CT core is made of CRGO (Cold Rolled Grain Oriented Silicon Steel). When the primary of CT is energized a working mmf is produced in the core. To produce a working MMF, excitation current Ie is taken. This MMF produces a flux inside the core of CT which links with the secondary winding and as per Faraday’s Law of Electromagnetic Induction, an emf is generated across the terminals of CT secondary. The emf induced in the CT secondary terminal is given as
E = 4.44fNØ
Where f is the frequency of supply, N is the number of secondary turns and Ø is flux in the core of CT.
But Ø is directly proportional to MMF and MMF in turn is directly proportional to current. Thus, if we increase the current, flux Ø generated in the core will increase till the core saturates. Thus, there must be a point where from the flux does not increase in the same proportion as the increase in current. This point is called Knee Point.
Limiting voltage of CT saturation. Beyond this voltage, CT gets saturated, and breakdown occurs. CT lost its linearity and was damaged.