Hi folks.
Here we are going to continue our discussion about differences in metering CT and protective CT.
Let us talk about the magnetising curve first.
This is typical magnetisation characteristics of a ferromagnetic material. Imagine it as leg of the person sitting on chair. The first point is similar to his ankle and second point is similar to his knee. The important point to note here is the region between these 2 points is linear. So we get output proportional to input (true reflection). Below ankle point and above knee point is not possible (due to magnetic inertia and due to saturation respectively, if you want to know).
For a metering CT, we want linear reflection of input from 20% to 120% of the load current. So this region should fall between the 2 markers we have set. Also when fault occurs, The currents values rise to very large extent (even 20 times the rated value). Under these conditions, CT should saturate and protect the measuring devices connected across it. So measuring CT is operated just below the knee point. The core material used is not of superior quality as it is supposed to get saturated at 120% of load.
For protective CT, we want linear reflection of the fault current. If we do not get linear reflection of normal current, not an issue. So 1.5 to almost 20 (or even higher) times of the rated secondary current should fall in the linear region. So protective CT is operated near ankle point. The core material used is of superior quality as we do not want saturation even the 50 times the rated secondary current of CT.
Thank you for your time. Please feel free to leave comments about your views. You can mail me your doubts at dnachiketa1010@gmail.com, I will be very glad to post about them.
Here we are going to continue our discussion about differences in metering CT and protective CT.
Let us talk about the magnetising curve first.
This is typical magnetisation characteristics of a ferromagnetic material. Imagine it as leg of the person sitting on chair. The first point is similar to his ankle and second point is similar to his knee. The important point to note here is the region between these 2 points is linear. So we get output proportional to input (true reflection). Below ankle point and above knee point is not possible (due to magnetic inertia and due to saturation respectively, if you want to know).
For a metering CT, we want linear reflection of input from 20% to 120% of the load current. So this region should fall between the 2 markers we have set. Also when fault occurs, The currents values rise to very large extent (even 20 times the rated value). Under these conditions, CT should saturate and protect the measuring devices connected across it. So measuring CT is operated just below the knee point. The core material used is not of superior quality as it is supposed to get saturated at 120% of load.
For protective CT, we want linear reflection of the fault current. If we do not get linear reflection of normal current, not an issue. So 1.5 to almost 20 (or even higher) times of the rated secondary current should fall in the linear region. So protective CT is operated near ankle point. The core material used is of superior quality as we do not want saturation even the 50 times the rated secondary current of CT.
Thank you for your time. Please feel free to leave comments about your views. You can mail me your doubts at dnachiketa1010@gmail.com, I will be very glad to post about them.
