ABB RELION 670 Series Applications Manual page 136

Section 6
Differential protection
Basic data:
Cable loop
resistance:
Max fault
current:
Calculation:
>
UR
EQUATION1213 V3 EN-US
Select a setting of
U>Trip =100 V.
The current transformer saturation voltage at 5% error can roughly be calculated from the
rated values.
The current transformer saturation voltage must be at least, twice the set operating voltage
U>Trip .
_ _
U CT Saturation
EQUATION1214 V2 EN-US
that is, greater than 2 ˣ
Check from the table of selected resistances the required series stabilizing resistor value to
use. Since this application it is not required to be so sensitive, select
ohm, which gives an IED current of 100 mA.
To calculate the sensitivity at operating voltage, refer to equation
acceptable value; in fact in this example the sensitivity is about 13% of the nominal current of
the CT:
= ×
IP n
EQUATION000020 V2 EN-US
Where
IR= 100mA
I = 15mA is the magnetizing current of the CT at the operating voltage
mag
I = 5 mA
res
130
2
<50 m 2.5mm (one way) gives 1 ˣ 0.42 ohm at 75° C.
Note! Only one way cable length is used as the power system
earthing in this example is limiting the earth-fault current to a
low level. If high earth-fault current exists use two way cable
length.
The maximum through fault current given by the transformer
reactance, for example, 28 kA.
28000
× + =
(5.5 0.42) 82.9
2000
> ( 10 5 5 + . ) × 20 310 =
V
U>Trip .
2000
( )
∑
+ + I =
IR I
res mag
1
is the current drawn by the IED circuit
is the current drawn by the non-linear resistor at the operating voltage
One way as the power system earthing in this
example is limiting the earth-fault current. If
the neutral solidly grounded, then a high-
earth-fault current exists, and a two way cable
length should be considered for the loop
resistance.
V
(
× 0 1 0 . ∠ ° + 0 005 0 . ∠ ° + × 4 0 015
1MRK 504 152-UEN B
(Equation 27)
(Equation 28)
SeriesResistor =1000
29 which gives an
)
. ∠ − 60 ; 1 1 70A °
U>Trip
U>Trip
Application manual
(Equation 29)