ABB RET670 Applications Manual page 160

Section 3
IED application
154
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
Select a setting of TripPickup =20 V.
The current transformer knee point voltage can roughly be calculated from the rated values. Considering
knee point voltage to be about 70% of the accuracy limit voltage.
(
VkneeANSI > +
2 0.1 100 0.7 147 V
EQUATION1779-ANSI V2 EN
TripPickup .
that is, greater than 2 ·
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 47, 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
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 system grounding is limiting the ground-fault
current. If high ground-fault current exists use two way cable.
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
1MRK504116-UUS C
(Equation 45)
(Equation 46)
(Equation 47)
Application manual