ABB RELION REX640 Technical Manual page 770

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