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.
potential transformer connection
Connection of VT / PT

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.
voltage transformer connection
voltage transformer connection for measuring

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.

residual voltage measurement
residual voltage measurement

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.
calculation of residual voltage with open delta
calculation of residual voltage with open delta

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.

Connection of potential transformer
Connection of potential transformer

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