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This paper describes the protection of a 138 kV, 300 MVA phase shifting transformer (PST), and the issues associated with differential protection stability during external faults that cause PST series winding saturation. In addition, this paper discusses PST modeling using the electromagnetic transients program (EMTP). EMTP modeling is necessary to verify the protective relaying settings for the particular application and their applicability to different fault scenarios and operating conditions. For the case study detailed in this paper, we verified the relay settings by playing the EMTP transient data back into the protective relays using an open loop approach. From the test data, we made setting changes and other enhancements that increased the PST protection system security for external faults.