Distance Element Performance Under Conditions of CT Saturation

Distance elements are widely used on transmission, subtransmission, and even distribution systems. Distance-based schemes are also a very economical and simple approach to protecting these lines. The ease of use of distance elements, especially in modern digital relays, has expanded the application to shorter and shorter lines. In the past it was unlikely that a distance-based scheme would be subject to CT saturation for remote fault or faults near the Zone 1 reach point. For the most part, CT saturation would occur for faults very near the relay terminal and the CT saturation would have very little impact on distance relay operation. This paper studies the impact of CT saturation on distance-based schemes. Saturation of the CT is evaluated for faults near to the relay location and at the reach setting. The impact to reach (underreach or overreach) and operating time are both evaluated. In addition, the behavior of directional elements is also studied. Recommendations are provided for CT sizing requirements.