ABB RELION REX640 Technical Manual page 768

Protection functions
4.3.5.9 Setting examples
Example 1
Figure 436: Restricted earth-fault protection of a transformer
The data for the protected power transformer are:
S = 20 MVA
n
U
2n
The longest distance of the secondary circuit is 50 m (the whole loop is 100
m) and the area of the cross section is 10 mm
I = S
n
I
kmax
In this example, the CT type is IHBF 12, the core size is 35 percent, the
primary current is 1200 A and the secondary current is 5 A.
R
in
U = 40 V (value given by the manufacturer).
k
I = 0.055 A (value given by the manufacturer).
e
R
m
U
=
s
According to the criterion, the value of U
V. It depends on if the stability of the scheme is achieved with U
Otherwise, it is possible to choose a bigger core of 65 percent with:
R
in
U = 81 V (value given by the manufacturer).
k
R
m
768
n
n2
= 11 kV
/ (√3 · U ) = 1050 A
n n
= 12 · I = 12600 A
n
= 0.26 Ω (value given by the manufacturer).
= 1.81 Ω/km · 2 · 0.05 km = 0.181 Ω ≈ 0.18 Ω
12600 ( . 0 26 0 18 . )
× +
V
≈
240
= 0.47 Ω (value given by the manufacturer).
= 0.18 Ω
m
u
s
2
.
23 V
should be 2 · U
k
1MRS759142 F
= 2 · 23 V = 46
s
= 40 V.
k
REX640
Technical Manual