Abstract: The output side of the static frequency converter (SFC) which is not equipped with step-up transformer can reduce the overall cost of installing the condenser, but then the conditions of grid connection will become harsh when the rotor speed is no longer controllable. In order to improve the grid-connected reliability of synchronization, firstly the relationship is established between the inrush current, frequency difference and angular difference, secondly the existence probability of synchronous point are researched on, then some suggestions that select a small lag angle in the positive frequency difference, a small advanced angle in the negative frequency difference and so on are put forward to improve the reliability of synchronization. Based on the example, the existence probabilities and permissible frequency differences are studied that verify the proposed methods can effectively improve the reliability of synchronization.