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 communication purposes. The capacitive element in the VT is used in conjunction with the inductive element in a line tuner to couple a high-frequency signal.
- 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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