What is meant by knee point voltage of CT?

Knee point voltage is calculated for electrical Current transformers and is very important factor to choose a CT. It is the voltage at which a CT gets saturated.(CT-current transformer).

The knee point is defined as the voltage at which a 10% increase in appliedvoltage increases the magnetizing current by 50%. For voltages greater than theknee point, the magnetizing current increases considerably even for small increments in voltage across the secondary terminals.

Knee point is that point, in the voltage vs magnetizing current characteristics of a CT,beyond which the CT current changes drastically with respect to change of voltage.
Upto Knee point the change nature is linear in charateristics and beyond knee point the curve  looses its linearity property.