Design of Counterpoise Grounding Electrodes for Transmission Towers in High Soil Resistivity Area

Abstract

This paper presents an approach based on numerical electromagnetic analysis for design of counterpoise grounding electrodes for transmission towers in high soil resistivity area. Frequency dependent soil parameter impulse characteristics of counterpoise electrodes are taken into account in the design process. The transient characteristics of the counterpoise electrodes calculated by a representative electromagnetic approach named the partial element equivalent circuit (PEEC) method are presented in the time and frequency domains. In cases of the time domain simulation, the standard lightning impulse current waveforms collected from IEC 62305 are taken into account. The soil ionization is neglected for the conservative cases. It is noticed that in cases of long counterpoise electrodes, the capacitive effect at the middle frequency range is found. This effect leads to the magnitude of the electrode impedance to be lower than its resistance. Neglecting frequency dependence of soil parameters, the impulse grounding resistance (impedance) of the 4-leg grounding electrode in the high resistivity soil area cannot be achieved to the specified utility’s value of 20 Ω due to effect of impulse effective length. However, when the frequency dependence of soil parameters is included in the design, the grounding resistance can be achieved to the specified utility’s value. Therefore, the grounding resistance of 20 Ω is still possible for the grounding system design according to the utility’s requirement.