ABB REL650 Applications Manual page 121

1MRK 506 334-UUS A
Application manual
The voltage on the healthy phases is generally lower than 140% of the nominal phase-to-
ground voltage. This corresponds to about 80% of the nominal phase-to-phase voltage.
The high zero-sequence current in solidly grounded networks makes it possible to use
impedance measuring techniques to detect ground 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 grounded networks
A network is defined as effectively grounded if the ground-fault factor f
The ground-fault factor is defined according to equation 36.
V
=
f max
e
V
pn
ANSIEQUATION1268 V1 EN
Where:
V is the highest fundamental frequency voltage on one of the healthy phases at single phase-
max
to-ground fault.
V is the phase-to-ground fundamental frequency voltage before fault.
pn
Another definition for effectively grounded network is when the following relationships
between the symmetrical components of the network impedances are valid, see
equation 37 and equation 38.
< ×
X 3 X
0 1
EQUATION2122 V1 EN
£
R
R
0 1
EQUATION2123 V1 EN
Where
R is the resistive zero sequence reach
0
X is the reactive zero sequence reach
0
R is the resistive positive sequence reach
1
X is the reactive positive sequence reach
1
Section 6
Impedance protection
is less than 1.4.
e
(Equation 36)
(Equation 37)
(Equation 38)
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