U
A
V
s
=
+
(
)
≈
25000
2000
15 75
1
209 37
.
.
.
Ω
Ω
(Equation 189)
In this case, the requirement for the current transformer knee point voltage is
fulfilled because U
kn
> 2U
s
.
The magnetizing curve of the CT is assumed to be linear. The magnetizing current at
the stabilizing voltage can be estimated as:
I
U
U
I
m
s
kn
e
=
⋅
(Equation 190)
I
V
V
mA
mA
m
=
⋅
≈
209 37
436
7
3 4
.
.
(Equation 191)
To obtain adequate protection stability, the setting current I
rs
must be at the
minimum of the sum of magnetizing currents of all connected CTs.
I
mA
mA
rs
=
⋅
≈
10 3 4
34
.
(Equation 192)
The resistance of the stabilizing resistor is calculated based on
.
R
V
A
S
=
≈
209 37
0 034
6160
.
.
Ω
(Equation 193)
The calculated value is the maximum value for the stabilizing resistor. If the value is
not available, the next available value below should be selected and the protection
relay setting current is tuned according to the selected resistor. For example, in this
case, the resistance value 5900 Ω is used.
I
V
mA
rs
=
≈
209 37
5900
35
.
Ω
(Equation 194)
The sensitivity of the protection is obtained as per
, assuming I
u
= 0.
I
A
A
A
A
prim
=
⋅
+
⋅
+
≈
2000 0 035
10 0 0034
0
140
( .
.
)
(Equation 195)
The power of the stabilizing resistor is calculated:
Protection functions
1MRS759142 F
782
REX640
Technical Manual