The strength indexes of the islanded LCC-HVDC sending power system

With the intensive exploitation of large-scale power plants, the LCC-HVDC sending power system may operate in an islanded mode, disconnected from the main ac system. Compared with the grid-connected mode, the frequency and voltage fluctuation of the islanded sending power system are more severe, the extent of which is fairly influenced by its inherent strength. However, there lack explicit strength indexes for it. Thus, it is necessary to study its appropriate strength indexes for planning and designing LCC-HVDC projects. In this paper, a two-stage equivalent model is established, in which the automatic voltage regulator (AVR) of generator and the studied timescale are specially considered. In addition, the accuracy of the two-stage model is verified by time-domain simulation. Furthermore, three strength indexes are proposed, including the transient impedance index for the transient overvoltage limit, the steady impedance index for the power transfer limit, and the mechanical inertia index for the transient frequency stability limit. Finally, the proposed strength indexes are verified by an actual HVDC project in China. It is concluded that the steady impedance index is more suitable for the islanded sending power system, compared with the previous SCR index.