unused taps, owing to auto-transformer action, voltages much higher
than design limits might be induced.
Basic data:
Current transformer ratio: 100/5 A (Note: Must be the same at all locations)
CT Class: C200
Secondary resistance: 0.1 Ohms (At 100/5 Tap)
Cable loop resistance: <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.
Max fault current: 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.
Calculation:
800
(0.1 0.1) 8
20
VR > × + =
EQUATION1768-ANSI V1 EN (Equation 45)
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.
( )
2 0.1 100 0.7 147 VVkneeANSI > + × × =
EQUATION1779-ANSI V2 EN (Equation 46)
that is, greater than 2 ·
TripPickup
.
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
(200 2 30) .5.2
5
IP approx A= × + × £
EQUATION1769-ANSI V1 EN (Equation 47)
Section 3 1MRK504116-UUS C
IED application
154
Application manual
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