What is the purpose of a voltage transformer?

Relays need to know the magnitude of the voltage in the system. A voltage transformer fulfills this role u2013 it steps down thousands of volts to 67V/115V which is then fed to a relay. At high voltage 138kV and above, VT's are built using capacitors and a small aux transformer. At 69kV and below, they are wound.

What is the cost of a voltage transformer?

345kV-67V/115V 0.6WXYZ accuracy-burden: ~$10,000 138kV-67V/115V CCVT: ~$5,500 14.4kV-120V wound VT: ~$1,000

What is the lead time to procure a voltage transformer?

~16 weeks for high voltage unit ~4 weeks for medium voltage unit

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.

Voltage transformer on a piece of bus. Image credit: FirstEnergy – Lorain county substation

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.

Voltage transformer used for capacitor bank protection. Image credit: FirstEnergy – Toledo Edison – Midway substation.

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.

Power line carrier - PEguru — GIF image showing how the carrier frequency travels from a relay at one substation to another.

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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What’s the purpose of a conventional power transformer?

Transform power, voltage, and current

Transform voltage and current

Transform voltage only

For which event should a surge arrester operate? Check all that apply.

Phase-to-ground short-circuit

Temporary overvoltage

Switching surge

Lightning strike

Which combination of devices would you use to implement power line carrier?

Wave trap and line tuner

Wavetrap, capacitor coupled voltage transformer, and line tuner

Wave trap, capacitor voltage transformer and line tuner

You are building an interconnect station, tying wind farm generation to the power grid. At the substation, the sources are tied through a tie-breaker. Which device would you install to make sure the sources are synchronized?

Current transformer

Power transformer

Voltage transformer

Ideal transformer

An industrial facility is plagued with voltage flicker and poor power factor at the service entrance. Which device would you install to fix both issues?

Install capacitor bank

Voltage regulator

Harmonic filter bank

In a distribution substation, the available ground-fault current on the low side of the step-down transformer is excessive, over-dutying the switching equipment. What is your cost-effective solution to reducing the fault current?

Add series inductors

Buy new transformer with higher impedance (%Z)

Add current limiting fuses in series on the switchgear

Add inductor in the transformer neutral

A transformer develops an internal fault. Which device would you install on the transformer high-side to interrupt fault current?

Circuit switcher

Disconnect switch

Either a breaker or a circuit switcher

Circuit breaker

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