ABB RELION REX640 Technical Manual page 359

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Figure 216: Examples of network rotating direction
4.1.2.8 Application
DPHxPDOC 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 protection 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 protection 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 protection relays in a
closed ring system.
In some applications, the possibility of obtaining the selectivity can be improved
significantly if DPHxPDOC 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
protection 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, DPHxPDOC 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 DPHxPDOC 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.
REX640
Technical Manual
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