ABB RER670 Applications Manual page 183

1MRK 506 375-UEN A
Railway application RER670 2.2 IEC
Application manual
Where
I is the capacitive earth fault current at a non-resistive phase-to-earth fault
j
X is the capacitive reactance to earth
c
In a system with a neutral point resistor (resistance earthed system) the impedance
Z can be calculated as:
0
 
j X 2 R

c n
Z
 
0
j X 2 R
c n
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
calculated as:
 
Z jX / /2 R / / 2 j X
0 c n
IECEQUATION16033 V1 EN-US
Where
X is the reactance of the Petersen coil. If the Petersen coil is well tuned we have 2X
n
this case the 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 58.
4 R X X

n n c

 
n
2 X X j R 2 2
n c n
will be: Z = 2R
0 0 n
Section 8
Current protection
(Equation 45)
0


X X
n c
(Equation 46)
n
can be
= X In
c
177