Control of renewables by Synchronous Power Controller (SPC) technology provides virtual damping and inertia which results to support phase stability on the grid and keep synchronism of interconnected areas tighter. Serving these benefits needs to have proper dynamic interaction between renewables and grid. In this paper, according to the dynamic modeling of Renewable Static Synchronous Generation unit based on SPC (RSSG-SPC) connected to an external grid, an index is introduced for the study of the dynamic interaction between RSSG-SPC and grid by analysis of synchronization power. Mathematical analysis for weak then stiff external grid confirms that the proposed index can be trustable for both single and two-dimensional design. Modal analysis and detailed time domain test approve that the suggested indices can clarify adjustment range for dynamic parameters aim to have optimal synchronization process between RSSG-SPC and grid, which leads to control this dynamic interaction.