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A novel reliability evaluation criterion, designated as consistent security, is proposed in this paper to compare the comprehensive performance between the current differential protections in various bias current designs. It is initially based on the investigations of the comparative analysis between the complex percentage differential criterion, which is a kind of compensated bias current design, and normal percentage differential criterion. A variety of fault scenarios is included in these comparisons (e.g., current-transformer (CT) saturation induced by a severe external fault resulting in false differential current, various internal fault scenarios leading to possible ratio, and phase differences of outgoing current to the incoming current). Based on investigations on the CT saturation due to external faults, a novel baseline, "consistent security", is thus put forward to evaluate the comprehensive performances of the differential protections implemented with different bias current designs. On the basis of consistent security, it can be proved that the complex differential criterion is better than normal differential criterion in most operation conditions.
Date of Publication: Jan. 2006