ABSTRACT
The kind of protection employed and the amount of protection installed for protecting cables and overhead lines depend mainly on the importance of the particular feeder and the type of power system grounding. The primary system quantity used for detecting the direction of ground fault energy is the phase relationship of the neutral current. Ground fault overcurrent protection is only applicable to ungrounded radial systems and only then providing the ground fault current is sufficiently higher than the relay setting. Time-overcurrent protection is used as back-up protection for small power transformers with ratings in the range of a few tens of MVA or even as the main protection on very small transformers to detect both internal and external phase faults. A current comparison function is frequently added to a phase comparison function to increase the reliability of a busbar protection scheme which is then referred to as directional current comparison.
