When existing recommendations for PST protection can let you down

This presentation describes a protection scheme that has been implemented on two symmetrical, double-core design, phase-shifting transformers (PSTs), each rated at 600 MVA, 400 kV, 866 A, 50 Hz, +40 with in total 64 tap positions (32 advance and 32 retard). Thus, every individual tap introduces a phase angle shift of 1.25. The PST protection scheme used follows the protection recommendations published by the IEEE. However, during the execution of this project, new challenges were faced due to (a) slightly different construction of the PST, and (b) numerical differential relays being used instead of the electro-mechanical ones that were originally suggested. Due to these differences, this presentation indicates which statements in the IEEE publication are wrong due to the slightly different PST design, and provides design and setting guidelines for the 87P and 87S functions when modern numerical transformer differential protection is used.