REG 316*4 1MRB520049-Uen / Rev. F ABB Switzerland Ltd
3-125
The value of the grounding resistor R
E
should be chosen such
that:
· the maximum E/F current I
E
£ 20 A
· 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
n
GN
1
3
= (U
GN
= generator rated voltage)
U
2n
= 100 V (should nothing else be specified)
The minimum value of the resistor R
Emin
:
R
U
I
E
GN
E
min
ma
³
3
where I
Emax
£ 20 A
The equation for determining the maximum value of the ground-
ing resistor R
Emax
(95 % scheme) can be derived from the
simplified circuit diagram of Fig. 3.5.11.3b:
R
U
CU
E
GN
HV
max
.
£
005
6
12
w
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
E
is chosen be-
tween R
Emax
and R
Emin
and rated for 10 s.
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