Recently, more attention has been paid to the large synchronous condenser as the problem of “strong DC and weak AC” is becoming increasingly serious. The 300 Mvar large synchronous condenser has been put into service in many converter stations in China. Because of the particularity of the start-up mode of a synchronous condenser, the start-up protection of the turbo-generator, hydro-generator and pumped-storage-generator is not fully applicable to the synchronous condenser. It is necessary to study the start-up process and protection configuration and algorithms of the synchronous condenser. Its start-up stage and characteristics of three-phase fault current are analyzed in the start-up process by comparing different types of generator units. A more reliable and faster start-up protection is proposed, including main transformer start-up quick-break protection and excitation transformer start-up differential protection. A method of phase angle and amplitude correction is presented. This is also suitable for wide frequency and wide voltage. The simulation results show that the error of amplitude calculation is clearly improved and meets the requirement of protection accuracy. This work is supported by National Natural Science Foundation of China (No. U1866603).