Medium Voltage Application Guide

SWITCHGEAR
Impedance Method Calculations
Case 1
Source (Network):
U p = 36 kV
= 1000 MVA
S sc
G
1
TR
Source
Feeder
Transformer (TR):
U p = 36 kV
U s = 11 kV @ 50 Hz
S = 20 MVA
Z% = 10%
I sc1
CB1
CB2
13851.A
I sc3
Generator (G):
U = 11 kV @ 50 Hz
S = 15 MVA
X d´´ = 15%
X d´ = 20%
For the purposes of this calculation, ignore all impedances of circuit breakers, cables and busbars.
The first step is to calculate the individual impedances.
2
CB3
Generator impedance (Z G ):
X d
U 2
Z d
S 100
11 2
15
15 100
1.21
The subtransient impedance is 1.2 .
Transformer impedance (Z TR ):
X d
U 2
S 100
11 2
20
15 100
1.6
The transient impedance is 1.6 .
Network impedance (Z NET ), as seen on the
secondary side of the transformer:
The permanent impedance is 0.6 .
36 2
1000
11 2
36
The permanent impedance is 0.12 .
710-12280-00A Medium Voltage Application Guide Page 159