I am working on the feasibility of measuring S-Parameters in a 500 kV Gas Insulated Voltage Transformer. Measuring S-parameters in HV equipment seems to be not that easy due to the dimensions involved and the influence of the leads used to connect the instrument to the equipment to be measured. So far, I have not been able to find papers on S-Parameters measurements applied to HV equipment.
The whole idea of S-parameters is to model very High frequency circuits such as those found in printed circuit boards, they are based on incident and reflected waves. Theoretically, it might be possible but you should have a larger set than the traditional S-parameter circuit. I hope this helps
regards
From work I did in the 90's in calibrating built in capacitive voltage dividers for measuring VFT,s in GIS it was possible to match sections of GIS bus bar using tapered transmission lines. This was successfully done in matching a coaxial BNC connection to a 275kV coaxial busbar section well up to 400MHz using a coaxial tapered transmission line section specially constructed for the job. Admittedly this was done using fast rising step voltages to determine the response of the dividers but the principal is the same. So doing in on the HV primary side connected to short section of GIS should be quite feasible as the geometries are coaxial. On the LV terminals this would also require some form of possibly planar or even two wire tapered transmission line arrangement. The exact shape of this would require some investigation; this could be possibly done using FEM modelling of the basic geometry of the terminals and the proposed tapered line looking at the response.
Hope this helps.