ABB REL670 2.2 IEC, Applications Manual

Page: 147 / 908

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 15)
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 20 524 E P V
> + × =
EQUATION1208 V1 EN-US
(Equation 16)
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 17)
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
U>Trip is taken.
For the voltage dependent resistor current the peak value of voltage 200 ˣ √2 is
used. Then the RMS current is calculated by dividing obtained current value from
1MRK 506 369-UEN B
Section 7
Differential protection
Line distance protection REL670 2.2 IEC 141
Application manual