Factors Which Influence the Behavior of Directional Relays

Very little material dealing with the factors (such as network dissymmetries) which lead to incorrect operation of directional relays has appeared in the technical literature. Practical experience has been the guide as to the relative merits of the 12 possible connections which utilize either line-to-neutral or line-to-line voltages and line currents or the differences of line currents to actuate the relay elements. In this article a graphical method for analyzing directional-relay operation is developed, utilizing circle diagrams and symmetrical components, and the method is applied to general cases to ascertain the effects of network dissymmetries. Since the relay volt-ampere expressions in terms of symmetrical components are too complicated to show the relative merits of the 12 connections, the expressions also are given in terms of the modified symmetrical components introduced by Edith Clarke and now widely used in network-analyzer studies of unbalanced faults. Circle diagrams based upon these modified components show conclusively that the ordinary 90-degree, 30-degree, and 60-degree connections are more free from disturbing influences than the other nine and therefore give more reliable operation in most practical applications of directional relays. The particular connection which will be most reliable at any given location can be determined by the methods given here when the constants of the power network are known.