Static frequency converter (SFC) is an important piece of equipment for the startup process of large synchronous units in pumped storage power plants. In the process of startup, the different current frequencies on both sides of the SFC make it difficult to configure the conventional differential protection based on power frequency phasors. Additionally, the existing protection schemes have deficiencies in terms of speed and reliability. To address these problems, this paper proposes a sampled value differential protection principle for an SFC based on model verification. First, the operating principles and control strategies of an SFC are thoroughly studied, and a mathematical model that aligns with the SFC’s working principles is constructed in the time domain using current sampled values. Then, a model verification method is applied to identify differences before and after a fault, and the corresponding protection criteria are established. Finally, an electromagnetic transient simulation model of the pumped storage unit’s SFC startup is built in PSCAD/EMTDC to verify the effectiveness of the protection principle. Results show that the proposed protection principle is not affected by the frequency differences on both sides of the SFC, and exhibits excellent selectivity, fast response, and reliability.