Sf6 insulated high voltage busducts. theoretical and experimental considerations of the effect of a ferromagnetic coating applied to busbars.

Abstract

Particularly at the higher system voltages, certain faults have been attributed to VFTs. This work presents a full frequency domain mathematical model of the effects likely to be introduced by the application of a thin ferromagnetic coating to the surfaces of the busbars as a means of attenuating these transients. Experimental verification shows the model to perform accurately as an analysis tool for a continuous coated co-axial system. While comparison between the model developed here (based on a planar conductor) and the very accurate Bessel function solution is presented, it must be remembered that it is not feasible to include the effects of coatings in the latter solution. Hence it is used merely as a bench-mark for improvements to the general model. Consideration is also given to the transient skin effect, and this is shown to result in an increased attenuation of VFTs. detectable during high voltage experimentation. The frequency domain model is extended to that of a full travelling wave model for VFTs in GIS. The Fourier technique is used to move between the frequency and time domains. Laboratory results obtained at high voltage.show that this technique will introduce both risetime increase and peak magnitude attenuation of VFTs. As the most frequent faults directly attributable to disconnector operation are known to be faults to earth at the switch, it is noted that the technique proposed in this work is uniquely suited for use in a design strategy that requires totally robust disconnectors at the expense of more severe VFTs in the GIS. The model is used to determine a range of physical characteristics of materials suitable for use in this application. While the characteristics of the most suitable materials have therefore been established, it is concluded that extensive metallurgical experimentation remains before this technique may be economically applied.