ABB RELION REX640 Technical Manual page 816

Protection functions
R
m
First the stabilizing voltage is calculated based on equation
U
=
s
In this case the requirement for the current transformer knee point voltage is
fulfilled because U
The magnetising curve of the CT is assumed to be linear. The magnetizing current at
the stabilizing voltage can be estimated.
I
=
m
I
=
m
The setting current I
currents of all connected CTs to obtain adequate protection stability.
I
= ⋅
rs
The resistance of the stabilizing resistor is calculated based on equation
R =
s
The calculated value 4629 Ω is the maximum value for the stabilizing resistor. If this
value is not available, the next available value downwards should be chosen and the
protection relay setting current is to be tuned according to the selected resistor. For
example in this case the resistance value 3900 Ω is used.
I
=
rs
The sensitivity of the protection is obtained as per equation (assuming I
816
Ω
m
= 0 00865 . ⋅ 200 ≈ 1 73 .
m
6 770 ⋅ A
(
15 3 . 1 73 .
⋅ Ω +
1000
> 2U
kn
U
s
I
⋅
e
U
kn
78 7 . V
35 mA 8 5 . mA
⋅ ≈
323 V
should be at the minimum of the sum of the magnetizing
rs
2 8 5 . mA 17 mA
≈
V
78 7 .
≈ 4629 Ω
A
0 017 .
78 7 . V
20 mA
≈
3900 V
Ω
)
78 7 . V
Ω ≈
.
s
1MRS759142 F
(Equation 222)
(Equation 223)
(Equation 224)
(Equation 225)
(Equation 226)
(Equation 227)
(Equation 228)
to be zero)
u
REX640
Technical Manual