ABB REC670 Applications Manual page 150

Section 7
Differential protection
144
but in the unused taps, owing to auto-transformer action, voltages much
higher than design limits might be induced.
Basic data:
Current transformer ratio:
CT Class:
Secondary resistance:
Cable loop resistance:
Max fault current:
Calculation:
800
VR > × + =
(0.1 0.1) 8
20
EQUATION1768-ANSI V1 EN-US
TripPickup =30 V.
Select a setting of
The current transformer knee point voltage must be at least, twice the set operating voltage
(
VkneeANSI > +
2 0.1 100 0.7 147 V
EQUATION1779-ANSI V2 EN-US
TripPickup .
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 R Series = 100
ohm, which gives an IED current of 200 mA.
To calculate the sensitivity at operating voltage, refer to equation 20, 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.
100
= × + ×
IP (200 2 30)
5
EQUATION1769-ANSI V1 EN-US
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.
)
× × =
£
approx .5.2 A
1MRK 511 401-UUS A
(Equation 18)
TripPickup .
(Equation 19)
(Equation 20)
Bay control REC670 2.2 ANSI
Application manual