ABB RELION RET670, Applications Manual

Page: 267 / 1016

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.
8.3.2.2 Fault infeed from remote end
M17048-83 v5
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 122 , 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-US
(Equation 115)
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-US
(Equation 116)
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 504 163-UUS A
Section 8
Impedance protection
Transformer protection RET670 2.2 ANSI 261
Application manual