ABB RELION RET670 Applications Manual page 264

Section 8
Impedance protection
258
ANSI05000215 V2 EN-US
Figure 120: 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 109:
×
3 V
=
3I A
0
+ + +
Z Z Z 3Z
1 2 0 f
EQUATION1710 V2 EN-US
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), is considered to be
2
equal to Z
1
Z is the zero sequence impedance (Ω/phase)
0
Z is the fault impedance (Ω), often resistive
f
Z is the ground-return impedance defined as (Z
N
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 solidly grounded networks makes it possible to use
impedance measuring techniques to detect ground faults. 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
1 N f
-Z
0
1MRK 504 163-UUS A
)/3
1
Transformer protection RET670 2.2 ANSI
Application manual
(Equation 109)