ABB Relion 670 Series Applications Manual page 170

Section 8
Current protection
= -
Z jX // 3R // j3X
0
EQUATION1947 V1 EN-US
Where
X
n
Now consider a system with an earthing via a resistor giving higher earth fault current than the
high impedance earthing. The series impedances in the system can no longer be neglected.
The system with a single phase to earth fault can be described as in Figure 82.
Phase to earth fault
IEC06000654 V1 EN-US
Figure 82: Equivalent of power system for calculation of setting
The residual fault current can be written:
=
3I
0
2 Z
EQUATION1948 V1 EN-US
Where
U
phase
Z
1
Z
0
R
f
The residual voltages in stations A and B can be written:
164
=
c n n
3X X
n
is the reactance of the Petersen coil. If the Petersen coil is well tuned we have 3X
impedance Z will be: Z = 3R
0 0
Source impedance
Z (pos. seq)
sc
Z (pos. seq)
T,1
R
Z (zero seq)
T,0
U
0A
3I
0
Z
lineAB,1
Z
lineAB,0
U
0B
Z
lineBC,1
Z
lineBC,0
3U
phase
× + + ×
Z 3 R
1 0 f
is the phase voltage in the fault point before the fault
is the total positive sequence impedance to the fault point. Z
is the total zero sequence impedance to the fault point. Z
is the fault resistance.
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9R X X
n n c
( )
+ × -
j3R 3X X
c n n c
n
N
Substation A
(pos. seq)
(zero seq)
Substation B
(pos. seq)
(zero seq)
en06000654.vsd
1MRK 511 401-UEN Rev. K
= X
n c
= Z +Z +Z +Z
1 sc T,1 lineAB,1 lineBC,1
= Z +3R +Z +Z
0 T,0 N lineAB,0 lineBC,0
Bay control REC670
Application manual
(Equation 53)
In this case the
(Equation 54)