Revised Tower Earthing Design in High-Voltage Transmission Network for High-Frequency Lightning Condition

Abstract

Tower earthing system for transmission grid is designed to protect the working personnel and equipment, as well as to maintain the quality of supply during fault. The existing tower earthing system for towers in TNB's grid is catered for normal power frequency (low-frequency) fault conditions. This earthing design is found to be inapt for transient nature of lightning condition to a certain extent, which involves a high-frequency domain. The current earthing practice of installing the electrodes radially in 60 m straight horizontal lines under the ground, in order to achieve the specified impedance value of less than 10 ?, was deemed ineffective in reducing the high-frequency impedance. This paper introduces a new earthing design that produces low impedance value at the high-frequency domain, without compromising the performance of low-frequency impedance. The performance of this new earthing design, as well as the existing design, are simulated for various soil resistivity values at varying frequency. The proposed concentrated earthing design is found to possess low tower footing resistance (TFR) value at both low and high-frequency. A good earthing design should have a fine balance between compact and radial electrodes under the ground.