ABB RET670 - Page 167

Z
0
is the zero sequence impedance (Ω/phase)
Z
f
is the fault impedance (Ω), often resistive
Z
N
is the ground-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-
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
e
is less than
1.4. The ground-fault factor is defined according to equation
52.
max
e
pn
V
f
V
=
ANSIEQUATION1268 V1 EN (Equation 52)
Where:
V
max
is the highest fundamental frequency voltage on one of the healthy phases at single phase-
to-ground fault.
V
pn
is the phase-to-ground fundamental frequency voltage before fault.
Another definition for effectively grounded network is when the following
relationships between the symmetrical components of the network impedances are
valid, see equation 53 and equation 54.
0 1
X 3 X< ×
EQUATION2122 V1 EN (Equation 53)
0 1
R R£
EQUATION2123 V1 EN (Equation 54)
Where
R
0
is setting of the resistive zero sequence reach
X
0
is setting of the reactive zero sequence reach
R
1
is setting of the resistive positive sequence reach
1MRK504116-UUS C Section 3
IED application
161
Application manual