The corona onset voltage gradient (COG) of conductors is a key parameter in the design of overhead transmission lines. The commonly used semi-empirical calculation formula proposed by Peek (1910s) at present can roughly estimate the COG of single conductors and is not applicable to large cross-section bundle conductors. In this paper

experiments are conducted to investigate the corona characteristics of a whole series of bundle conductors at different altitudes

and a prediction formula is proposed for the COG of large cross-section bundle conductors on AC transmission lines considering the altitude correction. The calculated values using the proposed prediction formula are compared with the experimental values of the COG in two cases: UHV eight-bundle conductors at an altitude of 19 m; 500 kV UHV four-bundle conductors in Wuhan

Xining

Geermu

and Nachitai at four different altitudes. The results show that the use of the formula can predict the COG of bundle conductors with the radius of 1.34 cm to 1.995 cm and the number of sub-conductors of 4 to 12 at altitudes of 19 m to 4000 m above the mean sea level. The research findings can provide a reference for the design of EHV and UHV overhead transmission lines and even those in higher voltage levels.