Basic data:
Secondary resistance: 0.66 ohms
Cable loop resistance:
<50 m 2.5mm
2
(one way) gives 2 ˣ 0.4 ohm at 75° C
Max fault current: The maximum through fault current is limited by the
transformer reactance, use 15 ˣ rated current of the
transformer
Calculation:
( )
250
15 0.66 0.8 18.25
300
UR V> × × + =
EQUATION1219 V1 EN (Equation 29)
Select a setting of
U>Trip
=40 V.
The current transformer saturation voltage at 5% error can roughly be calculated from the rated values.
( )
5 10 0.66 20 213.2E P V> + × =
EQUATION1220 V1 EN (Equation 30)
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 high sensitivity, select SeriesResistor=
1000 ohm which gives a current of 40 mA.
To calculate the sensitivity at operating voltage, refer to equation
31 which is
acceptable as it gives around 10% minimum operating current.
IP approx A= × ° + ° + × − °
( )
× ≤
−
300
5
40 0 5 0 4 20 60 10 33
3
EQUATION1221 V2 EN (Equation 31)
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 40 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
66.
7.2.4.6 Alarm level operation
The 1Ph High impedance differential protection HZPDIF function has a separate
alarm level, which can be used to give alarm for problems with an involved current
transformer circuit. The setting level is normally selected to be around 10% of the
operating voltage U>Trip.
Section 7 1MRK 502 071-UEN -
Differential protection
166 Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual
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