ABB Relion 620 Series, Technical Manual

Page: 606 / 1224

R
m
m
m
=
Ω
⋅ ≈
0 00865 200 1 73 . .
Ω
(Equation 116)
First the stabilizing voltage is calculated based on
Equation 106
.
U
A
V
s
=
⋅
⋅ +
( )
≈
6 770
1000
15 3 1 73 78 7 . . . Ω Ω
(Equation 117)
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 118)
I
V
V
mA mA
m
= ⋅ ≈
78 7
323
35 8 5
.
.
(Equation 119)
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 120)
The resistance of the stabilizing resistor is calculated based on
Equation 108
.
R
V
A
s
= ≈
78 7
0 017
4629
.
.
Ω
(Equation 121)
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 122)
The sensitivity of the protection is obtained as per
Equation 110
, (assuming I
u
to be
zero)
Protection functions 1MRS757644 H
606 620 series
Technical Manual