Phase angle regulating transformer protection

This paper describes the protection of a 345 kV, 450 MVA three phase, phase angle regulating (PAR) transformer. The transformer includes load tap changers which permit phase angle variations of 25° advance to 25° retard in a total of 32 steps (16 advance, 16 retard). The series unit and the exciting unit of the PAR are mounted in separate tanks, with four throat connections between the tanks (three-single phase primary connections and one-three phase secondary connection). The series unit is rated 86270 V/66122 V with the secondary windings delta connected and the exciting unit rated at 345000 V/67728 V grounded wye-grounded wye. The exciting unit is of three legged core type construction