ABB RELION 650 Series Applications Manual page 120

Section 7
Current protection
-
=
Z
0
-
EQUATION1946 V1 EN-US
Where
R
n
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 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 55.
Phase to earth fault
IEC06000654 V1 EN-US
Figure 55: Equivalent of power system for calculation of setting
The residual fault current can be written:
114
×
jX 3R
c n
+
jX 3R
c n
is the resistance of the neutral point resistor
=
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
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 504 158-UEN A
(Equation 38)
can be calculated as:
0
(Equation 39)
= X In this case the
n c
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