ABB REL670 2.2 IEC, Applications Manual

Page: 217 / 908

Z
0
is the zero sequence impedance (Ω/phase)
Z
f
is the fault impedance (Ω), often resistive
Z
N
is the earth-return impedance defined as (Z
0
-Z
1
)/3
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
M17048-38 v6
A network is defined as effectively earthed if the earth-fault factor f
e
is less than
1.4. The earth-fault factor is defined according to equation 102 .
f
U
U
e
pn
=
max
EQUATION1268 V4 EN-US
(Equation 102)
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, see equation
103 and equation 104 .
0 1
X 3 X
< ×
EQUATION2122 V1 EN-US
(Equation 103)
0 1
R
R
£
EQUATION2123 V1 EN-US
(Equation 104)
Where
R
0
is the zero sequence source resistance
X
0
is the zero sequence source reactance
R
1
is the positive sequence source resistance
X
1
is the positive sequence source reactance
1MRK 506 369-UEN B
Section 8
Impedance protection
Line distance protection REL670 2.2 IEC 211
Application manual