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.
- 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.
- 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.
- 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.
- 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.
Learn details of other major equipment
Attempt the quiz