Section 4 1MAC309294-MB F
Protection functions
114 RER620
Technical Manual
Figure 51: Examples of network rotating direction
4.1.2.7 Application
67/51P and 67/50P is used as short-circuit protection in three-phase distribution or sub
transmission networks operating at 50 or 60 Hz.
In radial networks, phase overcurrent relays are often sufficient for the short circuit
protection of lines, transformers and other equipment. The current-time characteristic
should be chosen according to the common practice in the network. It is recommended to
use the same current-time characteristic for all overcurrent relays in the network. This
includes the overcurrent protection of transformers and other equipment.
The phase overcurrent protection can also be used in closed ring systems as short circuit
protection. Because the setting of a phase overcurrent protection system in closed ring
networks can be complicated, a large number of fault current calculations are needed.
There are situations with no possibility to have the selectivity with a protection system
based on overcurrent relays in a closed ring system.
In some applications, the possibility of obtaining the selectivity can be improved
significantly if 67/51P and 67/50P is used. This can also be done in the closed ring
networks and radial networks with the generation connected to the remote in the system
thus giving fault current infeed in reverse direction. Directional overcurrent relays are also
used to have a selective protection scheme, for example in case of parallel distribution
lines or power transformers fed by the same single source. In ring connected supply
feeders between substations or feeders with two feeding sources, 67/51P and 67/50P is
also used.
Parallel lines or transformers
When the lines are connected in parallel and if a fault occurs in one of the lines, it is
practical to have 67/51P and 67/50P to detect the direction of the fault. Otherwise, there is
a risk that the fault situation in one part of the feeding system can de-energize the whole
system connected to the LV side.
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