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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
(Equation 47)
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
(Equation 48)
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
1MRK 511 286-UUS A
Section 6
Current protection
121
Application manual
Содержание REC650 ANSI
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