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Ferranti Effect PDF

1) The Ferranti Effect occurs when a long transmission line is open circuited or lightly loaded at the receiving end, causing the receiving end voltage to become greater than the sending end voltage. 2) This is due to the voltage drop across the line inductance being in phase with the sending end voltage. Both the capacitance and inductance of the long line contribute to this effect. 3) The Ferranti Effect can be modeled using a π-model equivalent circuit. For an open circuit line, the receiving end voltage will be greater than the sending end voltage due to the voltage drop across the line inductance.

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352 views2 pages

Ferranti Effect PDF

1) The Ferranti Effect occurs when a long transmission line is open circuited or lightly loaded at the receiving end, causing the receiving end voltage to become greater than the sending end voltage. 2) This is due to the voltage drop across the line inductance being in phase with the sending end voltage. Both the capacitance and inductance of the long line contribute to this effect. 3) The Ferranti Effect can be modeled using a π-model equivalent circuit. For an open circuit line, the receiving end voltage will be greater than the sending end voltage due to the voltage drop across the line inductance.

Uploaded by

PrakashAcharya
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© © All Rights Reserved
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Ferranti Effect

A long transmission line draws a substantial quantity of charging current. If such a line is open
circuited or very lightly loaded at the receiving end, the voltage at receiving end may become greater
than voltage at sending end. 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 sending end voltages. Therefore both
capacitance and inductance is responsible to produce this phenomenon.
The capacitance (and charging current) is negligible in short line but significant in medium line and
appreciable in long line. Therefore this phenomenon occurs in medium and long lines.
Represent line by equivalent -model.
C

P
N

Vs

Ic

Vr

Line capacitance is assumed to be concentrated at the receiving end.


OM = receiving end voltage Vr
OC = Current drawn by capacitance = Ic
MN = Resistance drop
NP = Inductive reactance drop
Therefore;
OP = Sending end voltage at no load and is less than receiving end voltage (Vr)
Since, resistance is small compared to reactance; resistance can be neglected in calculating Ferranti
effect.
From -model,
YZ
V s = 1 +
Vr + ZI r
2

For open circuit line; Ir = 0


YZ
V s = 1 +
Vr
2

{refer eq. 5, in -model ckt. derivation}

or;

YZ
V s V r = 1 +
Vr Vr
2

or;

YZ
Vs Vr =
Vr
2

YZ
= V r 1 +
1
2

( jCl ) (r + jL )l V
2

Neglecting resistance;
V s Vr =

Vr 2 l 2 LC
2

is constant in all line and is equal to velocity of propagation of electromagnetic


LC
waves (= 3 105 km/sec)
1
1
LC =
LC =
5
2
3 10
3 10 5
The quantity

Substituting the value in above equation;


Vr 2 l 2
Vs Vr =
2
2 3 10 5

V s Vr =

Vr 2 l 2 10 10
18

2 l 2 10 10
Vs = Vr 1

18

Now, from above expression;


2 l 2 10 10
1

18

< 1

Vs < Vr

Vr > Vs

So,
or;

i.e. receiving end voltage is greater than sending end voltage and this effect is called Ferranti Effect. It
is valid for open circuit condition of long line.

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