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 Coupling Capacitor Voltage Transformer.

Purpose of voltage transformers

  • Relays need to know the voltage magnitude – either for metering or to implement protective features. A voltage transformer provides this magnitude information – it steps-down system nominal voltage (i.e. thousands of volts) to either 67V or 115V (at 34.5kV and below, 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 (shown in cyan color) in a ring substation. These are suggested locations.
  • 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 implementing a communication channel. The capacitive element in the VT, in conjunction with the inductive element in a line-tuner, creates a resonant frequency equal to carrier signal frequency, ineffect creating a tight-path.
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

  • 4kV wound voltage transformer: ~$1,500 /phase
  • 12.47kV wound voltage transformer: ~$2,000 /phase
  • 34.5kV wound voltage transformer: ~$2,000 /phase
  • 69kV wound voltage transformer: ~$5,500 /phase
  • 138kV-67V/115V CVT relaying grade: ~$7,000 /phase
  • 138kV CT/PT combo wound transformer: ~$12,000 /phase
  • 230kV-67V/115V CVT relaying grade: ~$9,000 /phase
  • 345kV-67V/115V CVT relaying grade: ~$12,000 /phase
  • 500kV-67V/115V CVT relaying grade: ~$20,000 /phase
  • 765kV-67V/115V CVT relaying grade: ~$25,000 /phase

Note: The cost includes the price of structural steel for 69kV and above PTs/CVTs.

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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Voltage Transformer: Learn the Purpose, Cost, and Lead Time to Procure 2

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