Basic connection of Voltage / Potential transformer
- phase/phase: primary connected between two phases
- phase/earth: primary connected between a phase and the earth.
Metering connection arrangements for voltage/Potential transformer-
- star-connection of 3 transformers: requires 1 isolated MV terminal for each transformer.
- connecting to 2 transformers, so-called V-connection: requires 2 isolated MV
terminals per transformer.
Residual voltage metering – to measure residual voltage with PT/VT
The residual voltage which characterizes the voltage of the neutral point relative to earth, is equal to the vectorial sum of the three phase-earth voltages. The residual voltage is equal to 3 times the zero-sequence voltage V0.
It is impossible to measure a residual voltage with phase/phase VT’s.
The appearance of Residual voltage signifies the existence of an earthing fault. It is obtained by measurement or by calculation:
- Measuring by three voltage transformers whose primary circuits are star-connected and whose secondary circuits are open-delta connected, supplying the residual voltage
- Calculation by the relay based on three voltage transformers whose primary and secondary circuits are star-connected.
Connection of Voltage/Potential transformer-
VT with a double (or triple) secondary
A VT can have one or two secondaries (fi gure opposite), and more rarely three
secondaries for the appropriate applications (protection and/or metering).
A VT is connected across the terminals identifi ed according to the IEC:
- A for phase and N for earth on the MV side
- a and n on the corresponding secondary side. In the case of a double output,the first output is shown by 1a and 1n, the second by 2a and 2n.