ABB Relion 670 Series Applications Manual page 174

Section 8
Current protection
In many systems there is also a neutral point reactor (Petersen coil) connected to one or more
transformer neutral points. In such a system the impedance Z
= -
Z jX // 3R // j3X
0
EQUATION1947 V1 EN-US
Where
X
n
Now consider a system with an grounding via a resistor giving higher ground fault current than
the high impedance grounding. The series impedances in the system can no longer be neglected.
The system with a single phase to ground fault can be described as in Figure 81.
Z (pos. seq)
T,1
Z (zero seq)
T,0
Phase to ground fault
ANSI06000654 V1 EN-US
Figure 81: Equivalent of power system for calculation of setting
The residual fault current can be written:
168
=
c n n
3X X
n c
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
R
V
0A
3I
0
Z (pos. seq)
lineAB,1
Z (zero seq)
lineAB,0
V
0B
Z (pos. seq)
lineBC,1
Z (zero seq)
lineBC,0
en06000654_ansi.vsd
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9R X X
n n c
( )
+ × -
j3R 3X X
n n c
n
N
Substation A
Substation B
1MRK 511 401-UUS F
can be calculated as:
0
= X In this case the
n c
Bay control REC670
Application manual
(Equation 53)