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.