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:
0
2
2
c
n
c
n
jX
R
Z
jX
R
IECEQUATION16032 V1 EN-US
(Equation 45)
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:
0
4
/ /2
/ / 2
2
2
2
n
n
c
c
n
n
n
c
n
n
c
R X X
Z
jX
R
j X
X X
j R
X
X
IECEQUATION16033 V1 EN-US
(Equation 46)
Where
X
n
is the reactance of the Petersen coil. If the Petersen coil is well tuned we have 2X
n
= X
c
In
this case the impedance Z
0
will be: Z
0
= 2R
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
.
1MRK 506 375-UEN A
Section 8
Current protection
Railway application RER670 2.2 IEC
177
Application manual
Summary of Contents for RELION RER670
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