L7 - ECE 524 Transients in Power Systems Session 7 Page 1/9...

ECE 524:
Transients in Power Systems
Session 7; Page 1/9
Spring 2012
Transient R-L Circuit Example
Given a 230kV:34kV,  -Ygrounded, 100MVA transformer supplied supplied by a source
with a short circuit MVA of 2000. The transformer has a per unit reactance of 0.1pu on its
ratings base. Both the transformer and the source impedance have an X/R ratio of 12.
Calculate the complete phase A fault current if a fault occurs on the LV side
of the transformer at 70 degrees past the voltage peak.
Define Units:
MVA 1000kW  S
BASE
100MVA 
pu 1 
Transformer parameters:
S
rated
100MVA  XoverR 12 
V
hv
230kV  V
lv
34.5kV 
X
trans
0.1pu 
R
trans
X
trans
XoverR
 R
trans
0.01 pu  
Find the transformer impedances referred to the LV side since the
fault occurs on that side.
Z
BLV
V
lv
2
S
rated
 Z
BLV
11.9 Ω 
R
transLV
R
trans
Z
BLV
  R
transLV
0.0992 Ω 
X
transLV
X
trans
Z
BLV
  X
transLV
1.19 Ω 
L
transLV
X
transLV
2 π  60  Hz
 L
transLV
3.157 mH  