ABB RELION REX640 Technical Manual page 690

Section 4
Protection functions
684
Example 2a
n
n
GUID-58798798-AF49-4235-91A2-17B450382F7C V1 EN
Figure 378: Restricted earth-fault protection of a generator
In the protected generator:
S = 8 MVA
n
U = 6 kV.
n
I = 770 A
n
I = 6 · I = 6 · 770 A = 4620 A
kmax n
In this example, the CT type is KOFD 12 A 21 with:
I = 1000 A (value given by the manufacturer).
CT_1n
I = 1 A (value given by the manufacturer).
CT_2n
U = 323 V (value given by the manufacturer).
k
R = 15.3 Ω (value given by the manufacturer).
in
I = 0.012 A (value given by the manufacturer).
e
If the length of the secondary circuit is 100 m (the whole loop is 200 m) and the area
of the cross section is 2.5 mm
R = 7.28 Ω/km · 2 · 0.1 km ≈ 1.46 Ω
m
m
u
s
2
:
1MRS759142 C
REX640
Technical Manual