It is strongly recommended to use the highest tap of the CT whenever high
impedance protection is used. This helps in utilizing maximum CT capability,
minimize the secondary fault current, thereby reducing the stability voltage
limit. Another factor is that during internal faults, the voltage developed across
the selected tap is limited by the non-linear resistor but in the unused taps,
owing to auto-transformer action, voltages induced may be much higher than
design limits.
Setting example
Basic data:
Current transformer ratio: 2000/1 A
CT Class: 20 VA 5P20
Secondary resistance: 6.2 ohms
Cable loop resistance:
<100 m 2.5 mm
2
(one way) gives 2 ˣ 0.8 ohm at 75° C
Max fault current: Equal to switchgear rated fault current 40 kA
Calculation:
EQUATION1207 V2 EN-US (Equation 16)
Select a setting of
U>Trip
=200 V.
The current transformer saturation voltage must be at least twice the set operating voltage
U>Trip
.
( )
5 20 6.2 20 524E P V> + × =
EQUATION1208 V1 EN-US (Equation 17)
that is, bigger than 2 ˣ
U>Trip
Check from the table of selected resistances the required series stabilizing resistor value to
use. As this application requires to be so sensitive select
SeriesResistor
= 2000 ohm, which
gives an IED operating current of 100 mA.
Calculate the primary sensitivity at operating voltage using the following equation.
IP approx A= ° + ° + × − °
( )
× ≤
−
2000
1
100 0 20 0 3 10 60 10 275
3
EQUATION1209 V2 EN-US (Equation 18)
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
Section 6 1MRK 511 358-UEN A
Differential protection
96
Application manual
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