grounded networks, some system operators do not clear single phase-to- ground faults
immediately; they clear the line later when it is more convenient. In case of cross-country
faults, many network operators want to selectively clear one of the two ground-faults. To
handle this type phenomena, a separate function called Phase preference logic (PPLPHIZ)
is needed, which is not common to be used in transmission applications.
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 element 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.
8.9.2.2 Fault infeed from remote end
All transmission and most all 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.
With reference to figure
111, 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 (Equation 237)
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 (Equation 238)
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 506 369-UUS - Section 8
Impedance protection
Line distance protection REL670 2.2 ANSI 313
Application manual
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