Setting example
Basic data:
Current transformer ratio: 2000/5A
CT Class: C800 (At max tap of 2000/5A)
Secondary resistance: 0.5 Ohm (2000/5A tap)
Cable loop resistance:
2
Max fault current: Equal to switchgear rated fault current 40 kA
Calculation:
( )
40000
0.5 0.4 90
400
VR V> × + =
EQUATION1751-ANSI V1 EN-US (Equation 20)
Select a setting of
TripPickup
=100 V.
The current transformer saturation voltage must be at least twice the set operating voltage
TripPickup
.
(0.5 8) 100 * 0.7 595VkneeANSI V> + × =
EQUATION1752-ANSI V1 EN-US (Equation 21)
that is, bigger than 2 ˣ
TripPickup
Check from the table of selected resistances the required series stabilizing resistor
value to use. As this application requires to be so sensitive select R Series= 500 ohm,
which gives an IED operating current of 200 mA.
Calculate the primary sensitivity at operating voltage using the following equation.
( )
3
2000
200 0 3 50 60 10 .100
5
IP approx A
-
= ° + × - ° × £
EQUATION1887-ANSI V1 EN-US (Equation 22)
where
100 mA is the current drawn by the IED circuit and
10 mA is the current drawn by each CT just at pickup
20 mA is current drawn by metrosil at pickup
The magnetizing current is taken from the magnetizing curve for the current
transformer cores which should be available. The current value at T
ripPickup
is taken.
1MRK 504 163-UUS A Section 7
Differential protection
Transformer protection RET670 2.2 ANSI 169
Application manual
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