System Grounding - ABB RELION 670 SERIES Applications Manual

Section 8
Impedance protection
8.12.2.1
356

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.
Solidly grounded networks
In solidly grounded systems, the transformer neutrals are connected directly to ground
without any impedance between the transformer neutral and ground.
ANSI05000215 V2 EN
Figure 171: 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:
×
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
V
=
A
+ +
Z Z Z
1 N f
-Z
0
1MRK 506 369-UUS -
)/3
1
Line distance protection REL670 2.2 ANSI
Application manual
(Equation 289)