Evaluation of High Voltage Transmission Line Towers Footing Earth Resistance under High Impulse Currents

Abstract

In this paper, investigation of the performance of high transmission line towers footing earth resistances under high impulse currents was examined. The tower footings were properly earthed using the 3-point Fall-of-Potential and Multiple Earth Electrodes interconnected by both bare and insulated copper conductors to form ring electrode methods for the measurement of earth resistances. The earth resistances were simulated using CDEGS-HIFREQ software. Results show that the earth resistance of rod No.1 is 79.6Ω; whereas that for rod No. 8 is 122W. The differences were attributed to the lateral variation of the soil resistivity across the area occupied by the rods. Earth resistances measurements for a period of 13 months shown that the lowest values was 120Ω in June, 2017 and highest values of 165Ω in January same year for tower footing 1; Similarly, the earth resistance for the month of July 2016 was 66Ω, and 83Ω in February, 2017; 75Ω in August and 110Ω for January 2017; while the lowest and highest values were 60Ω and 90Ω in September 2016, and March, 2017 for tower footings 2, 3 and 4 over the periods. Earth resistance was lower in raining season during the months of June to November and higher in dry season during the months of December to May of the year. Using bare conductor ring, the earth resistance was 2.95Ω. While the calculated value of the bare ring electrode with eight (8) rods at its periphery was 2.3Ω. These values reduced considerably below the standard recommended value of 10Ω. Using ring electrodes replicate current distribution in the soil around the tower base during lightning strikes.