Where
I
j
is the capacitive ground fault current at a non-resistive phase-to-ground fault
X
c
is the capacitive reactance to ground
In a system with a neutral point resistor (resistance grounded system) the impedance Z
0
can be calculated as:
c n
0
c n
jX 3R
Z
jX 3R
- ×
=
- +
EQUATION1946 V1 EN-US (Equation 438)
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 439)
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 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
278.
1MRK 504 163-UUS A Section 9
Current protection
Transformer protection RET670 2.2 ANSI 559
Application manual
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