ABB Relion REC670 Applications Manual page 102

Section 6
Differential protection
Setting example
Basic data:
Current transformer ratio:
CT Class:
Secondary resistance:
Cable loop resistance:
Max fault current:
Calculation:
EQUATION1207 V2 EN-US
Select a setting of
The current transformer saturation voltage must be at least twice the set operating voltage
U>Trip .
E P 5
EQUATION1208 V1 EN-US
that is, bigger than 2 ˣ
Check from the table of selected resistances the required series stabilizing resistor value to
use. As this application requires to be so sensitive select
gives an IED operating current of 100 mA.
Calculate the primary sensitivity at operating voltage using the following equation.
2000
IP =
EQUATION1209 V2 EN-US
where
100 mA is the current drawn by the IED circuit and
10 mA
20 mA
96
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 current, 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 induced may be much higher than
design limits.
2000/1 A
20 VA 5P20
6.2 ohms
<100 m 2.5 mm
Equal to switchgear rated fault current 40 kA
U>Trip =200 V.
( )
> + × =
20 6.2 20 524 V
U>Trip
(
100 0 ° + 20 0 ° + × 3 10 60
1
is the current drawn by each CT just at pickup
is current drawn by metrosil at pickup
2
(one way) gives 2 ˣ 0.8 ohm at 75° C
SeriesResistor = 2000 ohm, which
− 3
)
− ° × 10 ≤ approx 275
1MRK 511 358-UEN A
(Equation 16)
(Equation 17)
A
Application manual
(Equation 18)