ABB REC670 Applications Manual page 190

Section 8
Current protection
184
- ×
jX 3R
=
Z c n
0
- +
jX 3R
c n
EQUATION1946 V1 EN-US
Where
R is the resistance of the neutral point resistor
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
calculated as:
= -
Z jX // 3R // j3X
0 c n n
EQUATION1947 V1 EN-US
Where
X is the reactance of the Petersen coil. If the Petersen coil is well tuned we have 3X
n
this case the impedance Z
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 80.
9R X X
=
n n c
(
+ × -
3X X j3R 3X
n c n n
will be: Z = 3R
0 0 n
1MRK 511 401-UEN A
)
X
c
Bay control REC670 2.2 IEC
Application manual
(Equation 49)
can be
0
(Equation 50)
= X In
n c