ABB REL670 2.2 IEC Applications Manual page 243

1MRK 506 369-UEN B
Line distance protection REL670 2.2 IEC
Application manual
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 f
1.4. The earth-fault factor is defined according to equation 149.
U
f
max
=
e
U
pn
EQUATION1268 V4 EN-US
Where:
U is the highest fundamental frequency voltage on one of the healthy phases at single
max
phase-to-earth fault.
U is the phase-to-earth fundamental frequency voltage before fault.
pn
Another definition for effectively earthed network is when the following
relationships between the symmetrical components of the network impedances are
valid, see equation 150
X ≤ ⋅ 3 X
0 1
EQUATION1269 V4 EN-US
£
R X
0 1
EQUATION1270 V4 EN-US
Where
R
is the zero sequence resistance
0
X is the zero sequence reactance
0
X
is the positive sequence reactance
1
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 should therefore always be
complemented with other protection function(s) that can carry out the fault
clearance in this case.
and equation 151.
Section 8
Impedance protection
SEMOD154453-40 v5
is less than
e
(Equation 149)
(Equation 150)
(Equation 151)
237