Applicable occasions of UHVAC/UHVDC transmission and their technology comparisons in China

From point view of the world,UHV transmission technology will develop continuously.In China,the planning UHV grid is composed of 1 000 kV AC transmission and ±800 kV DC transmission systems.Based on the differences of technical and economic characteristics of UHVAC/UHVDC,their applicable occasions in China are first compared.Then,some technical problems of UHVAC/UHVDC,such as lightning protection of transmission lines,reliability,stability,and electro-magnetic environment of power grid,insulator selection,and DC/AC coordination,are compared respectively.Main conclusions are therefore made as follows:UHVAC is mainly applicable to short-distance large-capacity electricity transmission and the network construction of a higher voltage grade.UHVDC is mainly applicable to long-distance large-capacity electricity transmission with clear transmission end and receiving end,and is also applicable to the interconnections of some regional and provincial power grids.The positive transmission lines of UHVDC are more likely to be damaged by lightning than the negative ones.The case that the electricity transmission to a certain region through several UHVAC transmission lines from the same large scale power plant,should be avoided.More attentions should be paid to the safety and stability status ofmulti-infeed HVDC transmission systems with UHVDC EHVDC transmission lines.It is suggested that composite insulators of large tonnage and high intensity are prior to be used in the UHV transmission lines in China.Pollution proof glass insulators and composite insulators should be combined in use,which will avoid the brittle fracture of composite insulators.