ABB Relion REG670, Applications Manual

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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
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 34 .
f
U
U
e
pn
=
max
EQUATION1268 V4 EN
(Equation 34)
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
35 and 36 .
0 1
X 3 X
< ×
EQUATION2122 V1 EN
(Equation 35)
0 1
R
R
£
EQUATION2123 V1 EN
(Equation 36)
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
1MRK 502 071-UEN -
Section 8
Impedance protection
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062 189
Application manual