kpv = RCT + RLEADS + RRELAY
Connect the circuit as shown. O/P of variac should be zero. Increase it to 5 Volts and take down the value of current from the ammeter. Now increase the voltage by 10% (5 + 10% = 5.5 V) and take the current reading. Now increase the voltage by 10% (5.5 V +0.55 V =6.05 V) and note down the current. Now keep on increasing voltage by 10% and note down current reading. At some value there will be 50% increase in current for 10% increase in voltage.
Example 40 V→ 0.2 A
40.4 V→ 0.3 A (0.2 + 50% = 0.3 A).
That point is the knee point voltage of that particular CT. From this point onwards a little increase in voltage will lead to a large increase in current, because the core is saturated fully. When we plot all the values on a graph taking current as X-axis and voltage as Y-axis, we will get the above graph. Protective relays operate between ankle point and knee point. Above this they cannot detect the fault correctly. Measuring CT operate in the ankle region.
