ABB REG316 4 Series Operating Instructions Manual page 168

ABB Switzerland Ltd
Designing a scheme for connection to the generator termi-
nals
If the generator star-point is inaccessible, the stator E/F protec-
tion is connected to three v.t's at the generator terminals (see
Fig. 3.5.11.6). In this case, the E/F current is fed by the three v.t.
primary windings. Assuming a permissible short-time current of
the primary windings of 5 A, the E/F current must be limited to a
maximum of 15 A.
The secondary rated voltage must be chosen such that it does
not exceed 300 V and, wherever possible, the secondary current
does not exceed 250 A.
Generator
U
GN
I 1 I
1
I E
U
1n
e I
3 C
Fig. 3.5.11.6 Stator E/F protection with grounding transformer
at the generator terminals
For an E/F at a generator terminal, the voltage of the phase
concerned becomes zero and the healthy phases are at phase-
to-phase potential with respect to ground. The vectorial addition
of the two phase-to-phase voltages produces three times the
rated voltage across the broken delta connection of the v.t. sec-
ondary windings:
U = 3 U
2n
where U is the rated secondary voltage. If U is greater than
2n
100 V, the E/F protection must be connected via an interposing
v.t.
3-130
REG 316*4 1MRB520049-Uen / Rev. F
Step-up transformer
LV
1
I E
1 I
I E = 3 I
1
/U U /U
2n 3n 4n
U>
R
e I
e
HV
2
U
HV
3 C
12
HEST 905 033 C