ABB RELION RET670 - Page 375

Where:
VA is the phase-to-ground 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 ground-return impedance defined as (Z
0
-Z
1
)/3
The voltage on the healthy phases during line to ground fault 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
GUID-39CAF169-315E-4E3E-9EE6-28CBF624B90E v5
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
259.
max
e
pn
V
f
V
=
ANSIEQUATION1268 V1 EN-US (Equation 259)
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
260 and 261.
0 1
X 3 X< ×
EQUATION2122 V1 EN-US (Equation 260)
1MRK 504 163-UUS A Section 8
Impedance protection
Transformer protection RET670 2.2 ANSI 369
Application manual