Technical description
2 Protection functions
2.3 General features of protection
stages
VM59.EN001
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47
relay plus the operating time of the outgoing feeder circuit
breaker.
Figure 2-1 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 2-2 shows an example of reset time i.e. release delay,
when the relay is clearing an overcurrent fault. When the
relay'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. 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 9.2.
For most stages it is less than 95 ms.
Figure 2-2 Reset time is the time it takes the trip or start relay contacts to
open after the fault has been cleared.
Hysteresis or dead band
When comparing a measured value against a pick-up value,
some amount of hysteresis is needed to avoid oscillation near
equilibrium situation. With zero hysteresis any noise in the
measured signal or any noise in the measurement itself would
cause unwanted oscillation between fault-on and fault-off
situations.
