ABB REG316 4 Series Operating Instructions Manual page 162

ABB Switzerland Ltd
where:
C capacitance between primary and secondary of the
12
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 rated voltage of the step-up transformer HV windings
HV
I E/F current
E
R grounding resistor.
E
The value of the grounding resistor R
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 must not be too small.
E
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):
Generator
U
GN
I
E
I
Emax
I
E
U /U
1n 2n
3 C R
E
I
E
Fig. 3.5.11.4 Stator E/F protection with a grounding resistor at
the star-point
3-124
REG 316*4 1MRB520049-Uen / Rev. F
E
LV
1
U>
determines the E/F cur-
Step-up transformer
HV
2
U
HV
3 C
12
HEST 905 031 C