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
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
dividing obtained current value from the metrosil curve with √2. Use the maximum value from
the metrosil curve given in Figure
6.1.4.5
Restricted earth fault protection
M16850-156 v10
For solidly earthed systems a restricted earth fault protection REFPDIF is often provided as a
complement to the normal transformer differential function. The advantage with the
restricted earth fault functions is the high sensitivity for internal earth faults in the
transformer winding. Sensitivities of 2-8% can be achieved whereas the normal differential
function will have sensitivities of 20-40%. The sensitivity for high impedance restricted earth
fault function is mostly dependent on the current transformers magnetizing currents.
1MRK 505 343-UEN B
Section 6
Differential protection
105
Application manual
Summary of Contents for Relion 670 series
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