8.4.2.2 System grounding
The type of system grounding plays an important role when designing the protection
system. In the following some hints with respect to distance protection are highlighted.
Solid grounded networks
In solid grounded systems the transformer neutrals are connected solidly to ground
without any impedance between the transformer neutral and ground.
ANSI05000215 V2 EN
Figure 123: Solidly grounded network
The ground-fault current is as high as or even higher than the short-circuit current. The
series impedances determine the magnitude of the ground-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
151:
3
×
V
A
3I
0
Z
1
+Z
2
+Z
0
+3Zf
=
V
A
=
Z
1
+ZN+Zf
EQUATION1748-ANSI V2 EN
(Equation 151)
Where:
V
A
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
Section 8 1MRK 506 369-UUS -
Impedance protection
252 Line distance protection REL670 2.2 ANSI
Application manual
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