Abstract: Based on the analysis of the requirements of SSSC demonstration project, this paper ascertains the compensation capacity of SSSC device, presents the access scheme and construction scale of the SSSC demonstration project through analysis and calculation. We study the selection principle of main equipment such as series transformer and converter valve, present the main electrical devices connection and the main equipment selection scheme of both the SSSC network side and valve side. Through simulation calculation, analyze over-voltage and insulation coordination of electrical devices, present the design scheme of over-voltage and insulation coordination of SSSC device. We divide the SSSC device according to different equipment and functions, implement technical and economic comparison. At last , we present the modular design of SSSC device, provide technical reference for the subsequent project construction.