High Impedance Differential CT

[charz]

If one of the CT is fully saturated for an external fault condition, there would be difference in the currents from the two CT's and if the difference is more than the Pickup, the differential relay trips, as in the following picture,

I read in a literature that, "The knee point voltage (Vk) for class PX CT's used in high impedance scheme is calculated for the worst condition that one of the CT's is fully saturated and the other CT has to develop enough voltage to drive current through the other CT circuit to ensure stability during external fault"

Why the other CT has to develop enough voltage to drive the current through the other CT?

 

[cranky108]

Why would you apply a high impedance differential on a transformer this way?

 

[HamburgerHelper]

I think you are looking at a picture for a low impedance relay. Below is a picture for a high impedance relay.

In a high impedance scheme, once one of the CTs saturate, the current is going to flow through that CT instead of the high impedance ,that you are measuring the voltage off of to determine if you should trip. The measured tripping voltage than drops due to the ease the current flows through the saturated CT. When a CT saturates, it basically decouples the primary and secondary and acts like an open air inductor.