Residual Voltage Transformer

RVTs are used for residual earth fault protection and for discharging capacitor banks. The secondary residual voltage winding is connected in open delta. Under normal conditions of operation, there is no voltage output across the residual voltage winding. When there is an earth fault, a voltage is developed across the open delta winding which activates the relay. 3 phase RVTs typically have 5 limb constructions.

Tests

A number of routine and type tests have to be conducted on VT s before they can meet the standards specified above. The tests can be d. classified as:

1. Accuracy tests to determine whether the errors of the VT are within specified limits.
2. Dielectric insulation tests such as power frequency withstand voltage test on primary and secondary windings for one minute.
3. Induced High Voltage test: The voltage is induced on the secondary winding at frequency of 100 or 150 Hz for 6000 cycles to verify the with stand ability of the V.T at higher voltages.
4. Impose tests with 1.2/50us wave Positive and Negative flashes of equal magnitude.
5. Temperature rise tests
6. Verification of terminal markings and polarity the routine tests are carried out in the test lab located in our factory premises itself. The type tests are conducted at NABL accredited labs such as ERDA or CPRI located in India.