ABB RET670 Applications Manual page 293

1MRK504116-UUS C
Application manual
preference logic (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 start 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 128, we can draw the equation for the bus voltage Va at left
side as:
= × × + +
V I p Z ( I
A A L A
EQUATION1273 V1 EN
If we divide Va by IA we get Z present to the IED at A side
V
I
= = × +
Z a p Z A
A L
I
A
EQUATION1274 V2 EN
The infeed factor (IA+IB)/IA can be very high, 10-20 depending on the differences in
source impedances at local and remote end.
V
A
Es
A
A
Z
SA
IEC05000217 V1 EN
Figure 128: Influence of fault infeed from remote end.
×
I ) R
B f
+
I
×
B R
f
I
A
p*ZL
(1-p)*ZL
I
A
ZL
Z <
R
IED application
VA
B
I
Z
B
Z <
f
en05000217.vsd
Section 3
(Equation 204)
(Equation 205)
Es
B
SB
287