ABB RELION 650 Series Applications Manual page 109

1MRK 505 363-UEN A
R
1
X
1
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 solidly
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
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
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 equation35.
=
3I
0
EQUATION1271 V3 EN-US
Where:
3I
0
IR
IL
IC
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:
w =
L
× ×
3
EQUATION1272 V1 EN-US
Application manual
is the resistive positive sequence of the source
is the reactive positive sequence of the source
( )
2
+ -
I I I
2
R L C
is the earth-fault current (A)
is the current through the neutral point resistor (A)
is the current through the neutral point reactor (A)
is the total capacitive earth-fault current (A)
1
w
C
Section 7
Impedance protection
GUID-02F306F5-1038-42AC-AFAE-3F8423C4C066 v4
) will
0
(Equation 35)
(Equation 36)
103