ABB RELION 620 Series Technical Manual page 185

1MAC504801-IB E
620 series ANSI
Technical Manual
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.
GUID-E92DF9EA-D7AD-4866-B426-08533D5E2973-ANSI V1 EN
Figure 82: Overcurrent protection of parallel lines using directional protection relays
67/51P and 67/50P 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 4
Protection functions
179