ABB RET670 - Page 163

Basic data:
Transformer rated current on HV winding: 250 A
Current transformer ratio: 600-300/5A A (Note: Must be the same at all locations)
CT Class: C200
Cable loop resistance: <50 ft AWG10 (one way between the junction point
and the farthest CT) to be limited to approx. 0.05
Ohms at 75deg C gives loop resistance 2 · 0.05 = 0.1
Ohms
Max fault current: The maximum through fault current is limited by the
transformer reactance, use 15 · rated current of the
transformer
Calculation:
( )
250
15 0.1 0.1 6.25
600 / 5
VR V> × × + =
EQUATION1775-ANSI V1 EN (Equation 48)
Select a setting of TripPickup =20 V.
The current transformerknee point voltage can roughly be calculated from the rated values.
Considering knee point voltage to be about 70% of the accuracy limit voltage.
(0.1 2) 100 210VkneeANSI V> + × =
EQUATION1770-ANSI V1 EN (Equation 49)
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 high sensitivity, select R series= 100 ohm
which gives a current of 200 mA.
To calculate the sensitivity at operating voltage, refer to equation
50 which is
acceptable as it gives around 10% minimum operating current, ignoring the current
drawn by the non-linear resistor.
( )
600
200 0 4 20 60 .5.4
5
IP approx A= × ° + × - ° £
EQUATION1885-ANSI V1 EN (Equation 50)
Where 200mA is the current drawn by the IED circuit and 50mA is the current drawn
by each CT just at pickup. The magnetizing current is taken from the magnetizing
curve for the current transformer cores which should be available. The value at
TripPickup is taken.
1MRK504116-UUS C Section 3
IED application
157
Application manual