ABB REL670 2.2 IEC, Applications Manual

Page: 504 / 908

c n
0
c n
jX 3R
Z
jX 3R
- ×
=
- +
EQUATION1946 V1 EN-US
(Equation 471)
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-US
(Equation 472)
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
274 .
Section 9
1MRK 506 369-UEN B
Current protection
498 Line distance protection REL670 2.2 IEC
Application manual