# 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 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.

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