ABB RELION 620 Series Technical Manual page 280

Section 4
Protection functions
274
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 V2 EN
Figure 135: Overcurrent protection of parallel lines using directional protection
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.
GUID-74662396-1BAD-4AC2-ADB6-F4A8B3341860 V2 EN
Figure 136: Overcurrent protection of parallel operating transformers
Closed ring network topology
The closed ring network topology is used in applications where electricity distribution
for the consumers is secured during network fault situations. The power is fed at least
from two directions which means that the current direction can be varied. The time
grading between the network level stages is challenging without unnecessary delays
in the time settings. In this case, it is practical to use the directional overcurrent
protection relays to achieve a selective protection scheme. Directional overcurrent
functions can be used in closed ring applications. The arrows define the operating
1MRS757644 F
620 series
Technical Manual