Calculation of electrical stress distribution and grading performance of 1 000 kV composite insulator for UHV a.c.overhead transmission line is described.Comparison among one-piece unit

strings of 1/3 and 2/3 units

1/6 and 5/6 units as well as 1/6 porcelain unit and 5/6 composite units is performed.The results show that two-units string gives no improvement in stress distribution

the structure of one-piece unit is better than two-unit string in design

and applying porcelain unit to live end and composite units to earth end can fully take advantage of arc-resistant performance of porcelain insulator and hydrophobic performance of composite performance to achieve better stress distribution than one-piece insulator

reducing the electrical stress at end sealing of composite insulator by 60%.Calculation and analysis of the electrical stress distribution of UHV insulator are conducted.After optimization of grading ring the electrical stress at live end of one-piece insulator is less than 320V/mm

and the maximum value at composite insulator of porcelain-composite units combination is less than 110 V/mm.