ABB RELION 620 Series Technical Manual page 504

Section 4
Protection functions
498
overcurrent-based schemes. In these kinds of networks, 21P is used to provide a fast and
selective protection for overhead lines and power cables. It can also be applied for radial
feeders to increase the sensitivity of the protection, especially if the short circuit power of
the source is low or if it is changing due to network operation.
From the selectivity point of view, it is advantageous that in the protection chain all
functions in different positions trip according to the same measuring principle. Therefore,
21P can also be applied for the backup protection of main transformers and buses. This
way, the selectivity with the distance protection of the outgoing lines is easier to achieve.
21P is suitable as a basic protection function against two-phase and three-phase faults in
all kinds of networks, regardless of the position of the neutral point.
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GUID-F623574D-4B5F-4DCA-8DFD-AE0D43FD1E5A V1 EN
Figure 238: Typical network configurations for 21P
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1MAC504801-IB E
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620 series ANSI
Technical Manual