ABB RET670 Applications Manual page 169

1MRK504116-UUS C
Application manual
IR
ANSI05000216 V1 EN
Figure 47: 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.
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 48, the equation for the bus voltage V
= × × + +
V I p Z ( I
A A L A
EQUATION1273 V1 EN
If we divide V by I we get Z present to the IED at A side.
A A
V I
= = × +
Z a p Z A
A L
I
A
EQUATION1274 V2 EN
Ic Ic
IL
×
I ) R
B f
+
I
×
B R
f
I
A
Section 3
IED application
Ic
en05000216_ansi.vsd
at A side is:
A
(Equation 57)
(Equation 58)
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