What is the Use Of Knee Point Voltage?

Knee point voltage

As per IEC, the knee point voltage is defined as the voltage at which a 10% increase in applied voltage increases the magnetizing current by 50%. For voltages greater than the knee point voltage, the magnetizing current increases considerably even for small increments in voltage across the secondary terminals.IEEE defines saturation as “the point where the tangent is at 45 degrees to the secondary exciting amperes”. Also known as “knee” point.

Knee point voltage shows the saturation limit of the current transformer. When CT goes into its saturation region, the core of the CT is no longer able to confine the magnetic flux in the core. Under saturation, the CT behaves as an open circuit and its output is zero. Therefore, the protection class CT must operate in its non-saturation region. To ensure the operation of the CT in the non-saturation region, we need to calculate the maximum allowable secondary winding voltage limit up to which CT remains in its non-saturation region.

Graphical presentation of knee point voltage

CT magnetization curve

Knee point voltage testing procedure

The secondary of the current transformer is connected to an alternating voltage source(AC) and the primary of CT is left open.  The KPV testing setup is given below.

As the secondary voltage is increased keeping the primary of CT open, the current increases in the secondary with an increase in voltage. At a particular voltage, the secondary current increases drastically, and that point is corresponding to the KPV of CT.

The secondary voltage is increased in the steps of 10 %  and the corresponding current is measured. The current and voltage is noted. The point where the 10 % increase in the voltage cause 50 % increase in the current is the knee point voltage of CT.

The  CT testing current and voltage are as tabulated below.

Secondary Voltage (Volt)Secondary Current (Ampere)
30.001
7.50.002
12.50.003
180.004
600.01
1500.02
2000.025
2350.03
3000.05
3560.08
3720.01
4000.15 ( CT Saturation Point)
4471.0

Significance of Knee Point Voltage

Knee point voltage parameter is very important for Protection class current transformers especially for Class-PS that is Special purpose current transformers used for the zone protection. The current transformer used for protection and differential protection has to carry 20 to 30 times of CT-rated current during fault conditions.

If the CT saturates under fault conditions the protection relay does not trip the breaker and it may cause heavy damage to an electrical network. Knee point Voltage is very important in view of instrument safety factor as well. The KPV of CT must be known for the protection class CT and PS((PS class- Protection Special) class CT. only.

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