ABB Switzerland Ltd REG 316*4 1MRB520049-Uen / Rev. F
3-124
where:
C
12
capacitance between primary and secondary of the
step-up transformer
C capacitance to ground of the stator windings, the ca-
bles with protection capacitors and the LV step-up
transformer winding
U star-point offset
U
HV
rated voltage of the step-up transformer HV windings
I
E
E/F current
R
E
grounding resistor.
The value of the grounding resistor R
E
determines the E/F cur-
rent. In view of the damage an E/F current can cause —
especially to the laminations of the stator core — the maximum
E/F current should be limited to 20 A for 10 s, i.e. the grounding
resistor R
E
must not be too small.
Tripping by the E/F protection is delayed by 0.5 s to avoid any
risk of false tripping during transient phenomena.
Designing a scheme for connection to the generator star-
point
Alternative 1 with grounding resistor and v.t. (see Fig. 3.5.11.4):
HEST 905 031 C
I
E
I
Emax
HV
LV
12
3 C
R
E
3 C
12
GN
U
U
HV
I
E
Generator Step-up transformer
I
E
U /U
1n 2n
U>
Fig. 3.5.11.4 Stator E/F protection with a grounding resistor at
the star-point
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