Ground Resistance Measurements on Transmission Towers with Overhead Ground Wires

Abstract

The main subject of this master s thesis is ground resistance measurements on interconnected transmission towers. Statnett, which is the Norwegian transmission system operator, initiated the assignment as a result of experienced problems with the currently used method. The purpose of the master s thesis is to select an adequate method for measurement of ground resistance at interconnected transmission towers, by evaluation of different methods. In this thesis six methods are explained, and their qualities are investigated by simulations in CDEGS as well as practical research measurements. Based on this investigation it was possible to identify a method that met the complex requirements for ground resistance measurements on interconnected transmission towers, as well as Statnett s preferences regarding time-consumption. The most adequate method for Statnett to use is one particular variation of the Fall of Potential method. This method involves a correction factor to find the real resistance of the ground electrode. When performing measurements on counterpoise electrodes, the current electrode is located perpendicular to the transmission line, at a distance equal to the ground electrode s extension in the measuring direction. Similarly, for ring electrodes it is located at a distance of four times the ground electrode s extension. The potential electrode is located in the middle of the ground electrode and the current electrode for both counterpoise and ring electrodes. Simulations performed using this measuring method, with soil conditions representative for Norwegian tower sites, show that it has an accuracy of ± 22%, which is acceptable. The method requires short test electrode distances. As a result of this it is less time-consuming and less sensitive to local variations in soil resistivity nearby the ground electrode, compared to the other methods. Other investigated methods also resulted in good accuracy when simulating the measurement methods. However, these methods are more time-consuming and more sensitive to soil conditions or the potential electrode s location. Some of the methods resulted in resistances that were substantially lower than the actual resistance of the ground electrode under test. An instruction for ground resistance measurements on interconnected transmission towers has been made. With the help of this, a work instruction will be written and the method will be implemented at Statnett during summer 2007.