Selection of transmission voltage

1. Arpit Raval (161310109050)
Rohan Kaushik (161310109051)
Rohit Chavda (161310109052)

2.  In order for the electricity to be transmitted safely and efficiently over long
distances, it must be at a high voltage and a low current .
 This is because if the current is too high, the lines would heat up too much and
even melt. If the voltage were too low, hardly any energy would be carried
 For every transmission line there is a superior limit fixed for the voltage to be
employed, beyond which nothing is gained in the matter of economy.
 This limit is reached when the cost of conductors, cost of insulators, supports,
transformers, switchgear, and lightning arrestors and also the cost of erection is
minimum.

3.  Transmission voltage is increased, the volume of conductor material required is
reduced. This decreases the expenditure on the conductor material. It may appear
advisable to use the highest possible transmission voltage in order to reduce the
expenditure on conductors to a minimum.
 However, it may be remembered that as the transmission voltages is increased,
the cost of insulating the conductors, cost of transformers, switchgear and other
terminal apparatus also increases. Therefore, for every transmission line, there is
optimum transmission voltages, beyond which there is nothing to be gained in the
matter of economy.
 The transmission voltages for which the cost of conductors, cost of
insulators, transformers, switchgear and other terminal apparatus is minimum is
called economical transmission voltage.

4.  If the power to be transmitted and the length of
transmission are known, calculations are made for
various transmission voltages.
 Initially, some standard transmission voltage is
selected and the relative total cost of equipment is
determined.
 A graph is drawn for the total cost of transmission
with respect to various transmission voltages as
shown in the figure at right. The lowest point on the
curve gives the optimum transmission voltage.
 As here in the graph, point P is the lowest and the
corresponding voltage OA is the optimum
transmission voltage.

5.  An economical transmission voltage for a 3 phase AC system is given as:
Where, V = line voltage in kV
P = maximum power per phase (in kW) to be delivered over single circuit
L = distance of transmission in km
 It may be noted here that in the above formula, power to be transmitted and
distance of transmission line have been taken into account. It is because both
these factors influence the economic voltage of a transmission line.
 If the distance of transmission line is increased, the cost of terminal apparatus is
decreased, resulting in higher economic transmission voltages.

6.  Efficient transmission of larger amounts of power:
In a 3 phase AC system, power is calculated as P=√3*VIcosɸ. It is clear that, for a large
amount of power to be transmitted at a lower voltage, the amount of current will be very
large.
Let's take an example, 200 MW of power is to be transmitted at 11kV and consider cosɸ =
0.8 lagging. In this case, the amount of current that will flow through the line would be
200,000,000 / (√3 * 11,000 * 0.8) ≈ 13,122 A. For safely carrying this much large current, a
conductor with very large diameter or much more number of conductors in bundled form
may be required.
And if the same power is transmitted at 220kV, the current would be 200,000,000 / (√3 *
220,000 * 0.8 ) ≈ 656 A. As the power lost in a conductor is given as I2R, you can see large
saving in losses can be achieved by transmitting electricity at higher voltages.
From this example, it is clearly not feasible and practical to transmit larger power at lower
voltages. Also, transmission of electricity at higher voltages is more efficient.

7.  Saving in conductor material:
For the same amount of power transmitted at a higher voltage the current will be relatively
lower. Current carrying capacity of a conductor depends on the diameter of the conductor
(conductor size) along with few other factors.
That means, for larger currents to be transmitted, the conductor size must be larger.
Hence, transmitting power at higher voltages will reduce the amount of current to be
carried and consequently the required conductor size would also be lesser.
 Improved voltage regulation:
Decreased current will also result in decreased voltage drop across the line. Voltage
regulation is defined as (VS - VR)/VS. As voltage drop is decreased, the difference between
sending end voltage and receiving end voltage is also decreased. Thus, voltage regulation is
improved.

8. With increase in the transmission voltage:
 cost of insulators increases
 cost of transformers increases
 cost of switchgear increases
 cost of lightning arrestor increases
 cost of support towers increases (as taller towers with longer cross arms are
required