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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-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
1MRK 504 163-UUS A
Section 9
Current protection
Transformer protection RET670 2.2 ANSI
559
Application manual
Содержание RELION RET670
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