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 difference, it indicates an internal fault, and the relay trips. During an external fault, CT saturation can cause the CTs to produce inaccurate secondary currents. If one CT saturates more than the others, it will output a lower secondary current than expected. This difference in secondary currents is interpreted by the differential relay as an internal fault, even though the fault is external. This unwanted tripping is called a "false trip" or "maloperation." Several mitigation strategies are implemented in relay design to prevent false trips due to CT saturation. One common technique is to use a "percentage differential relay." This type of relay uses a percentage restraint characteristic. The relay only trips if the differential current (the difference between the CT secondary currents) exceeds a certain percentage of the through current (the average of the CT secondary currents). This percentage restraint makes the relay less sensitive to inaccuracies caused by CT saturation during high through-current conditions. Another method is to use "harmonic restraint." CT saturation often introduces harmonics into the secondary current waveform. Harmonic restraint circuits in the relay detect these harmonics and desensitize the relay to prevent tripping. CT selection is also crucial. Choosing CTs with higher voltage ratings, larger core cross-sectional areas, and lower burden can reduce the likelihood of saturation. Furthermore, implementing air-gapped CTs can also mitigate saturation issues. Finally, using numerical relays with advanced algorithms for CT saturation detection and compensation can further improve the reliability of differential protection.