IEC05000216 V2 EN
Figure 124: High impedance grounding network
The operation of high impedance grounded networks is different compared to solid
grounded networks where all major faults have to be cleared very fast. In high impedance
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 Phase preference logic function (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.
8.4.2.3 Fault infeed from remote end
All transmission and most all sub-transmission networks are operated meshed. Typical for
this type of network is that we will have fault infeed from remote end when fault occurs
on the protected line. The fault 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
125, we can draw the equation for the bus voltage V
A
at left side
as:
( )A A L A B fV I p Z I I R= × × + + ×
EQUATION1273 V1 EN (Equation 157)
If we divide V
A
by IA we get Z present to the IED at A side
1MRK 506 369-UUS - Section 8
Impedance protection
Line distance protection REL670 2.2 ANSI 255
Application manual
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