Residual Voltage Transformer

RESIDUAL VOLTAGE TRANSFORMER is used to detect unbalanced voltage in three phase system and to supply voltage to directional earth-fault relay.
For directional earth-fault relay, it is necessary that the voltage applied to voltage coil of the relay corresponds in phase to that of the current in current coil. Such voltage will be the Residual voltage of the system and will be the phasor sum of the three line-to-earth voltages.
Residual voltage can be achieved by connecting secondaries of three single phase V.Ts connected in three different phase, in open-delta fashion. It is however, economical to use three phase V.Ts instead of Three nos. single phase V.Ts 'SE' manufactures three phase P.V.Ts suitable upto 33kv system voltage.