Siemens 7PG21 Solkor Rf - Page 34

7PG21 Solkor Rf Applications Guide
Example
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
==
19.05kV
Zs
17.5kA
Ω== 089.1
105.17
1005.19
3
3
x
x
Z
S
Also, since X/R at the busbar = 20,
We can evaluate the source impedance:
⎟
⎠
⎞
⎜
⎝
⎛
=
−
R
X
Tan
1
θ
o
87= θ
Ω== 00544cos
θ
SS
ZR &
Ω
=
=
0876.1sin
θ
SS
ZX (Primary)
X
R
Zs
O
R
L
= 0.07765 ohms/km (Primary)
R
L
= 10 x 0.07765 = 0.7765Ω (Primary)
X
L
= 0.28978 ohms/km
X
L
= 10 x 0.28978 = j2.8978Ω (Primary)
Total impedance for a through fault at the remote busbar =
Z
S
+ Z
L
= (R
S
+ R
L
) + (X
S
+ X
L
)
(0.0544 + 0.7765) + j(1.0876 +2.8978)
©2010 Siemens Protection Devices Limited Chapter 3 Page 14 of 15