ABB RET670 Applications Manual page 152

Section 3
IED application
146
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 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 may be much
higher than design limits might be induced.
Setting example
Basic data:
Current transformer ratio:
CT Class:
Secondary resistance:
Cable loop resistance:
Max fault current:
Calculation:
40000
(
> × +
VR 0.5 0.4
400
EQUATION1751-ANSI V1 EN
TripPickup =100 V.
Select a setting of
The current transformer saturation voltage must be at least, twice the set operating voltage
> + ×
VkneeANSI (0.5 8) 100 * 0.7
EQUATION1752-ANSI V1 EN
TripPickup
that is, bigger than 2 ·
Check from the table of selected resistances the required series stabilizing resistor
value to use. As this application requires to be so sensitive select R Series= 500 ohm,
which gives an IED operating current of 200 mA.
Calculate the primary sensitivity at operating voltage, ignoring the current drawn by
the non-linear resistor.
2000/5A
C800 (At max tap of 2000/5A)
0.5 Ohm (2000/5A tap)
2
<200 ft AWG10 (one way between the junction point and the
farthest CT) to be limited to approx. 0.2 Ohms at 75deg C gives
loop resistance 2 · 0.2 = 0.4 Ohms.
Equal to switchgear rated fault current 40 kA
)
=
90 V
=
595 V
1MRK504116-UUS C
(Equation 36)
TripPickup .
(Equation 37)
Application manual