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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What's the purpose of a conventional power transformer?

Transform voltage only

Transform voltage and current

Transform power, voltage, and current

Transforms voltage and current only. Power remains the same on both sides. Learn more.

Transforms voltage and current only. Power remains the same on both sides. Learn more.

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

Wave trap, capacitor voltage transformer and line tuner

Wave trap and line tuner

Wavetrap, capacitor coupled voltage transformer, and line tuner

You need a device that allows a certain frequency to pass (CCVT and line tuner) and a device that stops it (wavetrap). Note a capacitor voltage transformer does not come with coupling accessories as such it does not work for transmitting carrier frequencies.

Click here for more detail on wavetrap and here for CCVT.

You need a device that allows a certain frequency to pass (CCVT and line tuner) and a device that stops it (wavetrap). Note a capacitor voltage transformer does not come with coupling accessories as such it does not work for transmitting carrier frequencies.

Click here for more detail on wavetrap and here for CCVT.

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?

Buy new transformer with higher impedance (%Z)

Add current limiting fuses in series on the switchgear

Add inductor in the transformer neutral

Add series inductors

All methods mentioned help with limiting the fault current. The cheapest though is a neutral inductor, especially when mounted directly on the transformer. Learn more.

Current limiting fuses directly on the transformer secondary are rare. Mostly because of the continuous current requirement (1000amps or more) - fuses are rarely fabricated this large. This was a trap answer.

All methods mentioned help with limiting the fault current. The cheapest though is a neutral inductor, especially when mounted directly on the transformer. Learn more.

Current limiting fuses directly on the transformer secondary are rare. Mostly because of the continuous current requirement (1000amps or more) - fuses are rarely fabricated this large. This was a trap answer.

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

Voltage regulator

Install capacitor bank

Harmonic filter bank

Cap bank fixes the poor power factor only. It does provide a low impedance path for harmonics but it is not tuned to filter them out. A regulator does not remove the harmonics and neither does it fix the power factor. A filter bank addresses both - it is made up of inductors and capacitors therefore tunable and also capable of releasing VARs. (Note: one manifestation of harmonics is voltage flicker). Learn more.

Cap bank fixes the poor power factor only. It does provide a low impedance path for harmonics but it is not tuned to filter them out. A regulator does not remove the harmonics and neither does it fix the power factor. A filter bank addresses both - it is made up of inductors and capacitors therefore tunable and also capable of releasing VARs. (Note: one manifestation of harmonics is voltage flicker). Learn more.

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?

Voltage transformer

Current transformer

Power transformer

Ideal transformer

A synchronization check is done using a relay. It needs low voltage input (from both sides), obtained using a voltage transformer a.k.a. instrument transformer. Learn more.

A synchronization check is done using a relay. It needs low voltage input (from both sides), obtained using a voltage transformer a.k.a. instrument transformer. Learn more.

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

Circuit switcher

Disconnect switch

Circuit breaker

Either a breaker or a circuit switcher

Both the breaker and the switcher have an interrupting chamber that can safely interrupt the fault current. A switch does not. A switch will draw an arc, that is damaging and a safety hazard. Learn more about switches here and circuit switchers here.

Both the breaker and the switcher have an interrupting chamber that can safely interrupt the fault current. A switch does not. A switch will draw an arc, that is damaging and a safety hazard. Learn more about switches here and circuit switchers here.

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

Temporary overvoltage

Phase-to-ground short-circuit

Lightning strike

Switching surge

Surge arresters are specified to discharge lightning strikes and switching surges. The equipment/system insulation can handle temporary overvoltage. Learn more.

Surge arresters are specified to discharge lightning strikes and switching surges. The equipment/system insulation can handle temporary overvoltage. Learn more.

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