Normally this scheme is set to achieve a sensitivity of around 20 percent of the
used CT primary rating so that a low ohmic value can be used for the series
resistor.
It is strongly recommended to use the highest tap of the CT
whenever high impedance protection is used. This helps in utilizing
maximum CT capability, minimize the secondary fault current,
thereby reducing the stability voltage limit. Another factor is that
during internal faults, the voltage developed across the selected tap
is limited by the non-linear resistor but in the unused taps, owing to
auto-transformer action, voltages induced may be much higher than
design limits.
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 13)
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 524E P V> + × =
EQUATION1208 V1 EN-US (Equation 14)
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.
1MRK 505 388-UEN B Section 6
Differential protection
Busbar protection REB650 2.2 IEC 85
Application manual
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