Both parallel transformer and series transformer are core AC devices of unified power flow controller(UPFC), and special operating conditions of UPFC can easily affect transformer differential protection, which is the main protection for these two types of transformers. The differential protection could have various manifestations under different operation conditions and varied faults. In order to evaluate the affection for differential protection from different kinds of faults, the factors that may impact the sensitivity and reliability of differential protection are discussed when faults happen based on the control system of actual UPFC project. According to the simulation results of typical internal and external faults, the behavioral characteristics of differential protection are analyzed, and the adaptability analysis of differential protection is formed as well. The relationship between valve control protection and AC relay protection in aspect of protecting equipment are also discussed. A technical reference is provided for the implementation of AC relay protection of the UPFC project in terms of anti-misoperation and anti-rejection.