Seasonal Variations of Tower Footing Impedance in Various Transmission Line Grounding Systems

In transmission line grounding system design, spatial and temporal/seasonal variations of tower footing impedance should be considered. However, in real life, only spatial variations are taken into account at the design stage, seasonal variations have not been included. In this paper, seasonal variations of tower footing impedance of several types of transmission line grounding systems at Manitoba Hydro, Canada are investigated through field measurements for a whole year. The industrial practice uses simulated tower footing impedances by the software CDEGS in the design of transmission line grounding systems, however, the accuracy of simulated tower footing impedances as a critical design parameter has never been evaluated. In this paper, this industrial practice is evaluated by comparing simulated and measured tower footing impedances for various transmission line grounding systems. The importance to use proper tower footing impedance for lightening conditions is also demonstrated.