From a technical point of view, with ±800 kV UHV DC transmission, there is no need to drop points in the middle of the line, and a large amount of power can be sent directly to large load centres; in the case of parallel AC and DC transmission, dual-side frequency modulation can be used to effectively suppress regional low-frequency oscillations and improve the temporary (dynamic) stability limit of the cross section; to solve the problem of short-circuit current overruns in large recipient grids. The use of 1000 kV AC transmission, the middle can fall point, with grid function; strengthen the grid to support large-scale DC transmission; fundamentally solve the problem of short-circuit current exceedance of the large receiving end of the grid and 500 kV line transmission capacity is low, optimize the grid structure.
From the perspective of transmission capacity and stability performance, using ±800 kV UHV DC transmission, transmission stability depends on the effective short-circuit ratio (ESCR) and effective inertia constant (Hdc) of the grid at the receiving end and the structure of the grid at the sending end. With 1000 kV AC transmission, transmission capacity depends on the short-circuit capacity at each support point of the line and the transmission line distance (distance between two adjacent substation drop points); transmission stability (synchronisation capacity) depends on the size of the power angle at the operating point (power angle difference between the two ends of the line).
From the key technical issues requiring attention, the use of ±800 kV UHV DC transmission should focus on static reactive power balance and dynamic reactive power backup and voltage stability issues at the receiving end of the grid, and on the system voltage safety issues caused by simultaneous phase change failures in multiple DC feed-in systems. Using 1000 kV AC transmission, we should pay attention to the AC system phase adjustment and voltage regulation when the operation mode changes; we should pay attention to serious fault conditions, the relative weak section high power transfer and other issues; we should pay attention to the potential problems of large area blackout accidents and their preventive measures.
