ABB RELION REX640 Technical Manual page 782

Protection functions
U =
s
In this case, the requirement for the current transformer knee point voltage is
fulfilled because U
The magnetizing curve of the CT is assumed to be linear. The magnetizing current at
the stabilizing voltage can be estimated as:
I
=
m
I
=
m
To obtain adequate protection stability, the setting current I
minimum of the sum of magnetizing currents of all connected CTs.
I
=
rs
The resistance of the stabilizing resistor is calculated based on
R =
S
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 =
rs
The sensitivity of the protection is obtained as per
I
prim
The power of the stabilizing resistor is calculated:
782
25000 A
( )
15 75 . Ω + 1 . Ω ≈
2000
> 2U
kn
U
s
I
⋅
e
U
kn
209 37 . V
7 mA 3 4 . mA
⋅ ≈
436 V
10 3 4 . mA 34 mA
⋅ ≈
209 37 . V
≈ Ω
6160
A
0 034 .
209 37 .
V
≈ 35 mA
5900 Ω
2000 0 035 ( . A 10 0 0034 .
= ⋅ + ⋅
209 37 . V
.
s
A 0 A ) 140 A
+ ≈
1MRS759142 F
(Equation 189)
(Equation 190)
(Equation 191)
must be at the
rs
(Equation 192)
Equation 193
(Equation 193)
(Equation 194)
Equation 195
, assuming I
(Equation 195)
Technical Manual
.
= 0.
u
REX640