ABB RET670 Applications Manual page 323

1MRK504116-UUS C
Application manual
System grounding
The type of system grounding plays an important role when designing the protection
system. Some hints with respect to distance protection are highlighted below.
Solid grounded networks
In solidly grounded systems, the transformer neutrals are connected solidly to ground
without any impedance between the transformer neutral and ground.
ANSI05000215 V1 EN
Figure 139: 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 51:
×
3 V
=
3I A
0
+ + +
Z Z Z 3Z
1 2 0 f
EQUATION1710 V2 EN
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.
V
=
A
+ +
Z Z Z
1 N f
-Z
0
IED application
xx05000215_ansi.vsd
)/3
1
Section 3
(Equation 242)
317