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 18)
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 19)
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
20, 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 20)
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 dividing obtained current value from the
metrosil curve with √2. Use the maximum value from the metrosil curve given in
Figure 54.
Section 7 1MRK 506 369-UEN B
Differential protection
144 Line distance protection REL670 2.2 IEC
Application manual
To Next Page
Loading...
