ABB REL650 series, Applications Manual

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Ic Ic Ic
IL IR
en05000216_ansi.vsd
ANSI05000216 V1 EN
Figure 51: High impedance grounded 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 of phenomenon, a separate function called Phase preference logic
(PPLPHIZ) is needed in medium and subtransmission network.
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.
6.1.2.2 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 52 , the equation for the bus voltage V
A
at A side is:
( )
A A L A B f
V I p Z I I R
= × × + + ×
EQUATION1273 V1 EN
(Equation 41)
If we divide V
A
by I
A
we get Z present to the IED at A side.
1MRK 506 334-UUS A
Section 6
Impedance protection
117
Application manual