ABB RELION RET670 Applications Manual page 328

Section 8
Impedance protection
322
ANSI05000215 V2 EN-US
Figure 150: Solidly grounded network.
The ground-fault current is as high or even higher than the short-circuit current. The
series impedances determine the magnitude of the fault current. The shunt admittance
has very limited influence on the ground-fault current. The shunt admittance may,
however, have some marginal influence on the ground-fault current in networks with
long transmission lines.
The ground-fault current at single phase-to- ground in phase A can be calculated as
equation :
×
3 V
=
3I A
0
+ + +
Z Z Z 3Z
1 2 0 f
EQUATION1710 V2 EN-US
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 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 should therefore always be
complemented with other protection function(s) that can carry out the fault clearance in
those cases.
V
=
A
+ +
Z Z Z
1 N f
-Z
0
1MRK 504 163-UUS A
)/3
1
Transformer protection RET670 2.2 ANSI
Application manual
(Equation 200)