ABB Relion 670 series, Applications Manual

Page: 310 / 760

earthed 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
GUID-02F306F5-1038-42AC-AFAE-3F8423C4C066 v4
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 often operated radially, but can also be found operating as a meshed
network.
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 313 .
( )
2
2
R L C
0
3I I I I
= + -
EQUATION1271 V3 EN-US
(Equation 313)
Where:
3I
0
is the earth-fault current (A)
IR is the current through the neutral point resistor (A)
IL is the current through the neutral point reactor (A)
IC 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:
1
3
L
C
w
w
=
× ×
EQUATION1272 V1 EN-US
(Equation 314)
IEC05000216 V2 EN-US
Figure 186: High impedance earthing network
The operation of high impedance earthed networks is different compared to solid earthed
networks, where all major faults have to be cleared very fast. In high impedance earthed
networks, some system operators do not clear single phase-to-earth faults immediately; they
clear the line later when it is more convenient. In case of cross-country faults, many network
operators want to selectively clear one of the two earth faults.
Section 7 1MRK 505 343-UEN B
Impedance protection
304
Application manual