  Generalized short circuit ratio (gSCR) for gird strength assessment of
multi-infeed high voltage direct current (MIDC) systems is a rigorous
theoretical extension of traditional short circuit ratio (SCR), which allows
the considerable experience of using SCR to be extended to MIDC systems.
However, gSCR was originally derived based on the assumption of homogeneous
MIDC systems, where all HVDC converters have an identical control
configuration, which poses challenges to the applications of gSCR to
inhomogeneous MIDC systems. To weaken this assumption, this letter applies
modal perturbation theory to explore the possibility of applying gSCR in
inhomogeneous MIDC systems. Results of numerical experiments show that, in
inhomogeneous MIDC systems, the previously proposed gSCR can still be used
without modification, but critical gSCR (CgSCR) needs to be redefined by
considering the characteristics of HVDC converter control configurations.
Accordingly, the difference between gSCR and redefined CgSCR can effectively
quantify the pertinent ac grid strength in terms of static voltage stability
margin. The performance of our proposed method is demonstrated in a
triple-infeed inhomogeneous LCC-HVDC system.
