ABB REG316 4 Series Operating Instructions Manual page 94

ABB Switzerland Ltd
E. Setting instructions
Basic setting
Pick-up ratio
The purpose of the generator differential protection is to detect
phase faults in the stator zone. The protection is sensitive, fast
and absolutely discriminative.
Basic setting g
The basic setting g defines the pick-up of the differential protec-
tion for internal faults. It is the section of the operating charac-
teristic with a low restraint current I
The lowest possible value should be chosen for "g" (high sensi-
tivity) to enable it to detect the worst case faults, e.g. when exci-
tation is low. The protection cannot detect interturn faults on the
same winding, because they do not produce a differential cur-
rent.
Since, however, a small differential current flows during normal
operation, false tripping could result if "g" is set too low. The spu-
rious differential current is usually due to imbalances of c.t. er-
rors and c.t. burdens.
Allowing for an unwanted differential current, a typical setting is
0.1 I . Higher values have to be set for "g" should, for example,
N
the c.t's on opposite sides of the protected unit have different
accuracy classes or their burdens be too high.
The level of primary current at which the protection picks up de-
pends on the relay settings and the ratio of the c.t's. Assuming
that there is no compensation of the A/D channels by reference
value settings, it is calculated as follows:
Relay setting
(where I is the relay rated current)
N
Generator rated current
C.t. rated current
Calculated primary pick-up current
(referred to the generator rated current):
g I 5000
N 1
= =
i x 0 1 .
I I 4000
N GN
3-56
REG 316*4 1MRB520049-Uen / Rev. F
.
H
g = 0.1 I
I
GN
I
N1
=
0 125 .
g-Setting
v-Setting
N
= 4000 A
= 5000 A