ABB RER620 Technical Manual page 121

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RER620
Technical Manual
Figure 52: Overcurrent protection of parallel lines using directional 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.
Figure 53: 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 relays to achieve a
selective protection scheme. Directional overcurrent functions can be used in closed ring
applications. The arrows define the operating direction of the directional functionality.
The double arrows define the non-directional functionality where faults can be detected in
both directions.
Section 4
Protection functions
115