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Residual Voltage Transformers.
Residual Voltage transformer is a three phase, four pole Voltage transformer with an isolated neutral to measure earth faults in the system. RVT is of five limbed construction with three limbs for a three phase transformer with extra limbs on either side with no windings. Primary will be of star connection and secondary residual voltage winding will be of opendelta winding. An additional star connected winding may be also provided for measurement purpose.
An earth fault in any of the three phases of the system will result in the generation of single phase current which will flow in all three limbs with windings. A suitable return path of low reactance is provided for this flux in the additional limb. Due to open delta winding, single phase flux of each limb will produce a voltage on each coil. The total of three is a measure of the earth fault ( or Zero sequence) voltage produced by the fault. Under healthy conditions, it will be zero. In system with unearthed neutral, the resultant opendelta voltage will be 3 times of phase voltage. In case of solidly earthed system it will be phase voltage only. So it is necessary to mention system neutral condition to select phase voltage of the open delta winding.
Capacitors gets discharged through its primary winding, if RVT's are used in capacitor Banks. 'ECPL ' manufactures RVT's for both earth fault deduction and capacitor banks.