ABB REL670 2.2 IEC, Applications Manual

Page: 299 / 908

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
232 .
( )
2
2
R
L C
0
3I I I I
= + -
EQUATION1271 V3 EN-US
(Equation 232)
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 233)
IEC05000216 V2 EN-US
Figure 147: 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. To handle this type phenomena, a separate function called
Phase preference logic (PPLPHIZ) is needed, which is not common to be used in
transmission applications.
In this type of network, it is mostly not possible to use distance protection for
detection and clearance of earth-faults. The low magnitude of the earth-fault
1MRK 506 369-UEN B
Section 8
Impedance protection
Line distance protection REL670 2.2 IEC 293
Application manual