ABB RED670 Handbuch Seite 286

Abschnitt 4
IED Anwendung
282
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 (3U0) 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 the formula
below:
= + -
2
3 I I ( I I
0 R L C
EQUATION1271 V1 DE
Where:
3I0 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 that is:
1
w
=
L
w
× ×
3 C
EQUATION1272 V1 DE
IR
IEC05000216 V1 DE
Abb. 147: High impedance earthing network.
The operation of high impedance earthed networks is different compare 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 line 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
2
)
Ic Ic
IL
1MRK 505 186-UDE B
(Gleichung 189)
(Gleichung 190)
Ic
en05000216.vsd
Applikationshandbuch