Current Transformer Saturation Curves 6

[CarltonJohnCarr]

I have a question about the saturation of current transformers. I am a controls tech by trade and I was trying to make a comparison of the magnetic characteristics of saturable reactors to CTs. When the core of a saturable reactor is saturated, the permability of the core decreases (reluctance increases). This causes inductance of the core to decrease (L= uN2A/l). When inductance decreases, inductive reactance decreases (Xl=2pi fl), and for a given applied voltage, current will increase. This seems to be just the opposite of what happens to a CT when it saturates. Is there something different about the core material of the CT or am I just way off base with this whole thought process.

Thanks for your help.

 

[peterb]

You are right, Pentatek - this is a separate question. Of course, operation of the overcurrent relays will be affected by any saturation of the feeder CTs - you have to evaluate the CT requirements for each case.
In general, follow the manufacturer's specified CT requirements - my experience has been that the instantaneous element can usually be set to operate securely even with CT saturation. You may have to select higher ratio on multi-ratio CTs, to get the advantages of lower secondary fault current and higher kneepoint voltage, both of which work towards minimising transient saturation effects. With modern relays, that should not present any problem in selecting appropriate pickup settings, given the wide adjustment range generally available.
Of course, in an ideal world these factors would all be accounted for in the system design before you get to the operation phase.
Were there any fault recordings available for the incident in question? That would be a good way to evaluate any offset waveforms present.