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Fachkommission für Hochspannungsfragen

Aktuelle Publikation

Signal Delay Effects of Solid Dielectrics
on Time-of-Flight Measurements in GIS

So-called ‘time-of-flight’ (TOF) measurements have been employed for locating PD
sources within GIS for over 20 years. Portable digital sampling oscilloscopes have now
become available for reasonable cost whose input rise-times, bandwidths, and sampling
rates are theoretically sufficient to provide spatial resolution within approximately 10
centimeters. However, contrary to first impressions, GIS does not present an ideal
propagation environment for GHz bandwidth radio frequency (RF) signals; many
factors act to prevent reaching such high spatial resolution. In both laboratory
measurements and field simulations, the authors have recently observed that solid
dielectric insulators in the signal propagation path affect the accuracy of the location
determination. Therefore, it is important to take the delay effects of solid dielectrics
into account in order to increase accuracy of TOF measurements. To this end,
comparison measurements were made on a short section of GIS which clearly
demonstrate the delay time effects as UHF signal pass through solid insulators. In
addition, a sophisticated finite elements (FE) RF model was created for the same
section of GIS in order to simulate electromagnetic wave propagation within it. The
simulation results obtained were compared against the measurements for validation
purposes. In addition, results from a practical on-site measurement are presented along
with suggestions for others working in the field.

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