To deal with the tuning problem of the half-wavelength AC transmission line（HWACT）

we proposed a flexible frequency tuning device based on modular multi-level converter（MMC） to obtain different operating frequency so that the transmission line possessed the characteristics of half-wavelength.Moreover

we established an equivalent tuning system model by using PSCAD/EMTDC and designed a control strategy of the flexible tuning device.Under the background of global energy interconnection（GEI）

we put forward the fundamental scheme of half-wavelength AC transmission with variable frequency turning technology for transcontinental grid connection

and verified through simulation of a specific case.The simulation results show that the MMC-based flexible frequency tuning method can tune a transmission line with a length of 6 000 km to a half-wavelength line with an operating frequency of 24.484 Hz

and can realize the additional function of power factor compensation.The flexible frequency tuning method described in this paper shows better performances compared with the traditional tuning method

and is especially useful for the ultra long distance transmission line.It provides a novel alternative to solve the tuning problem of the half-wavelength transmission line.