Abstract: Based on the brief introduction of the unified power flow controller (UPFC) project in western Nanjing power grid, a blocking failure of the UPFC is analyzed in this paper. The blocking failure occurred since the “low grid voltage” was detected due to the evolved fault occurred on the 35kV distribution line, and the busbar automatic transfer of the shunt transformers was not completed. It is pointed out that the reason of the blocking failure is the unsuccessful coordination between the time delay of “low grid voltage” fault to block converter and the busbar automatic transfer logic of the shunt transformers. Besides, the countermeasure is proposed and have been proven to be effective.