Abstract: The series transformer is a very important part of the unified power flow controller（UPFC）.The primary windings of the series transformer are connected in series with the power system.The characteristics and operation mode are different from those of the common transformer,and a special protection configuration is needed.In view of the particularity of the series transformer,this paper proposes a series of problems for its protection.And an integral configuration is presented along with appropriative criteria.Then,a real time digital simulator（RTDS）is performed of actual parameters to prove the validity of theoretical analysis and the effectiveness of the protection criteria.