c n
0
c n
jX 3R
Z
jX 3R
- ×
=
- +
EQUATION1946 V1 EN (Equation 32)
Where
R
n
is the resistance of the neutral point resistor
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
0
can be
calculated as:
( )
n n c
0 c n n
n c n n c
9R X X
Z jX // 3R // j3X
3X X j3R 3X X
= - =
+ × -
EQUATION1947 V1 EN (Equation 33)
Where
X
n
is the reactance of the Petersen coil. If the Petersen coil is well tuned we have 3X
n
= X
c
In this
case the impedance Z
0
will be: Z
0
= 3R
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
62.
1MRK 505 291-UEN A Section 6
Current protection
125
Application manual
To Next Page
Loading...
