Rockwell Automation 865 Differential Protection Relay - Manual (Page 48)

3-4 Protection Functions
865-UM001A-EN-P – July 2009
Definition of 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 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 Allen-Bradley relays, the
retardation time is less than 50 ms.
Reset Time (Release Time)
Figure 3.2 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 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. For most stages it is less than 95 ms.
t
SET
t
CB
t
RESET
TRIP CONTACTS
ReleaseTime
Figure 3.2 – Reset time is the time it takes the trip or start relay contacts to
open after the fault has been cleared.