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.