ABB REL650 Applications Manual page 223

1MRK 506 334-UUS A
Application manual
Where
I is the capacitive ground-fault current at a non-resistive phase to ground-fault
j
X is the capacitive reactance to ground
c
In a system with a neutral point resistor (resistance grounded system) the impedance Z
can be calculated as:
- ×
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
= - =
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
the impedance Z will be: Z
0
Now consider a system with an grounding via a resistor giving higher ground-fault current
than the high impedance grounding. The series impedances in the system can no longer be
neglected. The system with a single phase to ground-fault can be described as in figure
109.
9R X X
n n c
(
+ × -
3X X j3R 3X X
n c n n c
= 3R
0 n
Section 7
Current protection
can be calculated as:
0
)
= X
n
0
(Equation 129)
(Equation 130)
In this case
c
217