The knee point voltage(KPV) of the current transformer tells the information about the magnetization characteristics of the current transformer. According to IEC,the knee point voltage is defined as a point where 10% increase in the secondary voltage cause an increase in the secondary current by 50%. The current transformer gets saturated at this point.
The calculation of the KPV is very important for the differential protection of the transformer, generator and the motor.
Beyond the knee point voltage the CT gets saturated and the CT is no more functional.The excitation current only heats up the core of the CT and eventually the core insulation ia apt to get failed in this situation.
The protection relay will not operate if the CT gets saturated.Therefore, the KPV of the CT must be checked before installing the same in the electrical network.
The KPV of the CT can be calculated using the following formula.
Vkp = K * If/CTR * (RCT + RL + RR)