ABB RELION REX640 - Page 816

R
m
m
m
=
Ω
⋅ ≈0 00865 200 1 73. . Ω
(Equation 222)
First the stabilizing voltage is calculated based on equation
U
A
V
s
=
⋅
⋅ +
( )
≈
6 770
1000
15 3 1 73 78 7. . .Ω Ω
(Equation 223)
In this case the requirement for the current transformer knee point voltage is
fulfilled because U
kn
> 2U
s
.
The magnetising curve of the CT is assumed to be linear. The magnetizing current at
the stabilizing voltage can be estimated.
I
U
U
I
m
s
kn
e
= ⋅
(Equation 224)
I
V
V
mA mA
m
= ⋅ ≈
78 7
323
35 8 5
.
.
(Equation 225)
The setting current I
rs
should be at the minimum of the sum of the magnetizing
currents of all connected CTs to obtain adequate protection stability.
I mA mA
rs
= ⋅ ≈2 8 5 17.
(Equation 226)
The resistance of the stabilizing resistor is calculated based on equation
R
V
A
s
= ≈
78 7
0 017
4629
.
.
Ω
(Equation 227)
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
V
V
mA
rs
= ≈
78 7
3900
20
.
(Equation 228)
The sensitivity of the protection is obtained as per equation (assuming I
u
to be zero)
Protection functions
1MRS759142 F
816 REX640
Technical Manual