GROUND FAULT PROTECTION AND ITS IMPORTANCE

An important aspect of transmission line protection is related to the fast detection and clearing of ground faults on transmission systems that have grounded neutrals. In the protection of transmission lines, ground faults are given special treatment. Ground faults are detected using different relays than those used for phase faults, although it is possible that phase relays may detect and properly clear ground faults. Ground relays, however, take advantage of unique features of the power system that make it possible to detect grounded conditions very quickly.

IMPORTANCE OF GROUND FAULT PROTECTION:

Most high-voltage and extra-high voltage transmission lines are grounded neutral transmission systems, either solidly grounded or grounded through a resistance or a reactance. It has been estimated that, on these high-voltage systems, over 90% of all transmission line faults are ground faults. It has been observed by one protection engineer that, on 500 kV transmission lines, one-line-to-ground faults "predominate to the extent that on many well designed circuits, no other type of fault has ever occurred, even after years of service". It may be noted that some faults involve phase-to-phase as well as ground short circuits, but the ground relays pick up these faults before the phase relays. On the system referenced, the ground relays are applied on the basis of two principles.

Install only those relays that are required to properly protect the line.
Provide redundancy in the form of two completely independent relay schemes at each line terminal.

The first principle refers to the dependability of the installed systems to properly perform correct detection and tripping to clear the fault, without unnecessary trips, and with all necessary speed. This means that the relays are not set to operate at the fastest possible speed, but as fast as is reasonably possible following detection and analysis of the observed system condition. The redundancy principle guards against the unobserved failure of one relay system by having a second system installed that is fully capable of performing the ground relaying function. These principles, or similar ones, are followed by many utilities.

Because of the high incidence of ground faults, it is important that transmission protection include a well-designed ground relaying system that embraces the two basic principles stated above.

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