Conventional directional overcurrent and earth fault protection-not always straightforward

Many important aspects concerning the application and limitations of directional overcurrent protection have been addressed in early protection papers and in many textbooks. Even so, apparently undocumented limitations have still been encountered. This class of protection is often applied as back up and so some performance limitations may lie hidden until a case of main protection failure exposes incorrect back-up relay responses. Based on actual incidents and analysis of actual applications, this paper highlights three forms of protection limitation that do not appear to have received previous attention. In two cases, unwarranted utility supply interruptions occurred. In the third case, a common sub-transmission application, directional earth fault protection was seen to be ineffective for its intended purpose. In all cases, protection setting guidelines or possible relaying solutions are proposed, which would be attainable with modern numerical relay technology.