ABB REG316 4 Series Operating Instructions Manual page 163

REG 316*4 1MRB520049-Uen / Rev. F
The value of the grounding resistor R
that:
· the maximum E/F current I
· the offset of the generator star-point for an E/F on the HV
side of the step-up transformer does not exceed half the relay
setting.
The star-point v.t. is designed in relation to the maximum con-
tinuous voltage resulting from an E/F, i.e. the phase-to-neutral
voltage of the generator. Arcing faults can cause higher transient
voltages and consequentially saturation of the v.t. The
specification of a relatively high overvoltage factor such as 1.9 is
therefore recommended.
V.t. rated voltages
U
=
U GN (U
1 n
3
U = 100 V (should nothing else be specified)
2n
The minimum value of the resistor R
U
³
R GN
E min
3 I
E max
£ 20 A
where I
Emax
The equation for determining the maximum value of the ground-
ing resistor R
Emax
simplified circuit diagram of
0 05 . U
£
R
E max
w
6 C U
12
where:
0.05 : protection sensitivity 5 % (95 % scheme)
6 : factor corresponding to 3 phases times 2 for half the
pick-up setting
The value of the effective grounding resistor R
tween R and R
Emax
E
£ 20 A
E
= generator rated voltage)
GN
Emin
(95 % scheme) can be derived from the
Fig. 3.5.11.3b:
GN
HV
and rated for 10 s.
Emin
ABB Switzerland Ltd
should be chosen such
:
is chosen be-
E
3-125