Power Transmission

Supply Systems
 Introduction
• In early days, there was a little demand for electrical energy so that
small power stations were built to supply lighting and heating loads.
• However, the widespread use of electrical energy by modern
civilization has necessitated to produce bulk electrical energy
economically and efficiently.
• The increased demand of electrical energy can be met by building
big power stations at favorable places where fuel (coal or gas) or
water energy is available in abundance.
• This has shifted the site of power stations to places quite away from
the consumers.
• The electrical energy produced at the power stations has to be
supplied to the consumers.
• There is a large network of conductors between the power station
and the consumers. This network can be broadly divided into two
parts viz., transmission and distribution.
• The purpose of this chapter is to focus attention on the various
aspects of transmission of electric power.
 7.1 Electric Supply System
An electric supply system consists of three principal components;
The power station (Generating systems)
The transmission lines
And the distribution system.

Fig 1. General Layout of Electric Supply System

• An electric supply system consists of three principal components
viz.,
The power station,
The transmission lines
And the distribution system.

• Electric power is produced at the power stations which are located at

favorable places, generally quite away from the consumers.
• It is then transmitted over large distances to load centers with the
help of conductors known as transmission lines.
• Finally, it is distributed to a large number of small and big
consumers through a distribution network.
• The electric supply system can be broadly classified into
(i) d.c. or a.c. system
(ii) overhead or underground system.
• Now-a days, 3-phase, 3-wire a.c. system is universally adopted for
generation and transmission of electric power as an economical
proposition.
• However, distribution of electric power is done by 3-phase, 4-wire
a.c. system.
• The underground system is more expensive than the overhead
system.
• Therefore, in Pakistan, overhead system is *mostly adopted for
transmission and distribution of electric power.
*In certain densely populated cities, the underground system is being employed for
distribution.
This is to eliminate the danger to human life which would be present with overhead
system and to avoid ugly appearance and inconvenience of pole lines running down
the main thorough fares.
 Typical AC Power Supply Scheme

• The large network of conductors between the power station and the
consumers can be broadly divided into two parts viz., transmission
system and distribution system.
• Each part can be further sub-divided into two—primary
transmission and secondary transmission and primary distribution
and secondary distribution.
• Fig. 7.1. shows the layout of a typical a.c. power supply scheme by
a single line diagram.
• It may be noted that it is not necessary that all power schemes
include all the stages shown in the figure.
• For example, in a certain power scheme, there may be no
secondary transmission and in another case, the scheme may be so
small that there is only distribution and no transmission.
 Typical a.c. Power Supply Scheme
(i) Generating station:
• In Fig 7.1, G.S. represents the generating station where
electric power is produced by 3-phase alternators operating in
parallel.
• The usual generation voltage is †11 kV. For economy in the
transmission of electric power, the generation voltage (i.e., 11
kV) is stepped upto 132 kV (or **more) at the generating
station with the help of 3-phase transformers.

† It
may be 6·6 kV or even 33 kV in certain cases.
** Depending upon the length of transmission line and the
amount of power to be transmitted.
• The transmission of electric power at high voltages has several
advantages including the saving of conductor material and high
transmission efficiency.
• It may appear advisable to use the highest possible voltage for
transmission of electric power to save conductor material and have
other advantages.
• But there is a limit to which this voltage can be increased.
• It is because increase in transmission voltage introduces insulation
problems as well as the cost of switchgear and transformer
equipment is increased.
• Therefore, the choice of proper transmission voltage is essentially a
question of economics.

 Generally the primary transmission is carried at 66 kV,

132 kV, 220 kV or 400 kV.
Figure 7.1. Typical AC Supply Scheme
(ii) Primary transmission
• The electric power at 132 kV is transmitted by 3-phase, 3-wire
overhead system to the outskirts of the city. This forms the primary
transmission.
(iii) Secondary transmission.
• The primary transmission line terminates at the receiving station
(RS) which usually lies at the outskirts of the city.
• At the receiving station, the voltage is reduced to 33kV by step-
down transformers.
• From this station, Electric power is transmitted at 33kV by 3-
phase, 3-wire overhead system to various sub-stations (SS) located
at the strategic Points in the city.
This forms the secondary transmission.
 (iv) Primary distribution
• The secondary transmission line terminates at the sub-station (SS)
where voltage is reduced from 33 kV to 11kV, 3-phase, 3-wire.
• The 11 kV lines run along the important road sides of the city.
This forms the primary distribution.
• It may be noted that big consumers (having demand more than 50
kW) are generally supplied power at 11 kV for further handling
with their own sub-stations.
 (v) Secondary distribution.

• The electric power from primary distribution line (11 kV) is

delivered to distribution sub-stations (DS).
• These sub-stations are located near the consumers’ localities and
step down the voltage to 400V, 3-phase, 4-wire for secondary
distribution.
• The voltage between any two phases is 400V and between any
phase and neutral is 230V.
• The single-phase residential lighting load is connected between
any one phase and neutral, whereas 3-phase, 400 V motor load is
connected across 3-phase lines directly.
• It may be worthwhile to mention here that secondary distribution
system consists of feeders, distributors and service mains.
• Fig. 7.2 shows the elements of
low voltage distribution
system.
• Feeders (SC or SA) radiating
from the distribution sub-
station (DS) supply power to
the distributors (AB, BC, CD
and AD).
• No consumer is given direct
connection from the feeders.
• Instead, the consumers are
connected to the distributors
through their service mains.

*A practical power system has a large number of auxiliary equipment's

(e.g., fuses, circuit breakers, voltage control devices etc.).
 7.3 Comparison of D.C. and A.C. Transmission
1. D.C. Transmission:
Advantages:
i. It requires only two conductors as compared to three for a.c.
transmission.
ii. There is no inductance, capacitance, phase displacement and
surge problems in d.c. transmission.
iii. Due to the absence of inductance, the voltage drop in a d.c.
transmission line is less than the a.c. line for the same load
and sending end voltage. For this reason, a d.c. transmission
line has better voltage regulation.
iv. There is no skin effect in a d.c. system. Therefore, entire
cross-section of the line conductor is utilized.
v. For the same working voltage, the potential stress on the
insulation is less in case of d.c. system than that in a.c.
system. Therefore, a d.c. line requires less insulation.
vi. A d.c. line has less corona loss and reduced interference with
communication circuits.
vii. The high voltage d.c. transmission is free from the dielectric
losses, particularly in the case of cables.
viii. In d.c. transmission, there are no stability problems and
synchronizing difficulties.
Drawbacks:
i. Electric power cannot be generated at high d.c. voltage due to
commutation problems.
ii. The d.c. voltage cannot be stepped up for transmission of
power at high voltages.
iii. The d.c. switches and circuit breakers have their own
limitations.
 2. A.C. Transmission
• Now-a-days, electrical energy is almost exclusively generated,
transmitted and distributed in the form of a.c.
Advantages
i. The power can be generated at high voltages.
ii. The maintenance of a.c. sub-stations is easy and cheaper.
iii. The a.c. voltage can be stepped up or stepped down by
transformers with ease and efficiency. This permits to
transmit power at high voltages and distribute it at safe
potentials.
Drawbacks:
i. An a.c. line requires more copper than a d.c. line.
ii. The construction of a.c. transmission line is more
complicated than a d.c. transmission line.
iii. Due to skin effect in the a.c. system, the effective resistance
of the line is increased.
iv. An a.c. line has capacitance. Therefore, there is a
continuous loss of power due to charging current even when
the line is open.
 Conclusion

• From the above comparison, it is clear that high voltage d.c.

transmission is superior to high voltage a.c. transmission.
• Although at present, transmission of electric power is carried
by a.c., there is an increasing interest in d.c. transmission.
• The introduction of mercury arc rectifiers and thyratrons have
made it possible to convert a.c. into d.c. and vice-versa easily
and efficiently.
• Such devices can operate upto 30 MW at 400 kV in single
units.
• The present day trend is towards a.c. for generation and
distribution and high voltage d.c. for transmission.
 Fig 3. Single line diagram of high voltage d.c. transmission

• The electric power is generated as a.c. and is stepped up to high

voltage by the sending end transformer TS.
• The a.c. power at high voltage is fed to the mercury arc
rectifiers which convert a.c. into d.c.
• The transmission of electric power is carried at high d.c. voltage.
• At the receiving end, d.c. is converted into a.c. with the help of
thyratrons.
• The a.c. supply is stepped down to low voltage by receiving end
transformer TR for distribution.
 Advantages of High Transmission Voltage
(i) Reduces volume of conductor material
Total volume of conductor material required is equal to

Where;
P is power transmitted in watts
V is line voltage in volts
Cosφ is power factor of the load
ρ is resistivity of conductor material
l is the line length in meters
W is the total power loss
The volume of conductor material required is inversely
proportional to the square of transmission voltage and power factor.
Hence, greater the transmission voltage, the lesser is the conductor
material required.
(ii) Increases transmission efficiency

Transmission efficiency

Where;
j is current density of conductor
ρ is resistivity of conductor material
l is length of line in meters
V is voltage of line in volts
As J, ρ and l are constants, therefore, transmission efficiency
increases when the line voltage is increased.
(iii) Decreases percentage line drop

Percentage of line drop =

As J, ρ and l are constants, therefore, percentage line drop

decreases when the transmission voltage increases.

Limitations of High Transmission Voltage:

i. the increased cost of insulating the conductors
ii. the increased cost of transformers, switchgear and other
terminal apparatus.
• Therefore, there is a limit to the higher transmission voltage which
can be economically employed in a particular case.
 7.5 Various Systems of Power Transmission
• It has already been pointed out that for transmission of electric
power, 3-phase, 3-wire a.c. system is universally adopted.
• However, the other systems can also be used for transmission under
special circumstances. The different possible systems of
transmission are :
1. D.C. system
(i) D.C. two-wire.
(ii) D.C. two-wire with mid-point earthed.
(iii) D.C. three-wire.

2. Single-phase A.C. system

(i) Single-phase two-wire.
(ii) Single-phase two-wire with mid-point earthed.
(iii) Single-phase three-wire.
3. Two-phase A.C. system
(i) Two-phase four-wire.
(ii) Two-phase three wire.

4. Three-phase A.C. system

(i) Three-phase three-wire.
(ii) Three-phase four-wire.

• The best system for transmission of power is that for which the
volume of conductor material required is minimum.
• Therefore, the volume of conductor material required forms the
basis of comparison between different systems.
7.8 Comparison of Conductor Material in Overhead
System