Abstract: With the development of HVDC and renewable-energy power generation， many typical power grids with high penetration of HVDCs and renewables are forming where there are of abundant clean energy in China. Under asynchronous transmission mode with multiple HVDCs， the inertia level of these systems have considerably decreased as a great amount of wind and photovoltaic power replaces the synchronous units， which impairs the frequency response performance for resisting dangerous faults. Meanwhile， the spatial and temporal distribution of mechanical inertia becomes more obvious than ever before， which makes the inertia response mechanism of the power system more complex to be detected. Thus， it is critical and essential to carry out inertia assessment of these low-inertia systems. This paper proposes a multi-dimensional framework for assessing the inertia level from three aspects: the overall size， the time-varying characteristics and the geographical distribution based on three quantitative indices， i.e.， the kinetic energy of units， the change rate of inertia and the inertia distribution index. Case study and application are carried out on Yunnan Power Grid， and the simulation results verify the effectiveness of the proposed method for assessing inertia level of power systems.

Key words:  inertia, rotational inertia, frequency stability, renewable, direct current transmission, power electronization