ABB REL670 2.2 IEC, Applications Manual

Page: 155 / 908

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 must, therefore, always
be complemented with other protection function(s) that can carry out the fault
clearance in those cases.
Effectively earthed networks
SEMOD168232-40 v2
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
22 .
f
U
U
e
pn
=
max
EQUATION1268 V4 EN-US
(Equation 22)
Where:
U
max
is the highest fundamental frequency voltage on one of the healthy phases at single
phase-to-earth fault.
U
pn
is the phase-to-earth 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, as shown in equation
23 and equation 24 .
X X
0 1
3 ≤ ⋅
EQUATION1269 V4 EN-US
(Equation 23)
R X
£
0 1
EQUATION1270 V4 EN-US
(Equation 24)
The magnitude of the earth fault current in effectively earthed networks is high
enough for impedance measuring element to detect earth-fault. However, in the
same way as for solid earthed networks, distance protection has limited
possibilities to detect high resistance faults and must, therefore, always be
complemented with other protection function(s) that can carry out the fault
clearance in this case.
8.1.2.3 Fault infeed from remote end
SEMOD168232-84 v2
All transmission and most all sub transmission networks are operated meshed.
Typical for this type of network is that we will have fault infeed from remote end
when fault occurs on the protected line. The fault infeed may enlarge the fault
impedance seen by the distance protection. This effect is very important to keep in
mind when both planning the protection system and making the settings.
1MRK 506 369-UEN B
Section 8
Impedance protection
Line distance protection REL670 2.2 IEC 149
Application manual