ABB REG316 4 Series Operating Instructions Manual page 95

REG 316*4 1MRB520049-Uen / Rev. F
Pick-up ratio v
The pick-up ratio "v" is decisive for the stability of the protection
during through-faults. This is the section of the operating charac-
teristic with restraint currents higher than 1.5 I
The value of "v" defines the pick-up current I
rent I in the moderately sloped region of the operating charac-
H
teristic. In the case of generator differential protection, "b" has a
fixed setting of 1.5 (compare this with
for transformer differential protection).
The "v" setting should be low enough to make the protection
sensitive to faults when load current is flowing, but high enough
to avoid false tripping during through-faults. A typical setting is
v = 0.25.
A higher setting (v = 0.5) is chosen in cases where the transient
behaviour of the c.t's during through-faults can cause large dif-
ferential currents. This is normally the result of under-sized c.t's
or widely differing c.t. burdens.
Typical settings:
g-Setting 0.1 I
v-Setting 0.25
Figures 3.5.1.2
N
ABB Switzerland Ltd
.
N
for a restrain cur-
D
and 3.5.1.3
3-57