What is Knee Point voltage?
Current transformers (CTs) are widely used in power systems for measuring and protection purposes. CTs work on the principle of electromagnetic induction and convert high currents into proportional low currents, which instruments can safely measure. However, CTs can saturate under certain conditions, leading to inaccurate measurements and potentially causing damage to the power system.
The knee point voltage (KPV) is a critical parameter in protection class current transformers (CTs) that is used to indicate the saturation limit of the CT. When the CT reaches its saturation region, the core of the CT is no longer able to confine the magnetic flux in the core.
At this point, the CT behaves as an open circuit, and its output becomes zero. Therefore, it is crucial to ensure that the CT operates in its non-saturation region to protect the electrical network from heavy damage during fault conditions.
To ensure the operation of the CT in the non-saturation region, it is necessary to calculate the maximum allowable secondary winding voltage limit up to which the CT remains in its non-saturation region. The KPV of the CT shows the point where the magnetizing current increases considerably even for small increments in voltage across the secondary terminals.
As per IEC, the KPV is defined as the voltage at which a 10% increase in applied voltage increases the magnetizing current by 50%.
What is the testing procedure for Knee point voltage?
The KPV is determined by performing a knee-point voltage testing procedure. In this procedure, the secondary of the CT is connected to an alternating voltage source, and the primary of CT is left open. 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 corresponds to the KPV of CT. The secondary voltage is increased in steps of 10%, and the corresponding current is measured. The point where the 10% increase in voltage causes a 50% increase in current is the KPV of the CT.
What is the significance Of Knee Point Voltage?
The KPV is of great significance for protection class CTs, especially for Class-PS CTs, which are special purpose CTs used for zone protection. During fault conditions, the CT used for protection and differential protection must carry 20 to 30 times of the CT-rated current.
If the CT saturates under such conditions, the protection relay will not trip the breaker, and it may cause heavy damage to the electrical network. Therefore, the KPV must be known for protection class CTs and PS class CTs to ensure the safety of the instrument.
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