ABB RELION REX640 - Page 769

U V V
s
=
× +
≈
12600 0 47 0 18
240
34
( . . )
U
k
= 2 · U
s
= 68 V (required value).
As mentioned earlier, I
m
= 0.5 · I
e
gives a realistic value for I
prim
in Equation
185 . If I
u
= 0 and I
rs
= m · 0.5 · I
o
, the value for the sensitivity is:
I
prim
= n · m · I
e
= 240 · 4 · 0.055 A ≈ 53 A
I
rs
= 4· 0.5 · 0.055 A = 0.11 A
The setting value can be calculated with:
Operate value
I
I
A
A
rs
CT n
=








=






≈
_
.
. %
2
0 11
5
2 2
The resistance of the stabilizing resistor can be calculated:
R
s
= U
s
/ I
rs
= 34 V / 0.11 A ≈ 309 Ω
However, the sensitivity can be calculated more accurately when the actual
values of I
u
and I
rs
are known. The stabilizing resistor of the relay is chosen
freely in the above example and it is assumed that the resistor value is not
fixed.
Example 2a
n
n
m
u
s
Figure 437: Restricted earth-fault protection of a generator
In the protected generator:
S
n
= 8 MVA
U
n
= 6 kV.
1MRS759142 F
Protection functions
REX640
Technical Manual
769