Residual Voltage Transformer (Rvt)

RESIDUAL VOLTAGE TRANSFORMER is used to detect unbalanced voltage in three phase system and to supply voltage to directional earth fault relay.
For directional full earth is necessary that the voltage applied to voltage coil of the relay corresponds in phase to that of 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 acheived by connecting secondaries of three phase V.Ts. Connected in three phase in open-delta is,however,economical to use three phase V.Ts. instead of three no.s single phase V.Ts. AE manufactures three phase R.V.Ts.suitable upto 33KkV system voltage.
In normal three phase limb V.T. when one phase experiences earth fault, the resultant flux due to two healthy lines return through transformer limb of faulty line, including a heavy short circuit current in the winding on the limb.Also th voltage included in the secondary open -delta winding is not true residual voltage. To overcome this difficulty two seperate limbs are provided in the magnetic circuit of three phase voltage Transformer to form Five limb Voltage Transformer as shown in fig.No.4.