Medium Voltage Application Guide

SWITCHGEAR
Current Transformers
A current transformer (CT) is designed to produce a secondary current which is accurately proportional to the
primary current. It consists of a single primary winding, which an external busbar or cable runs through, or it can
have a single primary bar, brought out to two ends for termination. A medium voltage current transformer can
have up to three independent secondary winding sets. The entire current transformer assembly is encapsulated in
resin, inside an insulated casing
Current transformers are used for metering or protection purposes. The accuracy class and size depends on the
individual application - for example, revenue metering would use high accuracy metering CTs.
NOTE
Never leave the secondary winding of a CT open circuit. This creates extremely high voltages which pose
a real danger to personnel.
Ring style CT
DIN style CT
IEC Ratings
Rated primary current, Ipr (A)
The primary current rating of a CT must be greater than the expected maximum operating current it is monitoring.
a metering CT's primary current rating should not exceed 1.5 times the maximum operating current
a protection CT's primary current rating needs to be chosen so that the protection pick-up level is attained
during a fault
Standard values for I pr : 10, 12.5, 15, 20, 25, 30, 40, 50, 60, 75 A, and decimal multiples of these values (source:
IEC 60044-1)
Rated secondary current, Isr
The secondary current rating of a CT is either 1 A or 5 A. CTs with a 5 A secondary rating are becoming less
common as more CT driven equipment becomes digital. For long secondary cable runs, CTs with 1 A secondary
windings can minimise the transformer and secondary cable size.
Transformer ratio, Kn
This is the ratio of secondary to primary winding turns.
Rated thermal short-time withstand current, Ith (kA)
This is the highest level of rms primary fault current which the CT can endure, both thermally and dynamically, for
1 second without damage. When used in a medium voltage enclosure, the I th rating should match the short-time
withstand rating of the entire switchgear.
Page 110 Medium Voltage Application Guide 710-12280-00A