This document compares DC and AC power transmission. It discusses the advantages and disadvantages of each system. DC transmission has advantages like requiring only two conductors, less voltage drops, and no skin effect. However, it has disadvantages like power cannot be generated at high DC voltages. AC transmission can generate and step up power to high voltages more easily using transformers, but has disadvantages like increased resistance from skin effect. Nowadays, power is almost exclusively transmitted using high voltage AC systems due to their advantages over long-distance transmission.

1. Comparison of D.C. and A.C. Transmission:
• The electric power can be transmitted either by means of d.c. or a.c. Each system has its own
merits and demerits.
• D.C. transmission:
• Advantages: The high voltage d.c. transmission has the following advantages over high voltage
a.c. transmission:
• It requires only two conductors as compared to three for a.c. transmission.
• There is no inductance, capacitance, phase displacement and surge problems in d.c.transmission.
• Due to the absence of inductance, the voltage drops 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.
• There is no skin effect in a d.c. system. Therefore, entire cross-section of the line conductor is
utilised.
• 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.
• A d.c. line has less corona loss and reduced interference with communication circuits.
• The high voltage d.c. transmission is free from the dielectric losses, particularly in the case of
cables.
• In d.c. transmission, there are no stability problems and synchronising difficulties.

2. • Disadvantages:
(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.

3. • A.C. transmission: Now-a-days, electrical energy is almost exclusively generated,
transmitted and distributed in the form of a.c.
• Advantages:
• The power can be generated at high voltages.
• The maintenance of a.c. sub-stations is easy and cheaper.
• 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.
• Disadvantages:
• An a.c. line requires more copper than a d.c. line.
• The construction of a.c. transmission line is more complicated than a d.c.
transmission line.
• Due to skin effect in the a.c. system, the effective resistance of the line is increased.
• An a.c. line has capacitance. Therefore, there is a continuous loss of power due to
charging current even when the line is open.

4. Difference between HVAC and HVDC Transmission
Systems
• Electrical power is produced at the generating plants, from where it is transmitted
over the long distances for utilization at the load points. Since, the transmission of
electrical power is performed with the help of transmission conductor. Hence, due to
the line parameter, some power loss occurs during the transmission.
• Hence, in order to reduce the power loss during the transmission, we need to take
some necessary actions. One major of them is to increase the transmission voltage to
higher values.
• The line voltage plays an important role in reducing the losses in transmission line. The
increase in the line voltage reduces the line current and hence the power losses.
• As we have two types of electric supplies namely AC (Alternating Current) and DC
(Direct Current). Hence, based in that the high voltage electrical transmission systems
are classified into two types as:
• HVAC (High Voltage Alternating Current) Transmission System
• HVDC (High Voltage Direct Current) Transmission System
• Read through this article to understand the major differences between HVAC (High
Voltage Alternating Current) and HVDC (High Voltage Direct Current) transmission
systems. Let's start with a basic overview of what HVAC and HVDC systems are.

5. • HVAC stands for High Voltage Alternating Current. When the supply
voltage of the transmission system is ranging from 33 kV AC to 230 kV
AC, it is called high voltage alternating current (HVAC) transmission.
• In the HVAC transmission, the power generated is stepped up to the high
voltages and then transmitted by the transmission lines. It requires at
least three line conductors for transmitting the three phase electrical
power. However, the HVAC voltage transformation and transmission is
simple and inexpensive.

6. • HVDC stands for High Voltage Direct Current. The type of high voltage
transmission system when the power is transmitted in the form of DC at
a voltage between 100 kV to 800 kV is called HVDC (High Voltage Direct
Current) transmission system.
• In this system, the electrical power produced in the form of AC is
converted into DC with the help of rectifiers and then transmitted
through the HVDC lines, and at the utilization end it is again converted
into AC. The major advantage of the HVDC is that it requires only two
conductors for transmission and has comparatively low power losses
over the long distances.

7. WEEK-4
• Electric power can be carried either by underground cables or
overhead transmission and distribution lines. The underground cables
are not typically used for power transmission due to two reasons.
• 1. Power is carried over long distances to remote load centres.
Obviously, the installation costs for underground transmission will be
huge.
• 2. Electric power has to be transferred at high voltages for economic
reasons. It is very difficult to achieve proper insulation to the cables
to withstand higher pressures.
• Therefore, power transfer over long distances is done by using
overhead lines.

8. Main components of overhead transmission lines
• Support: - Depending on the working voltage and the region, poles or towers are used. The function of the line support is obviously to
support the conductor, so as to keep them at a suitable level above the ground.
• Cross arms and clamps: -These are used on pole structures to support the insulators and conductors and are made of wood or steel angle
section.
• Insulators: - Pin, strain, or suspension types, depending on the application, for supporting conductors, taking strain, or suspending
conductors.
• Conductors: Copper, aluminium, ACSR, or any other material depending on the current carried and the line’s length.
• Guys and Stays: To resist lateral forces, braces or cables are fastened to the pole at the termination or angle poles.
• Lightning Arrestors: These devices are used to discharge excessive voltages built up on the line to the ground as result
• of lightning strikes.
• Fuses and Isolating Switches: Fuses and isolating switches are used to isolate various parts of the overhead system.
• Continuous Earth Wire: This is a wire that runs along the top of the towers to protect the line from lightning strikes.
• Vee Guards: -They are often provided below bare overhead lines running along or across public streets to make the line safe if it should
break.
• Guard Wires:
When crossing telephone or telegraph lines, guard wires are installed above or below the power lines. The earth is solidly connected to th
e guard wires and steel structures.
• Phase Plates: - They are used in order to distinguish the various phases.
• Bird Guards: - A rounded-top stick of ebonite is attached to the cross arm near the insulator to prevent flashover due to birds pecking on
the conductors (on lines with pin insulators)
• Danger Plate:-It is provided on each pole, as a warning measure indicating the working voltage of the line and the word “danger”. It is
provided at a height of 2.5 m from the ground.
• Barbed Wire: - Barbed wire is wrapped on a pole at a height of about 2.5 m from the ground for at least 1 metre. This prevents climbing by
unauthorised persons.
• Miscellaneous Items such as vibration dampers, top hampers, beads for jumpers etc.

11. • Conductor Materials: The conductor is one of the important items. The proper choice of
material and size of the conductor is very importance. The conductor material used for
transmission and distribution of electric power should have the following properties:
• high electrical conductivity.
• high tensile strength in order to withstand mechanical stresses.
• low cost so that it can be used for long distances.
• low specific gravity so that weight per unit volume is small
• Commonly used conductor materials. The most commonly used conductor materials for
overhead lines are copper, aluminium, steel-cored aluminium, galvanised steel and
cadmium copper. The choice of a particular material will depend upon the cost, the
required electrical and mechanical properties and the local conditions.
• All conductors used for overhead lines are preferably stranded in order to increase the
flexibility. In stranded conductors, there is generally one central wire and round this,
successive layers of wires containing 6, 12, 18, 24 ...... wires. Thus, if there are n layers,
the total number of individual wires is 3n(n + 1) + 1. In the manufacture of stranded
conductors, the consecutive layers of wires are twisted or spiralled in opposite directions
so that layers are bound together.

12. • Copper: Copper is an ideal material for overhead lines owing to its high electrical conductivity and
greater tensile strength. Copper has high current density i.e., the current carrying capacity of copper
per unit of cross sectional area is quite large. This has two advantages.
• smaller X-sectional area of conductor is required
• the area offered by the conductor to wind loads is reduced.
• This metal is quite homogeneous, durable and has high scrap value. copper is an ideal material for
transmission and distribution of electric power. However, due to its higher cost and non-availability, it
is rarely used for these purposes. Now-a-days the trend is to use aluminium in place of copper.
• Aluminium: It is cheap and light as compared to copper but it has much smaller conductivity and
tensile strength. The comparison of the two materials copper and aluminium is given below :
• The conductivity of aluminium is 60% that of copper. The smaller conductivity of aluminium means that for any particular transmission
efficiency, the X-sectional area of conductor must be larger in aluminium than in copper.
• For the same resistance, the diameter of aluminium conductor is about 1·26 times the diameter of copper conductor. The increased X-
section of aluminium exposes a greater surface to wind pressure and, therefore, supporting towers must be designed for greater
transverse strength. This requires the use of higher towers with consequence of greater sag.
• The specific gravity of aluminium (2·71 gm/cc) is lower than that of copper (8·9 gm/cc). Therefore, an aluminium conductor has almost
one-half the weight of equivalent copper conductor. For this reason, the supporting structures for aluminium need not be made so
strong as that of copper conductor.
• Aluminium conductor being light, is liable to greater swings and hence larger cross-arms are required.
• Due to lower tensile strength and higher co-efficient of linear expansion of aluminium, the sag is greater in aluminium conductors.
• Considering the combined properties of cost, conductivity, tensile strength, weight etc., aluminium is
better than copper. Therefore, it is being widely used as a conductor material. It is particularly
profitable to use aluminium for heavy-current transmission where the conductor size is large and its
cost forms a major proportion of the total cost of complete installation.

13. • 3.Steel cored aluminium: Due to low tensile strength, aluminium
conductors produce greater sag. This prohibits their use for larger
spans and makes them unsuitable for long distance transmission. In
order to increase the tensile strength, the aluminium conductor is
reinforced with a core of galvanised steel wires. The composite
conductor thus obtained is known as steel cored aluminium and is
abbreviated as A.C.S.R. (aluminium conductor steel reinforced).
• Steel-cored aluminium conductor consists of central core of
†galvanised steel wires surrounded by a number of aluminium
strands. Usually, diameter of both steel and aluminium wires is the
same. The X-section of the two metals are generally in the ratio of 1 :
6 but can be modified to 1 : 4 in order to get more tensile strength for
the conductor. Fig. shows steel cored aluminium conductor having
one steel wire surrounded by six wires of aluminium.

14. • Steel-cored aluminium conductor having one steel wire surrounded by six
aluminum wires
• The result of this composite conductor is that steel core takes greater
percentage of mechanical strength while aluminium strands carry the bulk
of current. The steel cored aluminium conductors have the following
advantages :
• The reinforcement with steel increases the tensile strength but at the same
time keeps the composite conductor light. Therefore, steel cored
aluminium conductors will produce smaller sag and hence longer spans can
be used.
• 2. Due to smaller sag with steel cored aluminium conductors, towers of
smaller heights can be used.

15. • 4. Galvanised steel: Steel has very high tensile strength. Therefore, galvanised steel
conductors can be used for extremely long spans or for short line sections exposed to
abnormally high stresses due to climatic conditions. They have been found very suitable
in rural areas where cheapness is the main consideration. Due to poor conductivity and
high resistance of steel, such conductors are not suitable for transmitting large power
over a long distance. However, they can be used to advantage for transmitting a small
power over a small distance where the size of the copper conductor desirable from
economic considerations would be too small and thus unsuitable for use because of poor
mechanical strength.
• 5. Cadmium copper. The conductor material now being employed in certain cases is
copper alloyed with cadmium. An addition of 1% or 2% cadmium to copper increases the
tensile strength by about 50% and the conductivity is only reduced by 15% below that of
pure copper. Therefore, cadmium copper conductor can be useful for exceptionally long
spans. However, due to high cost of cadmium, such conductors will be economical only
for lines of small X-section i.e., where the cost of conductor material is comparatively
small compared with the cost of supports.