Basic data:
Cable loop
resistance:
<100 ft AWG10 (one way between the junction point and the farthest
CT) gives loop resistance 2 · 0.05 = 0.1 Ohms
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 given by the transformer
reactance for example, 28 kA.
Calculation:
28000
(0.5 0.1) 42
400
VR V> × + =
EQUATION1766-ANSI V1 EN (Equation 42)
Select a setting of
TripPickup
=60 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.
(4 0.5) 100 0.7 315VkneeANSI V> + × × =
EQUATION1767-ANSI V1 EN (Equation 43)
that is, greater than 2 ·
TripPickup
.
Check from the table of selected resistances the required series stabilizing resistor
value to use. Since this application it is not required to be so sensitive, select R
series=150 ohm, which gives an IED current of 400 mA.
To calculate the sensitivity at operating voltage, refer to equation 44 which gives an
acceptable value; in fact in this example the sensitivity is about 13% of the nominal
current of the CT:
( )
2000
400 0 60 .140
5
IP approx A= × ° + £
EQUATION1886-ANSI V1 EN (Equation 44)
Where
IR= 100mA is the current drawn by the IED circuit
I
mag
= 15mA is the magnetizing current of the CT at the operating voltage
TripPickup
Ires = 5 mA is the current drawn by the non-linear resistor at the operating voltage
TripPickup
1MRK504116-UUS C Section 3
IED application
151
Application manual
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