Recommendation of HVDC transmission/interconnection for every strengthened regional grids,avoiding countrywide 1000kV interconnection

It is recommended to continue implementing HVDC transmission/interconnection for strengthened regional grids according to the zoning fundamental of System Security Stability Criteria,which is more secure and economic than countrywide 1 000 kV interconnection.For the same transmission capacity,it is concluded that,AC 1 000 kV line is worse than both HVDC line for long distance transmission and AC 500 kV line with compact double-circuit tower for short and middle distance transmission.AC 1 000 kV line is also worse than both HVDC and 500 kV lines in security and environmental protection.According to the analysis for long term development of worldwide power grids,including the acquisitions from the America Grid 2030 Vision,the Feasibility Report of European Grids and the Power Grids Document of United Nations,AC 1 000 kV grid overlaying the existing power grids would never be achieved in the future.Grid zoning fundamental by HVDC and delamination fundamental should be maintained,and the transmission capacity of networks and lines at different voltage levels should be strengthened.The problems of short circuit current/close-in HVDC inverters should be solved by effective solutions from planning stage.