Setting example
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, 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 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: 10 VA 5P20
Secondary resistance: 0.26 ohms
Cable loop resistance:
<50 m 2.5mm
2
(one way) gives 1 ˣ 0.4 ohm at 75° C
Note! Only one way as the tertiary power system earthing is limiting
the earth-fault current. If high earth-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:
UR > × + =
800
100 5
0 26 0 4 26 4
/
( . . ) ,
EQUATION1216 V2 EN-US (Equation 19)
Select a setting of
U>Trip
=30 V.
The current transformer saturation voltage must be at least, twice the set operating voltage
U>Trip
.
U CT Saturation V_ _ .> +
× × =
10
25
0 26 20 5 66
EQUATION1217 V2 EN-US (Equation 20)
that is, greater than 2 ˣ
U>Trip
.
Check from the table of selected resistances the required series stabilizing resistor value to
use. Since this application requires good sensitivity, select
SeriesResistor
= 300 ohm, which
gives an IED current of 100 mA.
To calculate the sensitivity at operating voltage, refer to equation
21, which gives an
acceptable value. A little lower sensitivity could be selected by using a lower resistance value.
IP approx A= × ° + ° + × − °
( )
× ≤
−
100
5
100 0 5 0 2 100 60 10 5
3
EQUATION1218 V2 EN-US
(Equation 21)
The magnetizing current is taken from the magnetizing curve of the current transformer cores,
which should be available. The current value at
U>Trip
is taken. For the voltage dependent
resistor current the peak value of voltage 30 ˣ √2 is used. Then the RMS current is calculated by
Section 6 1MRK 511 358-UEN A
Differential protection
98
Application manual
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