High Impedance Protection

[charz]

In High Impedance protection, for external fault conditions, the secondary current from the CT's of the unfaulted phases circulate through the secondary of the saturated CT (of the faulted phase), as the unfaulted CT's see the saturated CT secondary as a low impedance than the relay combined with the stabilising resistor.However for internal faults, how does the secondary current from the CT's circulate through the relay, why don't it pass through the CT which has the low impedance among them?

 

[cranky108]

CT's do work in reverse. If you pass current through the secondary of a CT, it will create current on the primary.

Take a little time and let that sink in.

 

[stevenal]

For in-zone faults, all the CTs saturate as they try to drive current in parallel through the high impedance relay input. You don't need much current through the relay, just voltage above the trip setting. See IEEE C37.234.