Voltage Transformer: Learn the Purpose, Cost, and Lead Time to Procure

voltage transformer
Capacitor Voltage Transformer. When equipped with carrier frequency coupling accessories they are referred to as Capacitor Coupling Voltage Transformer.

Purpose of voltage transformers

  • Relays need to know the voltage magnitude – either for metering or protection. A voltage transformer fulfills this role – it steps down thousands of volts to 67V/115V in high voltage stations (at the medium voltage the VT’s step it down to 120V) which is then fed to a relay.
  • VT’s are installed on every bus section.
Voltage transformers in a ring substation
Voltage transformers in a ring substation
  • In cases where two sources are tied through a breaker, checking the voltage magnitude and phase angle (using 25 sync check relay) on both sides before closing the breaker is necessary. VT’s are employed for this purpose.
Voltage Transformer: Learn the Purpose, Cost, and Lead Time to Procure 1
Voltage transformer on a piece of bus. Image credit: FirstEnergy – Lorain county substation
  • When a capacitor-can blows up, the voltage developed on the faulted phase is not the same as the other phases. Voltage transformers transmit this unbalanced voltage information to the relay.
Voltage transformer used for capacitor bank protection
Voltage transformer used for capacitor bank protection. Image credit: FirstEnergy – Toledo Edison – Midway substation.
  • On the transmission line exit, a coupling-capacitor VT is used for communication purposes. The capacitive element in the VT is used in conjunction with the LC element in a line tuner to couple a high frequency signal.
Power line carrier - PEguru
GIF image showing how the carrier frequency travels from a relay at one substation to another.
  • At high voltage, directional relays are programmed to “look” in one direction – maybe into the transformer or into the transmission line. The CVT’s can provide the polarizing potentials (3V_0).

Cost of voltage transformer

  • 345kV-67V/115V 0.6WXYZ accuracy-burden: ~$10,000
  • 138kV-67V/115V CCVT: ~$5,500
  • 14.4kV-120V wound VT: ~$1,000

Lead time to procure voltage transformer

  • ~16 weeks for high voltage unit
  • ~4 weeks for medium voltage unit

Information on cost and lead time is for your general knowledge only. Contact vendor with your equipment specifications for actual figures.


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Substation Major Equipment

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