This paper focuses on the uses of negative-sequence quantities in protective relaying. The emphasis is on numerical relays since they have facilitated the calculation of symmetrical components. Negative-sequence quantities (the voltage and current denoted by V2 and 12) are very useful quantities in protective relaying. The simplicity in the calculation of these quantities in modern numerical relays has reinforced their use in the theory and methods used by current protective relaying devices. The paper begins with discussion of some implementations of negative-sequence filters in older relays. Next is a brief review of symmetrical components and an analysis of unbalanced faults in power systems. This review leads to a discussion of the characteristics of negative-sequence quantities and illustrations of how these quantities are used in protective relaying. Because the discussion generally involves symmetrical component theory, the paper makes references throughout to the other two symmetrical components (positive-sequence and zero-sequence).
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