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 )