2.3 General features of protection
stages
2 Protection functions
Technical description
52
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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 2.3-1 shows an overcurrent 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.
Reset time (release time)
Figure 2.3-2 shows an example of reset time i.e. release delay,
when the device is clearing an overcurrent fault. When the
device's trip contacts are closed the circuit breaker (CB) starts
to open. After the CB contacts are open the fault current will
still flow through an arc between the opened contacts. The
current is finally cut off when the arc extinguishes at the next
zero crossing of the current. This is the start moment of the
reset delay. After the reset delay the trip contacts and start
contact are opened unless latching is configured. The reset time
varies from fault to fault depending on the fault size. After a
big fault the time is longer. The reset time also depends on the
specific protection stage. The maximum reset time for each
stage is specified in chapter 9.3. For most stages it is less than
95 ms.
Figure 2.3-2. Reset time is the time it takes the trip or start relay contacts
to open after the fault has been cleared.