Substation A
Substation B
Z
lineAB,1
(pos. seq)
Z
lineAB,0
(zero seq)
Z
lineBC,1
(pos. seq)
Z
lineBC,0
(zero seq)
U
0A
U
0B
3I
0
Phase to earth fault
R
N
Z
T,1
(pos. seq)
Z
T,0
(zero seq)
Source impedance
Z
sc
(pos. seq)
en06000654.vsd
IEC06000654 V1 EN-US
Figure 64: Equivalent of power system for calculation of setting
The residual fault current can be written:
phase
0
1 0 f
3U
3I
2 Z Z 3 R
=
× + + ×
EQUATION1948 V1 EN-US
(Equation 51)
Where
U
phase
is the phase voltage in the fault point before the fault
Z
1
is the total positive sequence impedance to the fault point. Z
1
= Z
sc
+Z
T,1
+Z
lineAB,1
+Z
lineBC,1
Z
0
is the total zero sequence impedance to the fault point. Z
0
= Z
T,0
+3R
N
+Z
lineAB,0
+Z
lineBC,0
R
f
is the fault resistance.
The residual voltages in stations A and B can be written:
( )
0 A 0 T ,0 N
U 3I Z 3R= × +
EQUATION1949 V1 EN-US
(Equation 52)
OB 0 T ,0 N lineAB,0
U 3I (Z 3R Z )= × + +
EQUATION1950 V1 EN-US (Equation 53)
The residual power, measured by the sensitive earth fault protections in A and B will be:
1MRK 511 358-UEN A Section 7
Current protection
137
Application manual
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