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Technical description
2 Protection functions
2.2 General features of protection
stages
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Reset time (release time)
Figure 2.2-2 shows an example of reset time i.e. release delay,
when the relay is clearing an overvoltage fault. When the
relay's trip contacts are closed the circuit breaker (CB) starts to
open. After the CB contacts are open current will still flow
through opened contacts through arc. The voltage is finally cut
off when the arc extinguishes at the next zero crossing of the
fault current. This is the start moment of the reset delay. After
the reset delay the trip contacts and start contact are opened.
Reset time varies from fault to fault depending on the fault
type. 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 2.2-2. 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.
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