2.2 General features of protection
stages
2 Protection functions
Technical description
46
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VM55.EN005
Figure 2.2-1. Definition for retardation time. If the delay setting would be
slightly shorter, an unselective trip might occur (the dash line pulse).
For example when there is a big fault in an outgoing feeder, it
might start i.e. pick-up both the incoming and outgoing feeder
relay. However the fault must be cleared by the outgoing feeder
relay and the incoming feeder relay must not trip. Although the
operating delay setting of the incoming feeder is more than at
the outgoing feeder, the incoming feeder might still trip, if the
operation time difference is not big enough. The difference
must be more than the retardation time of the incoming feeder
relay plus the operating time of the outgoing feeder circuit
breaker.
Figure 2.2-1 shows an overvoltage fault seen by the incoming
feeder, when the outgoing feeder does clear the fault. If the
operation delay setting would be slightly shorter or if the fault
duration would be slightly longer than in the figure, an
unselective trip might happen (the dashed 40 ms pulse in the
figure). In VAMP devices the retardation time is less than 50
ms.
