How does CT saturation affect differential relay operation during external faults, and what mitigation strategies are implemented in relay design?

CT saturation occurs when a current transformer (CT) can no longer accurately reproduce the primary current on its secondary side. This typically happens during high-magnitude fault currents, especially those with a significant DC component. The CT's core becomes fully magnetized, and it can't further increase its magnetic flux to reflect the increasing primary current. Differential relays operate based on the principle that under normal conditions or during external faults (faults outside the protected zone), the current entering the protected zone should equal the current leaving it. CTs on either side of the protected equipment measure these currents, and the relay compares them. If there's a significant d....