Abstract: In recent years, some major HVDC convertor stations and substations had installed large synchronous condensers. In the synchronizing cabinet, the designner prefered adding a sync-check relay after the synchronizing device, in order to prevent the fault-synchronizing of the synchronizing device and improve the reliability of the synchronization. However, because the imprecise voltage-difference algorithm of the sync-check relay, the unit had failed to be connected to the grid for several times in extreme cases. Therefore, the manufacturer subsequently upgraded the voltage-difference algorithm twice. Aiming at this problem, this paper analyses the influence of different voltage-difference algorithm and whether the system voltage tracking function of excitation regulator has been put into function on synchronization; Three optimization directions are put forward, including a reasonable voltage-difference algorithm, which thoroughly solves the problem that the voltage difference of the sync-check relay is inconsistent with the actual voltage difference, and the optimization method of the related setting(the allowable voltage difference and the allowable phase angle difference), so as to improve the reliability of synchronization。