Q.What is the knee point voltage?
A. It is the point at where 50% of secondary current increases in order to increase 10% of voltage in secondary winding.
Q. Why it is important in only PS class? what about metering class and over current and earth fault protection (5P class) ?
A. for metering and 5P class, secondary circuit is small so the voltage drop across burden is small,it implies that voltage across secondary winding is small,then it will not saturate in normal case.But in PS(the class used only for differential protection purpose) class secondary burden is very high so the voltage across secondary winding is also high which may be divert our CT to saturate position in normal condition also,so generally by designing specially(secondary current only 1amps, as to minimize ohimic loss in secondary circuit). So generally manufacture provide knee point voltage maximum compared to other classes).
Q. What is burden?
A. The load connected to secondary winding (sum of relay impedance or meter impedance and control cable resistance and CT secondary winding resistance).
Q. why we are doing this test?
A. We are just checking the manufacture specification is correct or not.
knee point test at field
PROCEDURE:
1.Isolate the CT from all external connections.
2. Apply 10% of knee point voltage (provided on name plate details) to the secondary circuit by using auto transformer.
3. measure secondary current by using leakage current tester.
4.increase the voltage by 10% step by step and measure current as said above.
5. Repeat the steps up 100% of knee point voltage.
6. After knee point voltage , current will not increase as starting (proportionally).
7. Repeat the same procedure for different tapping..
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