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

#### 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.
• 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 LC 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.

#### Attempt the quiz

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Substation Major Equipment

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