Purpose of current transformer
Relays need to know the current magnitude – either for metering or protection. A current transformer (CT) fulfills this role, stepping down 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.
For utilities, it is important to know how much power is imported or exported at its territory. A CT along with a PT is installed right where the transmission line enters the substation. This is typically the demarcation point of ownership.
At extra-high-voltage, the circuit breakers are designed as live-tank breakers. Due to the weight and space issues, CT’s cannot be mounted directly on the breaker. External stand-alone CT’s are employed.
The image below shows a CT mounted on a piece of medium-voltage bus.
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
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.