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Apparent power of transformer: S
n
= 10 MVA
Short circuit impedance of transformer: e
k
= 10%
Nominal voltage on transformer high voltage winding: U
n
= 138 kV
Calculating of fault current at HV (High Voltage) side of the taped transformer for three
phase fault on LV side
For convenience we choose calculation voltage to 138 kV.
ZsA
ZsB
Ztr
f
E
ZlA
ZlB
IEC14000046-1-en.vsd
IEC14000046 V1 EN-US
Figure 65: Thevenin equivalent for tap transformer
Converting of the sources into impedances gives:
Z
S
A
=
=
138
1700
11 2
2
.
Ω
EQUATION14000034 V1 EN-US
(Equation 34)
Z
S
B
=
=
138
1200
15 9
2
.
Ω
EQUATION14000035 V1 EN-US
(Equation 35)
Calculating of the short circuit impedance of the transformer gives:
Z
e
U
S
trf
k
n
n
=
×
=
×
=
100
0 1
138
10
190 4
2
2
.
.
Ω
EQUATION14000036 V1 EN-US
(Equation 36)
Based on the Thevenin equivalent below we calculate the fault current at HV side of the
transformer as:
If
Z
res
138
138
3
=
×
EQUATION14000037 V1 EN-US
(Equation 37)
where:
Z
Zs
Zl
Zs
Zl
Zs
Zl
Zs
Zl
Z
res
A
B
B
A
a
B
B
trf
a
=
+
(
)
×
+
(
)
+
+
+
+
=
198
Ω
EQUATION14000039 V1 EN-US
(Equation 38)
Section 6
1MRK 505 343-UEN B
Differential protection
138
Application manual
Summary of Contents for Relion 670 series
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