RFL Electronics RFL 9300 - Page 83

RFL 9300 RFL Electronics Inc.
August 25, 2000 3 - 14 (973) 334-3100
The calculations for external faults are similar to those for internal faults. First, the V
KPS
value is determined for
SLG faults:
V
KPS
= I
SLG
x R
2W
x TF
G
where
V
KPS
= Kneepoint voltage of the CT's B-H curve for worst-case SLG fault.
I
SLG
= Maximum expected SLG fault current (symmetrical, in secondary amperes rms).
R
2W
= "Two-way" secondary cable resistance, in ohms.
TF
G
= "Transient factor" for ground faults. This is equal to (1 + X
L
/R) where (L/R) is the pri-
mary system time constant.
Then calculate V
KPP
for three-phase faults:
V
KPP
= I
3PH
x R
1W
x TF
P
where
V
KPP
= Kneepoint voltage of the CT's B-H curve for worst-case three-phase fault.
I
3PH
= Maximum expected three-phase fault current (symmetrical, in secondary amperes
rms).
R
1W
= "One-way" secondary cable resistance, in ohms.
TF
P
= "Transient factor" for phase faults. This is equal to (1 + X
L
/R) where (L/R) is the primary
system time constant.
For external faults, the currents used will be smaller than for internal faults, because the protected line's imped-
ance helps to limit the current. The transient factors (TF
G
and TF
P
) also will be smaller, because line impedance
usually has a smaller angle (about 75 degrees) than source impedance (as high as 85 degrees, or more).
To select the proper CT class for external faults, take the larger of the calculated kneepoint voltages (V
KPS
or
V
KPP
), and divide it by four. The next-higher CT class will be adequate for RFL 9300 operation on external
faults.
Example: V
KPS
= 640 volts
V
KPP
= 720 volts
720 / 4 = 180
CT's rated "C200" should be used.