ABB RET670 Applications Manual page 166

Section 3
IED application
160
stringent demands on the fault clearing equipment in order to maintain an unchanged
or increased security level of the power system.
The distance protection function in the IED is designed to meet basic requirements for
application on transmission and sub-transmission lines (solid grounded systems)
although it also can be used on distribution levels.
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 V1 EN
Figure 46: 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
Table continues on next page
V
=
A
+ +
Z Z Z
1 N f
1MRK504116-UUS C
xx05000215_ansi.vsd
(Equation 51)
Application manual