Rockwell Automation 865 Differential Protection Relay - Manual (Page 49)

Protection Functions 3-5
865-UM001A-EN-P – July 2009
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.
> PICK UP
PICK UP LEVEL
hysteresis
Hysteresis_GT
Figure 3.3 – Behavior of a greater than comparator. For example in
overcurrent and overvoltage stages the hysteresis (dead band) acts according
this figure.
< PICK UP
PICK UP LEVEL
hysteresis
Hysteresis_LT
Figure 3.4 – Behavior of a less than comparator. For example in under-voltage
and under frequency stages the hysteresis (dead band) acts according this
figure.