Voltage instrument transformers (VT, sometimes also abbreviated with PT for potential transformer) are used on the primary side of medium- and high voltage distribution transformers. The purpose is to provide an isolated standardized low voltage value on the secondary side for energy metering and protection devices.
Up to 100 kV the VTs are usually operating with an inductive coupling. Above 100 kV the coupling is capacitive. Beside of maintaining the primary to secondary ratio also a low phase-displacement is required because the VTs are working together with current transformers (CT) for metering. Any secondary phase shift has impact on the measured power factor and finally on the billing.
The rated secondary voltage for VTs connected between phase/phase is 100 V or 110 V. For single-phase VTs connected between a phase and earth, the rated secondary voltage is divided by √3.
The pictures are for example only. The various instrument transformer manufacturers have a wide range of different sizes and shapes.
Voltage transformers are available in different accuracy classes. For energy metering purpose we use Cl. 0.2 or Cl. 0.5. The accuracy is guaranteed as long as the actual primary voltage is within 80 % and 120 % of the rated voltage and the actual load is within 25 % to 100 % with power factor 0.8 inductive.
The standard output power (burden) values are: 10, 15, 25, 30, 50, 75, 100, 150, 200, 300, 400, 500 VA.
The VT needs also to power-up the meter. You can make a complicated burden calculation or you look in the manufacturer data sheets. They have already simplified the selection. Typically, a VT with accuracy class 0.5 has a burden of 100 VA. If you are in doubt, ask your supplier.
The installation of voltage transformers require special skills and knowledge about working in the medium- or high voltage grid. Utilities are providing dedicated training and repeated safety briefings.
The voltage transformer burden can be verified with our portable meter test equipment RS350.
Thank you for reading.
Editor's note: This article was originally published in September 2020 and has been updated for comprehensiveness.