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

Purpose of current transformer

Relays need to know the current magnitude – either for metering or to implement protection schemes. A current transformer (CT) fulfills this role, stepping down hundreds or sometimes thousands of amps to (typically) 5A which is then fed to a relay.

A typical location for CT installation is on the breaker bushing or transformer bushing. These are donut CT’s and use the magnetic field generated by the current (flowing through the bushing) to induce a current in its winding.

At extra-high-voltage, the circuit breakers are designed as live-tank breakers. Due to the weight and size of CT’s, they cannot be mounted directly on the interrupter housing. External stand-alone CT’s are employed.

For utilities, it is essential to know how much power is imported or exported at its service boundary. A CT, along with a PT, is installed right where the transmission line enters the substation, a demarcation point of ownership.

The image below shows a CT mounted on a piece of medium-voltage bus.

Thus far, you’ve seen stand-alone and donut CTs. Take a look at the bar-type CT and Rogowski coil.

From protection and control stand-point, CT’s establish a zone of protection in the power system. A oneline taken from one of the PEguru ebooks is shown below. It shows how the CT’s on a breaker and a transformer are strategically chosen to implement line protection, transformer protection, and breaker protection.

Cost of current transformer

• 138kV wound stand-alone CT: ~$15,000 /phase
• 345kV 3000:5A MR C800 accuracy CT: ~$30,000 /phase

Lead time to procure current transformer

Extra high voltage stand-alone unit: ~1 year

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.

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

Ideal transformer

Current transformer

Power 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.

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?

Install capacitor bank

Voltage regulator

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.

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

Lightning strike

Phase-to-ground short-circuit

Switching surge

Temporary overvoltage

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.

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

Circuit breaker

Disconnect switch

Circuit switcher

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.

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

Wave trap, capacitor voltage transformer and line tuner

Wavetrap, capacitor coupled voltage transformer, and line tuner

Wave trap 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 inductor in the transformer neutral

Add current limiting fuses in series on the switchgear

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.

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