Design of grading ring for 624KV gapless zinc oxide surge arrester

Zinc oxide (ZnO) surge arresters are being used extensively in high voltage power systems for providing protection to transmission lines and associated substation equipment from over voltages caused due to lightning and switching. It is well known that the voltage distribution in a tall multi section arrester under normal operating conditions is non-uniform unless corrective measure is taken. 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 ageing of the blocks at the top if proper measures like providing suitable grading ring are not taken to make the voltage distribution uniform along the entire length of the surge arrester. In this study the adequacy of the grading ring proposed as first approximation by M./S. Lamco Electrical Industries Hyderabad, India for their 624kV surge arrester was evaluated. As the voltage distribution deviated very much from the acceptable deviation of 12 to 15%, the authors have attempted to arrive at an optimum grading ring diameter, tube diameter and tube position to control the deviation of voltage distribution to around 12%. The methodology and results of the optimization of the grading ring using Finite Element based 2-D Elecnet software is presented in this paper.