ABB Relion 670 series - Page 165

Z0 is the zero sequence impedance (Ω/phase)
Zf is the fault impedance (Ω), often resistive
ZN is the ground return impedance defined as (Z0-Z1)/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 solid grounded networks makes it possible to use
impedance measuring technique to detect ground-fault. However, distance protection has
limited possibilities to detect high resistance faults and must, 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 fe is less than 1.4.
The ground-fault factor is defined according to equation 25.
max
e
pn
V
f
V
=
ANSIEQUATION1268 V1 EN (Equation 25)
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, as shown in
equation
26 and equation 27.
X X
0 1
3≤ ⋅
EQUATION1269 V4 EN
(Equation 26)
R X£
0 1
EQUATION1270 V4 EN (Equation 27)
The magnitude of the ground fault current in effectively grounded networks is high
enough for impedance measuring element to detect ground-fault. However, in the same
way as for solid grounded networks, distance protection has limited possibilities to detect
1MRK 506 369-UUS - Section 8
Impedance protection
Line distance protection REL670 2.2 ANSI 159
Application manual