Grid-following (GFL) renewable energy devices are susceptible to sub-synchronous oscillations when connected to weak grids. In contrast

grid-forming (GFM) devices can actively provide voltage support and enhance grid strength

thereby effectively suppressing sub-synchronous oscillations. Consequently

the hybrid operation of GFL and GFM converters at the plant level has become an inevitable trend for enhancing the stability of weak-grid-connected renewable energy systems. Therefore

there is an urgent need for quantitative configuration methods that can accurately assess system stability margins and guide optimal plant design. This paper proposes an in-plant configuration method for sub-synchronous oscillation suppression based on the short-circuit ratio (SCR) margin. First

based on impedance modeling and the Nyquist stability criterion

theoretical calculation and experimental testing methods for the critical SCR of GFL converters are proposed

and an SCR margin index for renewable energy plants is subsequently defined. Then

the impact of integrating GFM devices on the plant SCR margin is quantitatively analyzed

and a capacity configuration method for GFM converters is developed. Finally

the effectiveness of the proposed method is verified through a practical photovoltaic power plant case study.