Research On Varying Frequency Flexible Tuning Method Of Half-wavelength AC Transmission System

Ultra-long distance,large capacity,reliable power transmission technology is essential to take full advantage of wind energy inside or near to the Arctic Circle and solar energy around the Equator during the development of global energy interconnection(GEI).Half-wavelength AC transmission(HWACT)technology,as a kind of point-to-point,ultra-long distance and large capacity transmission technology,is one of the effective technical ways to support transnational and intercontinental power transmission.However,there are still some key technical problems with HWACT remaining to be solved.This paper mainly studies the tuning problem of HWACT line.Firstly,the passive tuning methods of traditional half-wavelength transmission lines are comparatively studied,the tuning principle of the existing flexible tuning method is described,and the feasibility of variable frequency turning method is verified.Secondly,two specific variable frequency tuning methods are proposed,that is,a variable frequency tuning device based on variable frequency transformer(VFT)and a flexible frequency tuning device based on modular multi-level converter(MMC).Respectively,the corresponding control strategies for the proposed flexible tuning devices are designed.Finally,the cases where the electrical distance of the transmission line is less than and greater than one-half fundamental frequency wavelength are analyzed and simulation results show feasibility of the proposed flexible turning schemes.In the background of global energy interconnection,the fundamental scheme of HWACT with variable frequency turning technology based on MMC is put forward for transcontinental grid connection,and verified through simulation of a specific case.This paper compares the application of those two tuning methods,compared with the traditional tuning method,the flexible tuning methods described in this paper show better performance,which provide a useful reference for solving the tuning problem of the HWACT line.