ABB REL670 2.2 IEC, Applications Manual

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( )
2
2
R L C
0
3I I I I
= + -
EQUATION1271 V3 EN-US
(Equation 290)
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 291)
IEC05000216 V2 EN-US
Figure 170: 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.
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
current might not give start of the zero-sequence measurement elements or the
sensitivity will be too low for acceptance. For this reason a separate high sensitive
earth-fault protection is necessary to carry out the fault clearance for single phase-
to-earth fault. For cross-country faults and when using phase preference, it is
necessary to make sure that the distance protection is operating in the phase-to-
earth loops independently, whenever possible. See guidelines for setting
INReleasePE .
1MRK 506 369-UEN B
Section 8
Impedance protection
Line distance protection REL670 2.2 IEC 335
Application manual