ABB Relion 670 Series Applications Manual page 129

1MRK 511 401-UEN Rev. K
Setting example
Basic data:
Current transformer ratio:
CT Class:
Secondary resistance:
Cable loop resistance:
Max fault current:
Calculation:
UR
>
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
The magnetizing current is taken from the magnetizing curve for the current transformer
cores which should be available. The current value at
dependent resistor current the peak value of voltage 200 ˣ √2 is used. Then the RMS current is
calculated by dividing obtained current value from the metrosil curve with√2. Use the value
from the maximum metrosil curve given in Figure
Bay control REC670
Application manual
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
40000
( )
⋅ 6.2 1.6 + = 156
2000
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
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2
(one way) gives 2 ˣ 0.8 ohm at 75° C
V
− 3
)
− ° × 10 ≤
approx
U>Trip is taken. For the voltage
55
Differential protection
(Equation 18)
(Equation 19)
SeriesResistor = 2000 ohm, which
275
A
Section 7
(Equation 20)
123