Rockwell Automation 865 Differential Protection Relay - Manual (Page 47)

Protection Functions 3-3
865-UM001A-EN-P – July 2009
Blocking
Any protection function, except arc protection, can be blocked with
internal and external signals using the block matrix (Chapter 5).
Internal signals are for example logic outputs and start and trip
signals from other stages and external signals are for example digital
and virtual inputs.
Some protection stages have also inbuilt blocking functions. For
example under-frequency protection has inbuilt under-voltage
blocking to avoid tripping when the voltage is off.
When a protection stage is blocked, it won't pick-up in case of a fault
condition is detected. If blocking is activated during the operation
delay, the delay counting is frozen until the blocking goes off or the
pick-up reason, i.e. the fault condition, disappears. If the stage is
already tripping, the blocking has no effect.
Retardation Time
Retardation time is the time a protection relay needs to notice that a
fault has been cleared during the operation time delay. This parameter
is important when grading the operation time delay settings between
relays.
DELAY SETTING > t + t
FAULT RET
TRIP CONTACTS
t < 50 ms
RET
t
FAULT
RetardationTime
Figure 3.1 – Retardation Time