A A
0
1 2 0 f 1 N f
3 V
3I
Z Z Z 3Z Z Z Z
V×
= =
+ + + + +
EQUATION1710 V2 EN (Equation 335)
Where:
VA is the phase-to-ground 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 ground-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-
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.
Effectively grounded networks
A network is defined as effectively grounded if the ground-fault factor f
e
is less than 1.4.
The ground-fault factor is defined according to equation 25:
ANSIEQUATION1268 V1 EN
(Equation 336)
Where:
V
max
is the highest fundamental frequency voltage on one of the healthy phases at single phase-
to-ground fault.
V
pn
is the phase-to-ground fundamental frequency voltage before fault.
Another definition for effectively grounded network is when the following relationships
between the symmetrical components of the network impedances are valid, see equations
106 and 107:
1MRK 506 369-UUS - Section 8
Impedance protection
Line distance protection REL670 2.2 ANSI 385
Application manual
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