What is CT knee point test?

The “Knee” point is determined by observing a small voltage increase causing a large increase in current. Also known as “knee” point. This test verifies that the CT is of correct accuracy rating, has no shorted turns in the CT and no short circuits are present in the primary or secondary windings of the CT under test.

How is knee point of CT calculated?

Easy understanding of Knee Point voltage: Consider transformer Voltage transformation ratio Es= 4.44 * flux * Current * Number of conductor. From this equation, the flux transfer to the core is directly proportional to the applied voltage and the current is directly proportional to the mmf (ampere turns.).

What is CT saturation point?

CT Saturation When the primary current is so high that the core cannot handle any more flux, the CT is said to be in saturation. In saturation, there is no flux change when the primary current changes (as the core is already carrying maximum flux). Since there is no flux change there is no secondary current flow.

How do you test for CT and PT?

Testing and Commissioning of Current Transformer

1. Mechanical Check and Visual Inspection.
2. Insulation Resistance Test.
3. Polarity Test.
4. Secondary/Loop Resistance Test.
5. Burden Test (optional test)
6. Magnetization Curve Test (optional test)
7. Turns Ratio Test (optional test)
8. Primary Injection Test.

What is the knee point voltage on a Protection CT?

Every Protection class CT Name plate contains details of Knee point voltage as shown in below figure. The name plate is for 3-Core CT with middle taps. For this CT the knee point voltage for PS-Class core is V k >= 120V for 3S1-3S3 winding and V k >=60V for 3S1-3S2 winding.

How is the knee point of an excitation test determined?

Excitation tests are performed by applying an AC voltage to the secondary winding of the CT and increasing the voltage in steps until the CT is in saturation. The “Knee” point is determined by observing a small voltage increase causing a large increase in current. The test voltage is slowly decreased to zero to de‐magnetize the CT.

What is the knee point test for a current transformer?

Knee Point has its own significance for protection current transformers especially Class-PS current transformers. If CT Knee point changed for any reason there are severe effects occur like CT Saturation. Hence CT can’t sense the fault currents results relays won’t operate and huge damage occurs to switchgear panels.

Can a CT scan detect a knee infection?

None of the 31 patients with -iaCT had a knee infection at a mean follow-up of 291.0 ± 548.1 days. Based on these results, the sensitivity and specificity of the CT scan to detect +TAK and pw = (-TAK) was 100%.