Protection Engineering Basics and Schemes Protection Engineering | Page 18
V k = ALF( I s R CT + VA
I S
) V k = ALF ∗ I s( R CT + R lead + R burden)
V k = knee point Voltage ALF = Accuracy limit factor I s = I n = Nominal Secondary Current R CT = Current transformer Secondary Resistance
VA = Burden R lead = Lead Resistance which is very less And sometimes not considered in calculation
Always remember the knee point of metering CT is always less then Protection CT.
Before commissioning of any current test, one of the mandatory tests is to check the knee point voltage of current transformer.
For this purpose we required
� CT( whose knee point need to be check, manufacturer always mention the knee point voltage which we need to check in this test) � Voltmeter � Ammeter � 250v output supply-VARIAC( 1Phase)