ABB Relion REC670 Applications Manual page 104

Section 6
Differential protection
Setting example
Basic data:
Current transformer ratio:
CT Class:
Secondary resistance:
Cable loop resistance:
Max fault current:
Calculation:
UR >
EQUATION1216 V2 EN-US
Select a setting of
The current transformer saturation voltage must be at least, twice the set operating voltage
U>Trip .
U CT Saturation _ _
EQUATION1217 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 requires good sensitivity, select
gives an IED current of 100 mA.
To calculate the sensitivity at operating voltage, refer to equation 21, which gives an
acceptable value. A little lower sensitivity could be selected by using a lower resistance value.
100
IP =
5
EQUATION1218 V2 EN-US
The magnetizing current is taken from the magnetizing curve of the current transformer cores,
which should be available. The current value at
resistor current the peak value of voltage 30 ˣ √2 is used. Then the RMS current is calculated by
98
It is strongly recommended to use the highest tap of the CT whenever high
impedance protection is used. This helps in utilizing maximum CT capability,
minimize the secondary fault, thereby reducing the stability voltage limit.
Another factor is that during internal faults, the voltage developed across the
selected tap is limited by the non-linear resistor but in the unused taps, owing
to auto-transformer action, voltages much higher than design limits might be
induced.
100/5 A (Note: Must be the same at all locations)
10 VA 5P20
0.26 ohms
<50 m 2.5mm
Note! Only one way as the tertiary power system earthing is limiting
the earth-fault current. If high earth-fault current exists use two way
cable length.
The maximum through fault current is limited by the reactor
reactance and the inrush will be the worst for a reactor for example,
800 A.
800
× ( . 0 26 0 4 + . ) = 26 4
100 5 /
U>Trip =30 V.
10
 
> + 0 26 .  × 20 5 66 × =

25
 
U>Trip .
(
× 100 0 ° + 5 0 ° + × 2 100 60
2
(one way) gives 1 ˣ 0.4 ohm at 75° C
,
V
− 3
)
− ° × 10 ≤ approx A 5
U>Trip is taken. For the voltage dependent
1MRK 511 358-UEN A
(Equation 19)
(Equation 20)
SeriesResistor = 300 ohm, which
Application manual
(Equation 21)