7PG2113/4/5/6 Solkor Applications Guide
©2010 Siemens Protection Devices Limited
Chapter 7 Page 35 of 49
4.1.1 Example Fault Current Estimation
33kV Overhead line
10km long
X
L
= 0.28978 ohms/km R
L
= 0.07765 ohms/km (Primary)
CT ratio = 400:1
R
CT
= 2 ohms
CT wiring resistance, R
L
, 30m long 7/0.67mm 2.5mm sq. at 7.4 ohms/km = 0.22 ohms
VT ratio 33000:110V
Maximum X/R ratio at source busbar = 20
Maximum 3P fault level at busbar = 1000MVA
Consider 3P fault level based on maximum busbar levels.
V
Ph
= 33000/
√
3 = 19.05kV
Fault level per phase = 1000/3 = 333MVA
17.5kA
19.05x10
333x10
I
3
6
F
=
=
Ω
=
=
089
.
1
10
5
.
17
10
05
.
19
3
3
x
x
Z
S
Also, since X/R at the busbar = 20,
We can evaluate the source impedance:
19.05kV
Zs
17.5kA
X
Zs
O
R