ABB REC670 Applications Manual page 147

1MRK 511 401-UUS A
Bay control REC670 2.2 ANSI
Application manual
Figure 52: The protection scheme utilizing the high impedance function for the T-
feeder
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:
40000
(
> × +
VR 0.5 0.4
400
EQUATION1751-ANSI V1 EN-US
Select a setting of TripPickup =100 V.
The current transformer saturation voltage must be at least twice the set operating voltage
> + ×
VkneeANSI (0.5 8) 100 * 0.7
EQUATION1752-ANSI V1 EN-US
TripPickup
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 R Series= 500 ohm,
which gives an IED operating current of 200 mA.
Calculate the primary sensitivity at operating voltage using the following equation.
2000/5A
C800 (At max tap of 2000/5A)
0.5 Ohm (2000/5A tap)
2
Equal to switchgear rated fault current 40 kA
)
=
90 V
=
595 V
Section 7
Differential protection
(Equation 15)
TripPickup .
(Equation 16)
141