occurs on the protected line. The fault infeed may 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
61, we can draw the equation for the bus voltage Va at left
side as:
( )A A L A B fV I p Z I I R= × × + + ×
EQUATION1273 V1 EN (Equation 102)
If we divide Va by IA 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 (Equation 103)
The infeed factor (IA+IB)/IA can be very high, 10-20 depending on the differences in
source impedances at local and remote end.
21
ZL
21
EsA
VA
VA
A
B
EsB
IA
IB
Rf
p*ZL
(1-p)*ZL
ZSA
ZSB
en05000217_ansi.vsd
ANSI05000217 V1 EN
Figure 61: Influence of fault infeed from remote end
The effect of fault current infeed from remote end is one of the most driving factors to
justify complementary protection to distance protection.
Load encroachment
Sometimes the load impedance might enter the zone characteristic without any fault on
the protected line. The phenomenon is called load encroachment and it might occur
when an external fault is cleared and high emergency load is transferred on the
protected line. The effect of load encroachment is illustrated to the left in figure
62.
The entrance of the load impedance inside the characteristic is not allowed and the way
to handle this with conventional distance protection is to consider this with the settings
that is, to have a security margin between the distance zone and the minimum load
Section 3 1MRK504116-UUS C
IED application
192
Application manual
To Next Page
Loading...
