ABB REL670 2.2 IEC - System Earthing

8.4.2.2 System earthing
SEMOD154453-11 v3
The type of system earthing plays an important role when designing the protection
system. In the following some hints with respect to distance protection are
highlighted.
Solid earthed networks
SEMOD154453-14 v5
In solid earthed systems the transformer neutrals are connected solidly to earth
without any impedance between the transformer neutral and earth.
IEC05000215 V2 EN-US
Figure 121: Solidly earthed network
The earth-fault current is as high as or even higher than the short-circuit current.
The series impedances determine the magnitude of the earth-fault current. The
shunt admittance has very limited influence on the earth-fault current. The shunt
admittance may, however, have some marginal influence on the earth-fault current
in networks with long transmission lines.
The earth-fault current at single phase-to-earth in phase L1 can be calculated as
equation
148:
L1 L1
0
1 2 0 f 1 N f
3 U U
3I
Z Z Z 3Z Z ZZ
×
= =
+ + + + +
EQUATION1267 V3 EN-US (Equation 148)
Where:
U
L1
is the phase to earth 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 earth return impedance defined as (Z
0
-Z
1
)/3
The voltage on the healthy phases is generally lower than 140% of the nominal
phase-to-earth voltage. This corresponds to about 80% of the nominal phase-to-
phase voltage.
Section 8 1MRK 506 369-UEN B
Impedance protection
236 Line distance protection REL670 2.2 IEC
Application manual