ABB RET670 - Page 270

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.
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
114, we can draw the equation for the bus voltage V
A
at left
side as:
If we divide V
A
by IA we get Z present to the IED at A side
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.
Z <
ZL
Z <
EsA
VA
VA
A
B
EsB
IA
IB
Rf
p*ZL
(1-p)*ZL
ZSA
ZSB
en05000217.vsd
IEC05000217 V1 EN
Figure 114: Influence of fault current infeed from remote end.
The effect of fault current infeed from remote end is one of the most driving factors for
justify complementary protection to distance protection.
Section 3 1MRK504116-UUS C
IED application
264
Application manual