The primary function of protective devices is to remove faulted equipment from the electrical system. It is typically very advantageous for these devices to isolate as small of a section of the electrical system as possible. Overcurrent protection is the most common protection function used, as faulted equipment typically results in a large short circuit current. When overcurrent protection is employed, the overcurrent protection is typically coordinated with time so that the devices closest to the fault will operate and isolate only that section of equipment before devices farther from the fault operate and isolate larger pieces of the system. These time overcurrent devices must also be coordinated with damage curves for equipment such as buses and cables to clear the fault before the overcurrent can cause damage to the equipment. This concept of time coordination of overcurrent is well understood, and there are general guidelines applied to account for measurement error and other inaccuracies. The reduced signal levels provided to relays due to current transformer saturation is normally not considered during coordination studies. This reduced signal level will result in slower than desired operation of protective relays. Feeder relays could trip slower than upstream devices, isolating more of the power system than intended, or could allow primary equipment to be damaged before tripping. This paper will review the concept of coordinating time overcurrent relays, and then discuss the concept of loss of coordination associated with current transformer saturation. The paper will model current transformer saturation from actual installations using the IEEE PSRC CT Saturation Calculator tool. This model data will be used to estimate the performance of protective relays. The paper will discuss methods to improve coordination of overcurrent relays when faced with significant CT saturation.
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