What is the purpose of these PTs?
Looks like a wye primary with two wye secondaries and a open-delta secondary.

Without seeing schematics it's hard to say what each winding is used for. My guess:


Wye 1: Metering, synchronism check.
Wye 2: Protection.
Open delta: for residual voltage measurement (earth fault protection) or for connection of a damping resistor to mitigate ferroresonance. Note that the screen displays "3U0" which is the residual voltage, i.e. 3 × the "zero sequence" voltage U0.