ABB REF615 Applications Manual page 147

1MRS756378 D
REF615
Application Manual
In radial networks, the 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 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 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.
GUID-1A2BD0AD-B217-46F4-A6B4-6FC6E6256EB3 V1 EN
Figure 66: Overcurrent protection of parallel lines using directional relays
DPHxPDOC can be used for parallel operating transformer applications. In these
applications, there is a possibility that the fault current can also be fed from the LV-
side up to the HV-side. Therefore, the transformer is also equipped with directional
overcurrent protection.
Section 5
Protection functions
141