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Important Clarification: CT Knee-point and excitation current


Staff member
With reference to the question posted by @Brandon Baumberger on CT Equivalent Circuit Part 1b (Dated: 19th Sep 2018).

We wanted to correct a statement made in Equivalent circuit part 1b and provide more clarification. It was initially stated between 00:07:30 to 00:07:36 that

After crossing the knee point voltage, the excitation current becomes stabilized.
However, the correct word in this case is unpredictable, since the behaviour of the CT becomes non-linear after crossing the knee point voltage due to saturation of the core.

The linear relation between the excitation current and the secondary voltage ceases to exist beyond the knee point. A 10 percent increase in the secondary voltage causes a huge increase in the excitation current. In addition to this, it causes the ratio error to exceed the rated limits (e.g 5% or 10%).

The excitation current and the terminal voltage don't stabilize after the knee point voltage, and the CT starts to exhibit abnormal behaviour. Therefore, good engineering practice is to always stay below the knee point voltage to avoid CT malfunction caused by saturation.