Enter measured relay voltage, current, and their angles. Select the fault loop and relay characteristic. Use primary divided by secondary. Protected transmission line length. Zero sequence impedance per kilometer. Primary. Distance relays are applied as short-circuit protection in almost all transmission systems where overcurrent relays cannot be used for reasons of selectivity, fault detection requirements or where there is a need for improved fault clearing times. There are six loops: A-B, B-C, C-A, A-G, B-G, and C-G. Distance relaying is directional and typically utilizes four zones of protection, each of which reaches a fixed distance and operates in a set. A straightforward way of obtaining selective protection is to use time grading. The underreaching directly tripping application (Zone 1) is the focus of the paper, but the overreaching (Zone 2) and blocking (reverse zone) applications are discussed too.
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