Fast and Reliable CT Saturation Detection Using a Combined Method

Busbar differential relays may operate incorrectly for external faults due to current-transformer (CT) saturation. In this paper, two different techniques are suggested to detect CT saturation. The first proposed technique is based on the fact that the waveshape of the CT secondary current changes significantly at the instant of the saturation. Based on this feature, an algorithm which uses the second derivative of CT output current is developed. It uses an adaptive threshold to detect CT saturation fast. The second proposed technique utilizes two criteria based on the zero crossing principle. Utilizing the combination of the second derivative and zero crossing techniques results in a powerful and reliable scheme which is able to detect various CT saturation cases correctly and quite fast. The proposed method is able to detect even small CT saturation events. A real 400-kV busbar is simulated using PSCAD/EMTDC for evaluating the performance of the proposed algorithm. The obtained results demonstrate precise operation of the proposed algorithm in different conditions.