# Conection of Voltage / Potential transformer.

## 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:

1. Measuring by three voltage transformers whose primary circuits are star-connected and whose secondary circuits are open-delta connected, supplying the residual voltage
2. 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).

#### Identifying terminals

A VT is connected across the terminals identifi ed according to the IEC:

1. A for phase and N for earth on the MV side
2. 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.

## One thought on “Conection of Voltage / Potential transformer.”

1. curiosity says:

what is potential transformer (PT) selection scheme in control wiring ?