4.3.4.8
Setting examples
Example 1
n
n2
m
u
s
Figure 300: Restricted earth-fault protection of a transformer
The data for the protected power transformer are:
S
n
= 20 MVA
U
2n
= 11 kV
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
2
.
I
n
= S
n
/ (√3 · U
n
) = 1050 A
I
kmax
= 12 · I
n
= 12600 A
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
= 0.26 Ω (value given by the manufacturer).
U
k
= 40 V (value given by the manufacturer).
I
e
= 0.055 A (value given by the manufacturer).
R
m
= 1.81 Ω/km · 2 · 0.05 km = 0.181 Ω ≈ 0.18 Ω
U
V
V
s
=
×
+
≈
12600
0 26
0 18
240
23
( .
.
)
According to the criterion, the value of U
k
should be 2 · U
s
= 2 · 23 V = 46
V. It depends on if the stability of the scheme is achieved with U
k
= 40 V.
Otherwise, it is possible to choose a bigger core of 65 percent with:
R
in
= 0.47 Ω (value given by the manufacturer).
U
k
= 81 V (value given by the manufacturer).
R
m
= 0.18 Ω
Protection functions
1MRS757644 H
578
620 series
Technical Manual
