ABB REG670 Applications Manual page 366

Section 9
Current protection
360
- ×
jX 3R
=
Z c n
0
- +
jX 3R
c n
EQUATION1946 V1 EN
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
Where
X is the reactance of the Petersen coil. If the Petersen coil is well tuned we have 3X
n
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 190.
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
9R X X
=
n n c
(
+ × -
3X X j3R 3X
n c n n
will be: Z = 3R
0 0 n
1MRK 502 071-UEN -
0
)
X
c
Application manual
(Equation 198)
can be
(Equation 199)
= X In this
n c