Voltage and electric field distribution along porcelain long rod insulator string in AC 500kV transmission line

Porcelain long rod insulators are widely used in transmission lines while many of their features are lack of research. To determine the voltage and electric field distribution of porcelain long rod insulators helps to guide the existing operational strategy. In accordance with the insulator structure and the actual working conditions, the simulative calculation of porcelain long rod insulator string in 500kV transmission line is carried out through finite element method in three-dimensional model considering the effects of tower, bundle conductors, grading ring, shielding line and so on. The simulative calculation is in comparison with disk ceramic insulator string. The calculation is based on method of solving electrostatic field and all insulator strings are assumed to be clean and dry. The result shows that the voltage distribution of porcelain long rod insulator string vary through structure height, which presents a decline curve of three parts, and the electric potential of two ends declines faster than the middle. Each single section insulator also presents a decline curve that two ends declines faster than the middle. The voltage distribution of three insulators are about for 5:2:3, which means over 50% operating voltage concentrates on the single unit of high voltage polarity for the long rod insulator string. The electric field strength distribution of porcelain long rod insulator string presents a U-shaped curve and each single section insulator also presents a U-shaped curve. The maximum field strength lies on the first rod core areas of umbrella skirt root of the high voltage end. Compared with disk ceramic insulator string, porcelain long rod insulator has less fittings, leading to faster voltage decline in both ends. The surface electric field strength of porcelain long rod insulator in both ends is higher than disk ceramic insulator string, its grading ring could be optimized further. In summary, the voltage and electric field distribution of porcelain long rod insulator is more uneven, which may lead to higher risk of corona, flashover or even breakdown.