The magnitude of the ground-fault current in effectively grounded networks is high
enough for impedance measuring elements to detect ground faults. However
, in the
same way as for solidly grounded networks, 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
this case.
High impedance grounded networks
GUID-123D2D9D-8055-4A2F-9CC2-6EAE0C9F905B v2
In this type of network, it is mostly not possible to use distance protection for detection
and clearance of ground faults. The low magnitude of the ground fault current might
not give pickup of the zero-sequence measurement elements or the sensitivity will be
too low for acceptance. For this reason a separate high sensitive ground fault protection
is necessary to carry out the fault clearance for single phase-to-ground fault.
8.3.2.2 Fault infeed from remote end
GUID-DDFE8B5C-A064-4956-B8BB-49DEAC38558E v3
All transmission and most sub-transmission networks are operated meshed. Typical for
this type of network is that fault infeed from remote end will happen when fault occurs
on the protected line. The fault current infeed will enlarge the fault impedance seen by
the distance protection. This effect is very important to keep in mind when both
planning the protection system and making the settings.
The equation for the bus voltage V
A
at A side is:
( )A A L A B fV I p Z I I R= × × + + ×
EQUATION1273 V1 EN-US (Equation 80)
If we divide V
A
by I
A
we get Z present to the IED at A side:
a
A B
A L f
A A
V
I I
Z p Z R
I I
+
= = × + ×
EQUATION1274 V2 EN-US (Equation 81)
The infeed factor (I
A
+I
B
)/I
A
can be very high, 10-20 depending on the differences in
source impedances at local and remote end.
1MRK 502 071-UUS A Section 8
Impedance protection
Generator protection REG670 2.2 ANSI and Injection equipment REX060, REX061, REX062 231
Application manual
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