Where:
U
L1
is the phase-to-earth voltage (kV) in the faulty phase before
fault
Z
1
is the positive sequence impedance (Ω/phase)
Z
2
is the negative sequence impedance (Ω/phase)
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 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
GUID-39CAF169-315E-4E3E-9EE6-28CBF624B90E v4
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
f
U
U
e
pn
=
max
EQUATION1268 V4 EN-US
(Equation 310)
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
0
1
X
3 X
< ×
EQUATION2122 V1 EN-US
(Equation 311)
0
1
R
R
£
EQUATION2123 V1 EN-US
(Equation 312)
Where
R
0
is the resistive zero sequence of the source
X
0
is the reactive zero sequence of the source
R
1
is the resistive positive sequence of the source
X
1
is the reactive positive sequence of the source
The magnitude of the earth-fault current in effectively earthed networks is high enough for
impedance measuring elements to detect earth faults. However, in the same way as for solidly
1MRK 505 343-UEN B
Section 7
Impedance protection
303
Application manual
Summary of Contents for Relion 670 series
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