Measurment vs Protection Cores of CT

i) Based on application, current transformers are broadly classified as Measuring Current Transformers and Protective Current transformers. The basic difference is in their functional requirements. A measuring current transformer, for example, has to remain accurate only over its measuring range, may be, a few percentage more. If it maintains its accuracy for currents many times its rated current, such as fault currents, then the measuring instruments connected to them, may have to be designed to withstand this high current, for the duration of the coordinated fault clearing time. This will make the instruments bulky as also costly. So, a measuring CT may well have to saturate pretty quickly. On the other hand, the protective CT must maintain its accuracy, even for currents much beyond its rating, for, then only the protective devices can sense the fault and issue trip commands to isolate the faulty feeder. The level up to which it has to maintain its accuracy will be determined by many system parameters, such as the fault current, fault duration, short-time withstand levels of equipment downstream & upstream, etc.

Thus, as correctly guessed by you, the difference is in the core material. A protection CT core should have a very high permeability than a measuring CT Core.

ii) Current error (actually it should be ratio error) is the RMS Error (i.e.) the difference between theoretical CT Secondary current and the actual secondary current for the given primary current. Whereas composite error is the instantaneous error integrated over a period of one cycle.

iii) FS value is for Metering Cores and it should be as less as possible; otherwise the instruments connected to them may have to be designed to tolerate high values of secondary current, which would make the instruments bulky & costly.

Whereas ALF is for protection CTs and it shall be as per the actual fault current at the location of the CT.

thanks alot .that was very clear information.