ABB REL650 series, Applications Manual

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selecting the different characteristics for different loops of a zone makes ZQMPDIS (21)
highly effective distance protection function.
6.1.2.1 System grounding
The type of system grounding plays an important role when designing the protection
system. Some hints with respect to distance protection are highlighted below.
Solidly grounded networks
In solidly grounded systems, the transformer neutrals are connected directly to ground
without any impedance between the transformer neutral and ground.
xx05000215_ansi.vsd
ANSI05000215 V1 EN
Figure 50: 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
35 :
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 35)
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
Section 6
1MRK 506 334-UUS A
Impedance protection
114
Application manual