A sampling value network transmission bus protection independent of outside global synchronization system is designed. The protection has the advantages of simple structure, resource sharing and good expansibility, so it has very important significance in the construction of digital substation. This paper introduces the sampling transmission system of bus protection in digital substation, describes a point-to-point bus protection technology based on absolute delay, and proposes a network bus protection using voltage phase lock. The synchronization performance of bus voltage, digital phase locked loop and resampling interpolation are used to eliminate the synchronization dependence in network sampling system. Experiments show that the bus protection based on voltage phase lock is well synchronous, fast and reliable. This method is a new solution for the bus protection of digital substation.