Long transmission line draws substantial quantity of charging current. If such a line is
open circuited or lightly loaded at the remote end, the voltage at remote end may exceeds
local end voltage. This is known as Ferranti effect and is due to the voltage drop across the
line inductance (due to charging current) being in phase with the local end voltages. Both
capacitance and inductance are responsible for this phenomenon. The capacitance (and
charging current) is negligible in short line but significant in medium line and appreciable
in long line. The percentage voltage rise due to the Ferranti effect between local end and
remote end voltage is proportional to the length of the line and the properties of the
transmission line. The Ferranti effect is symmetrical between all three phases for normal
balanced load condition. The overvoltage caused by Ferranti effect can be reduced by
drawing larger load through the line or switching in the shunt reactor (connected either to
line or to remote bus) at the remote end. The calculated compensated voltage at the local
end can detect such overvoltage phenomenon.
The vector representation of local end and remote end voltages are shown below:
ANSI09000774-1-en.vsd
O
M
N
P
C
V
r
V
s
I
c
R
I
c
X
I
c
R
L
V
s
V
R
1
2
C
1
2
C
ANSI09000774 V1 EN
Figure 382: Vector diagram for local end and remote end voltage at no power transfer
conditions
Where:
OM Remote end voltage Vr
OP Local end voltage Vs
OC Current drawn by capacitance (Ic)
MN Resistance drop (IcR)
NP Inductive reactance drop (IcX)
If there is a transmission line that is opened at the remote end or radial or remote end source
is weak, then a fault anywhere on the line can result into undervoltage at the remote end.
There can be undervoltage at remote end also due to heavy loading or poor power factor
on lagging side. A fault in a line connected beyond the remote end bus can also produce
undervoltage at remote end. The compensated voltage calculated at the local end can
detect such undervoltages. The undervoltage caused by a fault can be asymmetrical while
that due to overloading is symmetrical.
1MRK 506 369-UUS - Section 16
Scheme communication
Line distance protection REL670 2.2 ANSI 779
Application manual
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