What is different between ct for protection and ct for measurement?

As the Main purpose of the Relay is "Protection"  so Protection CT means to protect the circuit when it has crossed the protection limits i.e when there comes a fault in circuit the relay cuts it off from the circuit and hence it is protect

CT Measurement means to measure the power distribution from the Generating end to Transmitting end

* Metering C.T:  will have to carry the fault currents which are10 times the normal full load current that’s why it is designed at much below the saturation point (knee) in order to avoid saturation.

These CTs have to protect against fault hence these CTs are not supposed to saturate during fault current and they should transform secondary current exactly replica of fault current. For example class5P20 means5 = % error, P = protection class,20 means =20 times of rated current means when20 times of rated current is flowing  through CT the error in sec. current may be +/-5%.

* Protection C.T: will have to carry only full load  current that’s why it is designed near to knee point. As per accuracy concern the metering Ct should be more accurate under normal operating condition.....i.e. under  normal operating condition the protection CT accuracy not important.

It will function up to full load current only then it will be saturated. It will not follow the CTR Above the full load current. Take the example of a train, if bogies are connected to engine it will follow the speed of engine but if coupling is broken bogies will not follow the engine  

Never connect metering CT for protection purpose as it will saturate during fault and will not provide any protection at all.

CT for the protection has the ability to measure and convert that current value to the low value and provide it to the relay circuit for isolation of faulty element from the system.

Protection CT core will be designed, in such a way that during fault condition it will not get saturated for huge primary current. unlike instrument transformer core get saturated for a small increase in primary current (based on instrument limit factor like1.5,2....).

At the same time there is limit factors for Accuracy limit current like10,15,20,30...for protection CT also, beyond which core starts saturate, secondary current will not increase. Hence relay is safe guarded for maximum current limit for that connected relay.

Protection CT is use to produce the output according to fault current wich could be many times high then its rating. that's why it is designe below saturation point. So its accuracy is low.

Measuring CT is use to produce most accurate output accoring input cuurnt so, it is designe near saturation point.