ABB RED670 Handbuch Seite 285

1MRK 505 186-UDE B
Applikationshandbuch
The voltage on the healthy phases is generally lower than 140% of the nominal phase-
to-earth voltage. This corresponds to about 80% of the nominal phase-to-phase
voltage.
The high zero sequence current in solid earthed networks makes it possible to use
impedance measuring technique to detect earth fault. However, 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 those cases.
Effectively earthed networks
A network is defined as effectively earthed if the earth fault factor fe is less than
1.4. The earth fault factor is defined according to equation 186.
V
max
=
f
e
V
pn
EQUATION1268 V1 DE
Where:
Vmax is the highest fundamental frequency voltage on one of the healthy phases at single line
to earth fault.
Vpn is the phase to earth fault fundamental frequency voltage before fault.
Another definition for effectively earthed network is when the following
relationships between the symmetrical components of the network impedances are
valid, see equation 187
< ×
X 3 X
0 1
EQUATION1269 V1 DE
£
R R
0 1
EQUATION1270 V1 DE
The magnitude of the earth fault current in effectively earthed networks is high
enough for impedance measuring element to detect fault. However, in the same
way as for solid 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 many times operated in radial, but can also be found
operating meshed.
and equation 188.
Abschnitt 4
IED Anwendung
(Gleichung 186)
(Gleichung 187)
(Gleichung 188)
281