ABB REF615 - Page 181

X
d
’ transient reactance of the generator of the power station.
If
x
d
' 0.15
S
n
5 MVA
U
n
10 kV
X
kG
2 Ω
the setting is then calculated:
X x
U
S
V
VA
d d
n
n
’ ’
.
( )
( )
= = × =
2
2
0 15
10000
5000000
3Ω
GUID-D691182A-75BD-4204-8C1D-8C5AC95333F2 V1 EN
U
X
X X
G
kG
kG d
1
0 5 0 6 0 5 0 6
2
2 3
0 74=
+
+ = +
+
=. . . . .
’
GUID-0BE5AFDB-2BFD-40BC-A7D7-1B3902E2B157 V1 EN
The actual setting should be slightly, for example, 10 percent higher than the value
calculated to allow some marginal for the fault resistance and other factors that may
make the U
1
value higher:
Voltage start value 1.1 × 0.74 = 0.81
1MRS756378 D Section 5
Protection functions
REF615 175
Application Manual