The AC grid strength of power systems integrated by renewable power generations via power electronic devices should meet the requirement due to the risk of system oscillation. However
due to the strong interaction among the power electronic devices
it is difficult to evaluate the AC grid strength. This paper explored the evaluation of grid strength from small-signal stability perspective. Firstly
we constructed the linearized model of multiple power electronic devices infeed systems(MPEIS) and derived the characteristic equation for the MPEIS. Then
based on eigenvalue perturbation theory
it is proved that the dominant characteristic locus of an inhomogeneous MPEIS can be equivalently represented by that of a homogeneous MPEIS which has the similar AC grid structure as the inhomogeneous MIPES. Finally
we extended the application of generalized short-circuit ratio(g SCR) from homogeneous MIPES to inhomogeneous MIPES
as well as two equivalent definitions and the calculation method of critical generalized short-circuit ratio(CgSCR). The theoretical analysis results showed that the definition of gSCR of a homogenous MIPES is still valid for an inhomogeneous MIPES
while their critical values are different. The case studies illustrated the effectiveness of the proposed methods.