Voltage transformer is often consists of two cores. The first core is connected in star, and the stare point must be earthed. The second core is connected as open delta.
Voltage coils of protection relays and measuring meters are connected in parallel with the star core. Open delta core is used fore directional earth fault relay.
Connection of stare and open delta core with protection relays and measuring meters are shown in figure (5), and figure (6) respectively.
