Optimization of Grading Ring of 624 kV Zinc Oxide Surge Arrester using Finite Element Technique

Zinc oxide (ZnO) surge arresters are being used extensively in high voltage power systems for providing protection to the transmission lines and the associated substation equipment from over voltages caused due to lightning and switching surges. It has been observed in practice that the voltage distribution in a tall multi-section arrester under normal operating conditions is quite non-uniform. As a result, the ZnO blocks at the top section share higher voltage and hence higher thermal stresses than the ZnO blocks at the bottom section. This leads to accelerated thermal aging of the blocks at the top if proper measures are not taken to make the voltage distribution uniform along the entire length of the arrester. Generally, provision of grading rings is a common method for achieving uniform voltage distribution through out the length of arrester. In this paper, the results of the optimization of the grading ring dimensions for a 765 kV system arrester with a rated voltage of 624 kV using Finite Element based 2D Elecnet software are presented.