The measuring core of current transformer is connected to current coils of Ammeters, Wattmeters, Watt-hour meters, Var-Meters, and Var-Hour-Metes. The zone of work of all these relays below the rated of secondary of current transformer.
The measuring cores must have high to accurate monitor of the system quantities accuracy in the range of load levels. In fault conditions,Current is very high and may reach 20 times the rated value, So measuring core must be saturated to avoid the damage of measuring instruments.
Protection cores are designed to work at high levels of current (at fault). The accuracy of these cores may be less comparing with measuring cores. The saturation level of these cores are very higher than measuring cores.
Magnetization curve of current transformer with measuring and protection cores is shown in figure (3), and different regions of magnetization curve is shown in figure (4).
Note :
The knee point can be defined as the point at which a 10% increase flux density causes 50% increase in exciting ampere -turns.
