More attention is paid to synchronous condenser nowadays with developing of DC transmission, new energy resources, large HV power grid and EHV technology. 300 Mvar synchronous condenser (CS) will be put into service soon in China. This paper summarizes CS present applying status, as well as requirement background and SFC (static frequency converter) start mode of large CS. Some special relay protective functions for CS are analyzed in detail. Relay configuration scheme, principle and implement method of the 300 Mvar CS are put forward.