ABB REL670 2.2 IEC, Applications Manual

Page: 154 / 908

8.1.2.2 System earthing
SEMOD168232-11 v2
The type of system earthing plays an important roll when designing the protection
system. In the following sections, some hints with respect to distance protection are
highlighted.
Solid earthed networks
SEMOD168232-14 v2
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 55: Solidly earthed network
The earth 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 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
21 :
L1 L1
0
1 2 0 f 1 N f
3 U U
3I
Z Z Z 3Z Z Z Z
×
= =
+ + + + +
EQUATION1267 V3 EN-US
(Equation 21)
Where:
UL1 is the phase-to-earth 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 earth return impedance defined as (Z0-Z1)/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
148 Line distance protection REL670 2.2 IEC
Application manual