ABB Relion 611 Series, Technical Manual

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GUID-BF32C1D4-ECB5-4E96-A27A-05C637D32C86 V2 EN
Figure 55: Examples of network rotating direction
4.1.2.7 Application
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.
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.
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.
1MRS757454 D
Section 4
Protection functions
611 series 121
Technical Manual