ABB RELION RET670 Applications Manual page 297

1MRK 504 163-UUS A
Transformer protection RET670 2.2 ANSI
Application manual
ANSI05000215 V2 EN-US
Figure 137: 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
equation156:
×
3 V
=
3 I A
0
+ + +
Z Z Z 3
1 2 0
EQUATION1782-ANSI V1 EN-US
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
The voltage on the healthy phases is generally lower than 140% of the nominal phase-
to-ground voltage. This corresponds to about 80% of the nominal phase-to-phase
voltage.
The high zero sequence current in solid grounded networks makes it possible to use
impedance measuring technique to detect ground fault. However, distance protection
has limited possibilities to detect high resistance faults and should therefore always be
complemented with other protection function(s) that can carry out the fault clearance in
those cases.
V
=
A
+ +
Z Z Z Z
f 1 N f
Section 8
Impedance protection
(Equation 156)
291