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 being in phase with
the sending end voltages. Therefore both capacitane 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 in long line by equivalent π model

It is
proportional to the square of lengths of lines, that is, ∆V α kx2

, where x is the
length of line and k is a constant for all voltage levels

In
general practice we know, that for all electrical systems electric current
flows from the region of higher potential to the region of lower potential, to
compensate for the electrical potential
difference that exists in the system. In all practical cases the sending
end voltage is higher than the receiving end, so electric current flows from
the source or the supply end to the load. But Sir S.Z. Ferranti, in the year
1890, came up with an astonishing theory about medium distance transmission line or long distance transmission linessuggesting that in
case of light loading or no load operation of transmission system, the
receiving end voltage often increases beyond the sending end voltage, leading
to a phenomena known as Ferranti effect in power system.

Why Ferranti Effect
occurs in a Transmission Line?

A long transmission line can be
considered to composed a considerably high amount of capacitance and inductance
distributed across the entire length of the line. Ferranti Effect occurs when
current drawn by the distributed capacitance of the line itself is greater than
the current associated with the load at the receiving end of the line( during
light or no load). This capacitor charging current leads to voltage drop across
the line inductance of the transmission system which is in phase with the
sending end voltages. This voltage drop keeps on increasing additively as we
move towards the load end of the line and subsequently the receiving end
voltage tends to get larger than applied voltage leading to the phenomena
called Ferranti effect in power system. It is illustrated with the help of a
phasor diagram below.

Thus both the capacitance and inductance effect
of transmission line are equally responsible for this particular phenomena to
occur, and hence Ferranti effect is negligible in case of a short transmission lines as the
inductance of such a line is practically considered to be nearing zero. In
general for a 300 Km line operating at a frequency of 50 Hz, the no load
receiving end voltage has been found to be 5% higher than the sending end
voltage.

Now for analysis of Ferranti effect let us
consider the phasor diagrame shown above.

Here V

_{r}is considered to be the reference phasor, represented by OA.
This is represented by the phasor OC.

Now in case of a long transmission line, it has been
practically observed that the line electrical resistance is negligibly small
compared to the line reactance, hence we can assume the length of the phasor I

_{c}R = 0, we can consider the rise in the voltage is only due to OA - OC = reactive drop in the line.
Now if we consider c

_{0}and L_{0}are the values of capacitance and inductance per km of the transmission line, where l is the length of the line.
Since, in case of a long transmission line, the capacitance is
distributed throughout its length, the average current flowing is,

Thus the rise in voltage due to line inductance
is given by,

From the above equation it is absolutely
evident, that the rise in voltage at the receiving end is directly proportional
to the square of the line length, and hence in case of a long transmission line it keeps
increasing with length and even goes beyond the applied sending end voltage at
times, leading to the phenomena called Ferranti effect in power system.

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