ABB REL670 2.2 IEC Applications Manual page 217

1MRK 506 369-UEN B
Line distance protection REL670 2.2 IEC
Application manual
Z is the zero sequence impedance (Ω/phase)
0
Z is the fault impedance (Ω), often resistive
f
Z is the earth-return impedance defined as (Z
N
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 solidly earthed networks makes it possible to use
impedance measuring techniques to detect earth faults. 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 102.
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 103
< ×
X 3 X
0 1
EQUATION2122 V1 EN-US
£
R
R
0 1
EQUATION2123 V1 EN-US
Where
R is the zero sequence source resistance
0
X is the zero sequence source reactance
0
R is the positive sequence source resistance
1
X is the positive sequence source reactance
1
-Z
0
and equation 104.
Section 8
Impedance protection
)/3
1
is less than
e
M17048-38 v6
(Equation 102)
(Equation 103)
(Equation 104)
211