What is the "Knee Point" during CT testing?
Think of a CT like a towel soaking up water. At first, it absorbs water (voltage) easily. But eventually, it gets soaking wet (saturated) and can't hold any more. The Knee Point is the transition into that "full" state.
The Concept:
• Normal: Iron core is "empty." Voltage increase → small exciting current increase.
• Knee Point: The iron core is almost "full" of magnetism.
• Saturation: Core is "overflowing." Huge voltage increase → almost no more flux.
Standard Definition (IEC):
• The point where a 10% increase in voltage causes a 50% increase in exciting current.
Why it matters: If a CT saturates during a fault, it "lies" to the protection relay. The secondary current stops following the primary, and the relay won't trip.