ABB RELION REX640 Technical Manual page 769

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