Hitachi Relion 670 Series Applications Manual page 166

Section 8
Current protection
The impedance Z
apparatus) the impedance is equal to the capacitive coupling between the phase conductors and
earth:
 
Z
0
IECEQUATION16031 V1 EN-US
Where
I
j
X
c
In a system with a neutral point resistor (resistance earthed system) the impedance Z
calculated as:


Z

0
IECEQUATION16032 V1 EN-US
Where
R is the resistance of the neutral point resistor
n
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
 
Z
0
IECEQUATION16033 V1 EN-US
Where
X is the reactance of the Petersen coil. If the Petersen coil is well tuned we have 2X
n
impedance Z
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 64.
160
is dependent on the system earthing. In an isolated system (without neutral point
0

2 U
 
phase
jX j
c
I
j
is the capacitive earth fault current at a non-resistive phase-to-earth fault
is the capacitive reactance to earth

j X 2 R
c n

j X 2 R
c n

jX / /2 R / / 2 j X
c n n
2
will be: Z = 2R
0 0 n
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4 R X X
n n c

  
X X j R 2 2 X
n c n n
1MRK506375-UEN Rev. N
can be calculated as:
0

X
c
= X In this case the
n c
Railway application RER670
(Equation 45)
can be
0
(Equation 46)
(Equation 47)
Application manual