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

Current transformer
High accuracy C800 wound current transformer typically used for revenue metering.

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.

Current transformer
Current transformer on a circuit breaker. Image credit: FirstEnergy – Ohio Edison – Lisbon station.

For utilities, it is important 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. This is typically the demarcation point of ownership.

Current Transformer: Learn the Purpose, Cost, and Lead Time to Procure 1
A current transformer near the dead-end structure where a transmission line enters the substation. Image credit: Western Area Power – Test track substation.

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.

External current transformers near live tank breaker
External current transformers near a live tank breaker

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

External current transformer used near a switch
External current transformer used near a switch

At this point it is needless to say, there are many kinds of CTs. What’s shown thus far are stand-alone and donut CTs. There are other variations such as bar-type, Rogowski coil, etc.

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.

Substation oneline drawing
Relay oneline for a ring bus substation.

Cost of current transformer

365kV 3000:5A MR C800 accuracy CT: ~$30,000

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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Current Transformer: Learn the Purpose, Cost, and Lead Time to Procure 2

Substation Major Equipment

It’s only stupid-simple, let’s begin.

For which event should a surge arrester operate? Check all that apply.
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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?
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A transformer develops an internal fault. Which device would you install on the transformer high-side to interrupt fault current?
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What’s the purpose of a conventional power transformer?
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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?
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