Design of optimum toroid configuration for the 1200KV system solid core bus post insulator

Extra High Voltage and Ultra High Voltage porcelain solid core post insulators for use in up to 1200kV AC systems are being manufactured by various insulator manufacturers in India. One application is to use as bus support insulator in 1200kV substations. The post insulators shall have superior corona free performance for reliable and safe substation operation. Generally corona activity starts at an energized hardware like external metal fittings if the electric-field intensity close to its surface exceeds the dielectric strength of the surrounding air. At higher operating voltages, corona rings are necessary to reduce the voltage gradient on external metal fittings to reduce corona discharges as well as associated radio, television and audible noise interference levels to acceptable levels. Corona ring also helps to smoothen the voltage profile distribution along the insulator preventing concentration of voltage stresses. The authors have made an attempt to evaluate the adequecy of corona ring of a solid core bus post insulator comprising of a three toroidal ring system designed by the manufacturer M./S. Birla Ngk Ltd., India for use in 1200kV system and then arrive at an optimum corona ring design using a 2-D Finite element software. In the optimization study, the electric field on the surface of toroids was computed for various design parameters such as number of toroids, toroid diameter, tube diameter, and the arrangement of different toroids and the same was compared with the corona onset gradient of the corresponding toroids. Finally a six toroid configuration was successfully designed by the authors and the same was commissioned in the 1200kV experimental station at Bina, Madhya Pradesh state in India. The results of the study are presented in this paper.