ABB REL670 2.2 IEC - Page 218

The magnitude of the earth-fault current in effectively earthed networks is high
enough for impedance measuring elements to detect earth faults. However, in the
same way as for solidlyearthed networks, distance protection has limited
possibilities to detect high resistance faults and should therefore always be
complemented with other protection function(s) that can carry out the fault
clearance in this case.
High impedance earthed networks
M17048-57 v6
In high impedance networks, the neutral of the system transformers are connected
to the earth through high impedance, mostly a reactance in parallel with a high
resistor.
This type of network is many times operated in radial, but can also be found
operating meshed networks.
What is typical for this type of network is that the magnitude of the earth-fault
current is very low compared to the short circuit current. The voltage on the
healthy phases will get a magnitude of √3 times the phase voltage during the fault.
The zero sequence voltage (3U
0
) will have the same magnitude in different places
in the network due to low voltage drop distribution.
The magnitude of the total fault current can be calculated according to equation
105.
( )
2
2
R
L C
0
3I I I I= + -
EQUATION1271 V3 EN-US (Equation 105)
Where:
3I
0
is the earth-fault current (A)
I
R
is the current through the neutral point resistor (A)
I
L
is the current through the neutral point reactor (A)
I
C
is the total capacitive earth-fault current (A)
The neutral point reactor is normally designed so that it can be tuned to a position
where the reactive current balances the capacitive current from the network that is:
1
3
L
C
w
w
=
× ×
EQUATION1272 V1 EN-US
(Equation 106)
Section 8 1MRK 506 369-UEN B
Impedance protection
212 Line distance protection REL670 2.2 IEC
Application manual