The fault current, in the fault point, can be calculated as:
phase
j
0
0
f
3 U
I
3I
Z
3 R
×
=
=
+ ×
EQUATION1944 V1 EN-US
(Equation 492)
The impedance Z
0
is dependent on the system earthing. In an isolated system (without neutral
point apparatus) the impedance is equal to the capacitive coupling between the phase
conductors and earth:
phase
0
c
j
3 U
Z
jX
j
I
×
= -
= -
EQUATION1945 V1 EN-US
(Equation 493)
Where
I
j
is the capacitive earth fault current at a non-resistive phase-to-earth fault
X
c
is the capacitive reactance to earth
In a system with a neutral point resistor (resistance earthed system) the impedance Z
0
can be
calculated as:
c
n
0
c
n
jX 3R
Z
jX
3R
-
×
=
-
+
EQUATION1946 V1 EN-US
(Equation 494)
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 495)
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 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
.
Section 8
1MRK 505 343-UEN B
Current protection
436
Application manual
Содержание Relion 670 series
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