The distance protection function in 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.
8.5.2.2 System grounding
The type of system grounding plays an important roll 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 135: 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 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 L1 can be calculated as
equation
187:
0
1 2 0 1
3
3
3
A A
f N f
V V
I
Z Z Z Z Z Z Z
×
= =
+ + + + +
EQUATION1782-ANSI V1 EN (Equation 187)
Where:
VA is the phase-to-ground voltage (kV) in the faulty phase before fault
Z1 is the positive sequence impedance (Ω/phase)
Z2 is the negative sequence impedance (Ω/phase)
Z0 is the zero sequence impedance (Ω/phase)
Zf is the fault impedance (Ω), often resistive
ZN is the ground return impedance defined as (Z0-Z1)/3
1MRK 506 369-UUS - Section 8
Impedance protection
Line distance protection REL670 2.2 ANSI 279
Application manual
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