ABB RELION RET670 Applications Manual page 375

1MRK 504 163-UUS A
Transformer protection RET670 2.2 ANSI
Application manual
Where:
VA is the phase-to-ground voltage (kV) in the faulty phase before fault
Z is the positive sequence impedance (Ω/phase)
1
Z is the negative sequence impedance (Ω/phase)
2
Z is the zero sequence impedance (Ω/phase)
0
Z is the fault impedance (Ω), often resistive
f
Z is the ground-return impedance defined as (Z
N
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
A network is defined as effectively grounded if the ground-fault factor f
1.4. The ground-fault factor is defined according to equation 259.
V
=
max
f
e
V
pn
ANSIEQUATION1268 V1 EN-US
Where:
V is the highest fundamental frequency voltage on one of the healthy phases at single
max
phase-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 260 and 261.
< ×
X 3 X
0 1
EQUATION2122 V1 EN-US
Impedance protection
-Z )/3
0 1
GUID-39CAF169-315E-4E3E-9EE6-28CBF624B90E v5
Section 8
is less than
e
(Equation 259)
(Equation 260)
369