TYPES OF GROUND RELAYS
There are three basic types of relays that are used for ground relaying; overcurrent relays, distance relays, and pilot relays. Ground relays are almost always completely independent of phase relays, even though any fault current, including ground fault current, will flow through one or more of the phase relays. The ground relays, however, can be provided with much greater sensitivity to the zero-sequence currents by using higher tap settings. This means that the ground relays will pick up much faster than phase relays for a fault involving the ground.
1) OVERCURRENT RELAYS:
Directional or non-directional overcurrent relays are widely used at most voltage levels because of their low cost and reliable service record. Many relay engineers prefer an overcurrent relay with an inverse or very inverse time-current characteristic. This means that the pickup will be very fast for close-in faults and delayed for faults at the end of the transmission line.
This delay makes coordination with adjacent lines relatively easy because of the rapid change in fault current for the more remote faults. The ground relay must coordinate with bus differential relays, as well as ground relays on any outgoing lines at the remote end.
In systems with multiple grounds, which is usually the case, the ground overcurrent relays will need to be directional relays. The ground relays in a looped system must be coordinated all around the loop in both directions, in exactly the same way that phase relays are coordinated. This is a cut-and-try process.
Instantaneous overcurrent relays are usually applied to supplement the ground fault protection when overcurrent relays are used. Instantaneous ground relays can reduce the fault clearing time to about one cycle in many cases, for faults on a large fraction of the line length.
2) DISTANCE RELAYS:
Directional ground distance relays are responsive to impedance or reactance between the relay and the fault. These relays, although more expensive than overcurrent relays, can provide almost instantaneous protection for most of the line length. For many years, distance relays were not widely used for ground protection due to the inherent problem of measuring zero-sequence impedance or reactance in the presence of a fault. Ground faults usually involve fault resistance of widely varying magnitude. This may prevent the relay from responding to a ground fault. Some relay engineers back up ground distance relays with overcurrent relays to make sure that all faults are recognized in a timely way. Many of the problems associated with ground distance relays have been solved by newer devices, making this a good alternative where overcurrent or directional overcurrent coordination is a problem.
3) PILOT RELAYS:
Pilot relaying is used for ground protection in special cases where the other methods are inadequate for reasons of security or dependability. Pilot relays use either a directional comparison or phase comparison to determine if the fault is within the protected zone. This might be a good solution for a three-terminal line, for example, where other types of relays are difficult to coordinate. Pilot relaying is sometimes selected on the more important lines because of the high speed and security offered by the pilot schemes. Some engineers argue that pilot relays are not required for ground fault protection, but should be used where stability or other considerations make it necessary to have both terminals of the transmission line tripped at the same time.
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