ABB REC670 Applications Manual page 145

1MRK 511 401-UEN A
Bay control REC670 2.2 IEC
Application manual
Basic data:
Current transformer ratio:
CT Class:
Secondary resistance:
Cable loop resistance:
Max fault current:
Calculation:
800
UR > × ( . 0 26 0 4 +
100 5 /
EQUATION1216 V2 EN-US
Select a setting of U>Trip =30 V.
The current transformer saturation voltage must be at least, twice the set operating voltage
10

U CT Saturation _ _ > + 0 26 .

25

EQUATION1217 V2 EN-US
U>Trip .
that is, greater than 2 ˣ
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.
100
(
IP = × 100 0 ° + 5 0 ° + ×
5
EQUATION1218 V2 EN-US
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.
100/5 A (Note: Must be the same at all locations)
10 VA 5P20
0.26 ohms
2
<50 m 2.5mm (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.
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.
. ) = 26 4 ,

 × 20 5 66 × = V

− 3
)
2 100 60 − ° × 10 ≤ approx A
Section 7
Differential protection
(Equation 18)
U>Trip .
(Equation 19)
5
(Equation 20)
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