35kv electrical main wiring, what are those three coils for?

This is the symbol of three-phase voltage transformer (PT). The primary winding of PT is star-shaped and has two secondary windings. One winding is also star-connected, with an output voltage of100V, and the other winding is open triangle-connected. When the system is short-circuited to the ground, the third harmonic will appear at the opening, and protection can be introduced to make it act.