ABB REF615 E - Page 449

Example 2a
n
n
m
u
s
GUID-787D9DE6-961E-454A-B97A-FAFC6F9701F0 V1 EN
Figure 237: Restricted earth-fault protection of a generator
In the protected generator:
S
n
= 8 MVA
U
n
= 6 kV.
I
n
= 770 A
I
kmax
= 6 · I
n
= 6 · 770 A = 4620 A
In this example, the CT type is KOFD 12 A 21 with:
I
CT_1n
= 1000 A (value given by the manufacturer).
I
CT_2n
= 1 A (value given by the manufacturer).
U
k
= 323 V (value given by the manufacturer).
R
in
= 15.3 Ω (value given by the manufacturer).
I
e
= 0.012 A (value given by the manufacturer).
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
2
:
R
m
= 7.28 Ω/km · 2 · 0.1 km ≈ 1.46 Ω
1YHT530004D05 D Section 4
Protection functions
615 series 443
Technical Manual