Aiming at the problem of poor grid connection condition and low success rate of large synchronous condenser, the traditional phase angle difference time function is improved. The improved curve can reflect the change rule of phase angle difference between 0 and 2 π, and directly show whether there are synchronization points and their number in a certain slip speed range and the corresponding closing time range in a certain phase angle difference range. It is very convenient to predict the time when the closing pulse is sent out. When the initial moment of inert deceleration is 0 and the acceleration starts to decelerate with -0.167 r/s² as the starting point of inert deceleration, the number of simultaneous points within the maximum allowable slip range and the closing time range allowed by the synchronous condensers are analyzed through examples, and the correctness of the example analysis results is verified by Matlab/Simulink simulation software. At the same time, taking the impulse current of the phase modulator as the setting basis of the maximum allowable phase angle difference can effectively reduce the impact on the synchronous condenser when connected to the grid, and improve the success rate of connected to the grid.