Theoretical Fundamentals and Implementation of Novel Self-Adaptive Distance Protection Resistant to Power Swings

To deal with the maloperation of distance protection in the case of power swings, the criterion of “concentric circle” was widely used. This criterion is implemented based on the time difference that the apparent impedance locus passes through boundaries of these two impedance circles. In many cases (e.g., a very fast power swing), it will fail to block. Actually, with regard to different moving loci, the time difference staying within the gap between two circles and the corresponding time staying within the internal operating circle have an inherent relationship. In this paper, the aforementioned relationship is deliberately assumed and demonstrated. The worst condition for the correct action of this criterion is presumed and proven, leading to the invention of novel distance protection which can operate rapidly during power swings. The necessary issues on the implementation of the proposed criterion are discussed. Based on the result of the discussion, complete self-adaptive distance protection is put forward. On the basis of the aforementioned analysis, the availability and the feasibility of this novel scheme are verified with the results of EMTDC simulations.