With the development of large power grid and the concept of global energy interconnection being proposed

half-wavelength alternating current transmission（HWACT） technology is increasingly concerned.The overhead lines combined with cable is expected to become a new way for power transmission.In order to further study the characteristics of hybrid HWACT line

we researched the optimal proportion and interconnection performance of hybrid HWACT line.Firstly

the mathematic and physical half-wavelengths（HWs） of a single line were derived.Subsequently

the HW of a hybrid line was analyzed

and the concept and definition of optimal proportion were proposed.Then

the optimal function was constructed based on the characteristics of HWACT line so as to solve the value of optimal proportion.Finally

in order to compare the hybrid HWACT system with the conventional HWACT system

simulations were carried out based on the case of planning UHV power grids of the State Grid Corporation of China for the year 2020.The interconnection performances of the hybrid HWACT line was evaluated based on power-angle curves under different fault conditions.The results indicate that the hybrid HWACT line combined with the principle of optimal proportion helps to improve the system damping and build a strong synchronous power grid.