ABB Relion 670 Series Applications Manual page 123

1MRK 511 358-UUS A
Setting example
Basic data:
Current transformer ratio:
CT Class:
Secondary resistance:
Cable loop resistance:
Max fault current:
Calculation:
800
VR >
EQUATION1768-ANSI V1 EN-US
Select a setting of
The current transformer knee point voltage must be at least, twice the set operating voltage
TripPickup .
VkneeANSI >
EQUATION1779-ANSI 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
current of 200 mA.
To calculate the sensitivity at operating voltage, refer to equation 21, which gives an acceptable
value, ignoring the current drawn by the non-linear resistor. A little lower sensitivity could be
selected by using a lower resistance value.
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, 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)
C200
0.1 Ohms (At 100/5 Tap)
<100 ft AWG10 (one way between the junction point and the
farthest CT) to be limited to approximately 0.1 Ohms at 75deg C
Note! Only one way as the tertiary power system grounding is
limiting the ground-fault current. If high ground-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.
× + =
(0.1 0.1) 8
20
TripPickup =30 V.
( )
+ ×
2 0.1 100 0.7 147 V
TripPickup .
× =
R Series = 100 ohm, which gives an IED
Section 6
Differential protection
(Equation 19)
(Equation 20)
117