ANSI05000215 V2 EN
Figure 148: 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
104:
A A
0
1 2 0 f 1 N f
3 V
3I
Z Z Z 3Z Z Z Z
V×
= =
+ + + + +
EQUATION1710 V2 EN (Equation 231)
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 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.
Section 8 1MRK 506 369-UUS -
Impedance protection
310 Line distance protection REL670 2.2 ANSI
Application manual
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