Current Transformers

Current transformers are used to supply information to the protective relays and/or current, power and energy metering “instruments”. For this purpose they must supply a secondary current proportional to the primary current flowing through them and must be adapted to network characteristics: voltage, frequency and  current.They are defined by their ratio, power and accuracy class. Their class (accuracy as a function of CT  load and of over current) is chosen according to the application.

• A “protection” CT must saturate sufficiently high to allow a relatively accurate measurement of the fault current by the protection whose operating threshold can be very high. Current transformers are thus expected to have an Accuracy Limit Factor (ALF) that is usually fairly high. Note that the associated “relay” must be able to withstand high over currents.
•  An “instrument” CT requires good accuracy around the nominal current value. The metering instruments do not need to withstand currents as high as the protection relays. This is why the “instrument” CTs, unlike the “protection” CTs, have the lowest possible Safety Factor (SF) in order to protect these instruments through earlier saturation.
• Some CTs have secondary windings dedicated to protection and metering. These “instrument” and “protection” CTs are governed by standard IEC 60044-1 (in France NF C 42-502). The matching of CTs with protection relays calls for a thorough knowledge of CTs. The following section gives a few reminders of CTs corresponding to this use.