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The voltge at which CT core saturates or Excitation/Magnetising current exceed CT's design data ( Im) is called Knee Point voltage of a current transformer.
It is the significance of saturation level of a CT core mainly used for protection purposes. It is defined as the voltage at which 10 % increase in voltage of CT secondary results in 50 % increase in secondary current.
Knee Point Voltage of a Current Transformer is defined as the voltage at which 10 % increase in voltage of CT secondary results in 50 % increase in secondary current
We know that 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 the Faraday’s Law of Electromagnetic Induction, an emf is generated across the terminals of CT secondary by Transformer action. The emf induced in the CT secondary terminal is given as E = 4.44fNØ Where f is frequency of supply, N is 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 do not increase in the same proportion as the increase in current. This point is called Knee Point. After discussing this much, we can at least say that Knee Point is something related with Saturation of CT core. If there is something called saturation, then we must draw saturation curve of the CT core to have more insight.
