ABB RELION REX640, Техническое руководство, Страница: 770 / 1960

I
n
= 770 A
I
kmax
= 6 · I
n
= 6 · 770 A = 4620 A
In this example, the CT type is KOFD 12 A 21 with:
I
CT_1n
= 1000 A (value given by the manufacturer).
I
CT_2n
= 1 A (value given by the manufacturer).
U
k
= 323 V (value given by the manufacturer).
R
in
= 15.3 Ω (value given by the manufacturer).
I
e
= 0.012 A (value given by the manufacturer).
If the length of the secondary circuit is 100 m (the whole loop is 200 m) and
the area of the cross section is 2.5 mm
2
:
R
m
= 7.28 Ω/km · 2 · 0.1 km ≈ 1.46 Ω
The required knee-point voltage can be calculated using equation
U
k
= 2 · ( 4620 A / 1000 ) · ( 15.3 + 1.46 ) ≈ 155 V.
The value 155 V is lower than the value 323 V, which means that the value of U
k
is high enough.
As mentioned earlier, I
m
= 0.5 · I
e
gives a realistic value for I
prim
in
Equation
185
. If I
u
= 0 and I
rs
= m · 0.5 · I
e
, the value for the sensitivity is:
I
prim
= n · m · I
e
= 1000 · 4 · 0.012 A = 48 A ( ≈ 6 % x I
n
).
I
rs
= 4 · 0.5 · 0.012 A = 0.024 A.
The setting value can be calculated with:
Operate value
I
I
A
A
rs
CT n
=





 =





 ≈
_
.
. %
2
0 024
1
2 4
The resistance of the stabilizing resistor can now be calculated:
R
s
= U
s
/ I
rs
= 78 V / (2 · I
e
) = 78 V / (2 · 0.012 A) = 3250 Ω.
Example 2b
In this example, I
rs
= 4 x 12 mA = 48 mA and I
u
= 30 mA. This results in the
sensitivity:
I
prim
= n · ( I
rs
+ I
u
+ m · I
m
) = 1000 · (48 + 30 + 24) mA = 102 A
The setting value can be calculated with:
Operate value
I
I
A
A
rs
CT n
=





 =





 ≈
_
.
. %
2
0 048
1
4 8
The resistance of the stabilizing resistor is now:
R
s
= U
s
/ I
rs
= 78 V / 48 mA ≈ 1630 Ω
In this example, the relay is of such a type that the stabilizing resistor can be
chosen freely.
Protection functions 1MRS759142 F
770 REX640
Technical Manual