ABB REC670 Applications Manual page 142

Section 7
Differential protection
136
Normally this scheme is set to achieve a sensitivity of around 20 percent of the
used CT primary rating so that a low ohmic value can be used for the series
resistor.
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.
Setting example
Basic data:
Current transformer ratio:
CT Class:
Secondary resistance:
Cable loop resistance:
Max fault current:
Calculation:
EQUATION1207 V2 EN-US
U>Trip =200 V.
Select a setting of
The current transformer saturation voltage must be at least twice the set operating voltage
( )
> + ×
E P 5 20 6.2 20 524
EQUATION1208 V1 EN-US
U>Trip
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 SeriesResistor=
2000 ohm, which gives an IED operating current of 100 mA.
Calculate the primary sensitivity at operating voltage using the following equation.
2000/1 A
20 VA 5P20
6.2 ohms
2
<100 m 2.5 mm (one way) gives 2 ˣ 0.8 ohm at 75° C
Equal to switchgear rated fault current 40 kA
=
V
1MRK 511 401-UEN A
(Equation 15)
U>Trip .
(Equation 16)
Bay control REC670 2.2 IEC
Application manual