in the 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 tertiary power system grounding is limiting the
ground-fault current. If high ground-fault current exists use two way cable
length.
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 21)
Select a setting of
TripPickup
=30 V.
The current transformer knee point voltage must be at least, twice the set operating voltage
TripPickup
.
( )
2 0.1 100 0.7 147 VVkneeANSI > + × × =
EQUATION1779-ANSI V2 EN (Equation 22)
that is, greater than 2 ˣ
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
23, 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 23)
Section 7 1MRK 506 369-UUS -
Differential protection
150 Line distance protection REL670 2.2 ANSI
Application manual
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