Transmission & distribution of electrical power

Transmission &
Distribution of
Electrical Power
PRESENTED BY : PRIYANKA SOLANKI
ASSISTANT PROFESSOR

Brief Introduction of course
Module I: Basics Of Transmission
Module II: Transmission Line Components
Module III: Transmission Line Parameters
Module IV: Performance Of Transmission Line.
Module V: Extra High Voltage Transmission.
Module VI: Components Of Distribution System.
Module VII: Underground Cables.
Module VIII: Substations.

Module I: Basics Of Transmission
Introduction to transmission.
Necessity of transmission of electricity.
Classification & comparison of different transmission

Introduction to transmission.
The process by which large amounts of electricity produced are transported over long
distances for eventual use by consumers.
Electric power transmission is the bulk movement of electrical energy from a generating site,
such as a power plant, to an electrical substation.
These generating stations are not necessarily situated at the load center.
Load center is the place which consumes maximum power. Hence there must be some means
by which we can transmit the generated power to the load center..

NECESSITY OF TRANSMISSION
Electricity is supplied to consumers through the National Grid at a very high voltage to reduce
energy losses during transmission.
Transformers are used to increase or decrease the voltage of the supply. Electricity is charged in
units. Increasing the voltage decreases the current, which decreases the power loss.
The advantage of transmitting electricity at high voltage is that is minimizes the power loss due
to resistance in conductors. Power is dissipated as heat. .
The power loss in the transmission line is be proportional to I^2.

CLASSIFICATION & COMPARISON
OF DIFFERENT TRANSMISSION:
1.High voltage DC electrical transmission system.
2. High AC electrical transmission system.

Module II
Transmission Line Components
 Introduction to line components.
 Types of conductors-Copper, Aluminum & state their trade names.
 Line insulators – requirements, types, and field of applications.
 Distribution of potential over a string of suspension insulators.
 Concept of string efficiency.
 Corona – corona formation, advantages & disadvantages, factors affecting corona, important terms related to corona.
 Spacing between Conductors.
 Calculation of Span length & sag Calculation

Introduction to line components.
A transmission line has resistance, inductance and capacitance uniformly distributed along the
whole length of the line.
Resistance. It is the opposition of line conductors to current flow. The resistance is distributed
uniformly along the whole length of the line
Inductance. When an alternating current flows through a conductor, a changing flux is set up
which links the conductor. Due to these flux linkages, the conductor possesses inductance.
Capacitance. We know that any two conductors separated by an insulating material constitute a
capacitor. As any two conductors of an overhead transmission line are separated by air which
acts as an insulation, therefore, capacitance exists between any two overhead line conductors.

Types of conductors
An ideal conductor has following features.
· It has maximum electrical conductivity.
· It has high tensile strength so that it can withstand mechanical stresses.
· It has least specific gravity i.e. weight / unit volume.
· It has least cost without sacrificing other fact

1. AAC (All
Aluminum
Conductor)
It has lesser strength and more sag per span
length than any other category.
· Therefore, it is used for lesser span i.e. it is
applicable at distribution level.
· It has slightly better conductivity at lower
voltages than ACSR i.e. at distribution level
· Cost of ACSR is equal to AAC.

ACAR (Aluminium Conductor,
Aluminium Reinforce)
It is cheaper than AAAC but pro to corrosion.
· It is most expansive.

AAAC (All Aluminium Alloy
Conductor)
It has same construction as AAC except the alloy.
· Its strength is equal to ACSR but due to absence of steel it is light in weight.
· The presence of formation of alloy makes it expensive.
· Due to stronger tensile strength than AAC, it is used for longer spans.
· It can be used in distribution level i.e. river crossing.
· It has lesser sag than AAC.

ACSR (Aluminium Conductor Steel
Reinforced)
It is used for longer spans keeping sag minimum.
· It may consist of 7 or 19 strands of steel surrounding by aluminum strands concentrically. The
number of strands are shown by x/y/z, where ‘x’ is number of aluminum strands, ‘y’ is number of
steel strands and ‘z’ is diameter of each strand. Strands provide flexibility, prevent breakage and
minimize skin effect..
 If the Al and St strands are separated by a filler such as paper then this kind of ACSR is used in
EHV lines and called expanded ACSR.
· Expanded ACSR has larger diameter and hence lower corona losses.

Types of insulator
There are mainly three types of insulator used as overhead insulator likewise
1. Pin Insulator
2. Suspension Insulator
3. Strain Insulator

Pin Insulator
The pin insulator is used in power distribution for the voltage up to 33kV. It is placed on the
cross arm of the supporting tower.
The pin insulator has grooves on the upper end for keeping the conductor.
The conductor is tied to the insulator on the top groove on straight line positions and side
groove in angle positions by annealed binding wire of the same material as that of the
conductor.
A lead thimble is cemented into the insulator body to receive the pin.

Advantages of Pin Insulator
1.It has high mechanical strength.
2.The pin type insulator has good creepage distance.
3.It is used on the high voltage distribution line.
4.The construction of the pin type insulator is simple and requires less
maintenance.
5.It can be used vertically as well as horizontally.

Disadvantages of Pin Insulator
1.It should be used with the spindle.
2.It is only used in the distribution line.
3.The voltage rating is limited, i.e., up to 36kV.
4.The pin of the insulator damaged the insulator thread.

Post Insulator
Post insulators are similar to Pin insulators, but post insulators are more suitable for
higher voltage applications.
Post insulators have a higher number of petticoats and a greater height compared to
pin insulators.
We can mount this type of insulator on supporting structure horizontally as well as
vertically
The insulator is made of one piece of porcelain and it has clamp arrangement are in
both top and bottom end for fixing.

S.No. Pin Insulator
Post Insulator
1 It is generally used up to 33KV system It is suitable for lower voltage and also for higher
voltage
2 It is single stag It can be single stag as well as multiple stags
3 Conductor is fixed on the top of the insulator by
binding
Conductor is fixed on the top of the insulator with
help of connector clamp
4 Two insulators cannot be fixed together for
higher voltage application
Two or more insulators can be fixed together one
above other for higher voltage application
5 Metallic fixing arrangement provided only on
bottom end of the insulator
Metallic fixing arrangement provided on both top and
bottom ends of the insulator

Advantages of Suspension Type Insulator
.
1.Each unit operates the voltage of about 11kV and hence depending upon
the voltage the appropriate number of discs are connected in series with the
string.
2.If one of the units is damaged, then it is replaced by the new one and hence
no need of replacing the whole string.
3.The string is free to swing in any direction and, therefore, great flexibility is
provided to the transmission line.
4.The conductors are placed below the suspension insulators and hence it
partly protects the conductor from lightning.

Advantages of Strain Insulator
The advantages of strain insulators include the following.
•These are used for low voltages upto 11kv
•These are insulated from the ground for low-voltage lines.
•These are designed with porcelain
•If the insulator is damaged, the stay or guy wires will not drop to the ground.

Distribution of potential over a string of suspension insulators.

Concept of
string
efficiency.
The string efficiency is defined as the
ratio of voltage across the string to the
product of the number of strings and
the voltage across the unit
adjacent string.

Methods of improving string efficiency:
Use of Long Cross Arm
Capacitance Gradings or Grading of the unit
Uses of Grading Rings or Static Shielding

Corona Effect
The phenomenon of ionisation of surrounding air around the conductor due to which luminous
glow with hissing noise is rise is known as the corona effect.
Air acts as a dielectric medium between the transmission lines.
The electric field intensity also increases because of the charging current.

Factors affecting Corona Effect
Effect of supply voltage : Directly proportional with corona effect
The condition of conductor surface : rough surface decrease corona
Air Density Factor: inversely proportional to corona loss
Effect of system voltage : Directly proportional
The spacing between conductors : Directly proportional

Disadvantages of corona discharge:
1.The glow appear across the conductor which shows the power loss occur on it.
2.The audio noise occurs because of the corona effect which causes the power loss on the
conductor.
3.The vibration of conductor occurs because of corona effect.
4.The corona effect generates the ozone because of which the conductor becomes corrosive.
5.The corona effect produces the non-sinusoidal signal thus the non-sinusoidal voltage drops occur
in the line.
6.The corona power loss reduces the efficiency of the line.
7.The radio and TV interference occurs on the line because of corona effect.

Minimizing corona:
Conductor diameter
The voltage of the line
Spacing between conductors

Spacing between the conductors
The minimum distance between bottom conductor and ground of a 400KV transmission line is
8.84 meter.
The minimum ground clearance of 33KV uninsulated electrical conductor is 5.2 meter.

Calculation of Span length & sag
Calculation
The sag in overhead line is a length in between support point of conductor and the lowest point
on the conductor of transmission and distribution overhead line.
Calculation of Sag:

Sag calculation when support are at
unequal level:

Module III: Transmission Line Parameters
R,L & C of 1-ph & 3-ph transmission line.
Skin effect, proximity effect & Ferranti effect.
Concept of transposition of conductors & necessity.

Classification of Transmission line
Short transmission line – The line length is up to 60 km and the line voltage is comparatively
low less than 20KV.
Medium transmission line – The line length is between 60 km to 160 km and the line voltage is
between 20kV to 100kV.
Long transmission line – The line length is more than 160 km and the line voltage is high
greater than 100KV.

Skin Effect in transmission lines
The phenomena arising due to unequal
distribution of current over the entire
cross section of the conductor being used
for long distance power transmission is
referred as the skin effect in transmission
lines.

Factors affecting skin effect
Frequency
Diameter
The shape of the conductor
Type of material

Proximity Effect
When the conductors carry the high alternating
voltage then the currents are non-uniformly
distributed on the cross-section area of the
conductor. This effect is called proximity effect.
The proximity effect results in the increment of
the apparent resistance of the conductor due to
the presence of the other conductors carrying
current in its vicinity.

Factors Affecting the Proximity Effect
1.Frequency – The proximity increases with the increases in the frequency.
2.Diameter – The proximity effect increases with the increase in the
conductor.
3.Structure – This effect is more on the solid conductor as compared to the
stranded conductor (i.e., ASCR) because the surface area of the stranded
conductor is smaller than the solid conductor.
4.Material – If the material is made up of high ferromagnetic material then
the proximity effect is more on their surface.

Ferranti Effect
The Ferranti effect is a
phenomenon that describes the
increase in voltage that occurs at
the receiving end of a long
transmission line relative to the
voltage at the sending end

How to reduce Ferranti effect:
Shunt reactor is an inductive current element connected between line and neutral to
compensate the capacitive current from transmission line
Electrical devices are designed to work at some particular voltage

Transposition of Conductors
The transposition is a physical rotation of the conductors so that the conductor is moved to
take up the next physical position in the regular sequence.
The transposition of the conductor equalizes the mutual inductance and capacitance between
the lines.
The transposition is mainly done in the switching station and the substations.

Needs of Transposition
The inductance of unsymmetrical line causes voltage drops even if the voltage is in a balanced
condition.
Because of the inducing voltages, the magnetic field exists in the conductor which causes the
interference in the line.
This can be reduced by continually exchanging the position of the conductor, which can be
done by transposition the conductors.

Disadvantage of Transposition
Frequently changing the position of conductors weakens the supportive structure which
increases the cost of the system.

Module IV
Performance Of Transmission Line.
 Classification of transmission lines.
 Losses, Efficiency & Regulation of line.
 Performance of single phase short transmission line(Numerical based on it )
 Effect of load power factor on performance.
• General circuit & Generalized Circuit Constants( A, B, C, D )

Classification of transmission lines

Losses, Efficiency & Regulation of line

Parameter Specification Unit
A = VS / VR Voltage ratio Unit less
B = VS / IR Short circuit resistance Ω
C = IS / VR Open circuit conductance mho
D = IS / IR Current ratio Unit less

Module V:
Extra High Voltage Transmission.
 Introduction & Requirement.
 EHVAC Transmission, Reasons for adoption &limitations.
• HVDC Transmission – Advantages, Limitations.

INTRODUCTION
The term high voltage characterizes electrical circuits in which the voltage used is the cause of
particular safety concerns and insulation requirements.
High voltage is used in electrical power distribution, in cathode ray tubes, to generate X-rays and
particle beams, to demonstrate arcing, for ignition, in photomultiplier tubes, and high power
amplifier vacuum tubes and other industrial and scientific applications.
WHAT IS EHV TRANSMISSION ?
Two factors considered in the classification of a "high voltage" are the possibility of causing a
spark in air, and the danger of electric shock by contact or proximity. In electric power
transmission engineering, high voltage is usually considered any voltage over approximately
35,000 volts.

NECESSITY OF EHVAC TRANSMISSION
With the increase in transmission voltage, for same amount of power to be transmitted current in
the line decreases which reduces I2R losses. This will lead to increase in transmission efficiency.
With decrease in transmission current, size of conductor required reduces which decreases the
volume of conductor.
The transmission capacity is proportional to square of operating voltages. Thus the transmission
capacity of line increases with increase in voltage.
With increase in level of transmission voltage, the installation cost of the transmission line per
km decreases.
It is economical with EHV transmission to interconnect the power systems on a large scale.
The no. of circuits and the land requirement for transmission decreases withthe use of higher
transmission voltages.

ADVANTAGES
Reduction in the current.
Reduction in the losses.
Reduction in volume of conductor material required.
 Decrease in voltage drop & improvement of voltage regulation.
 Increase in Transmission Efficiency.
 Increased power handling capacity.
The no. of circuits & the land requirement reduces as transmission voltage increases.
The total line cost per MW per km decreases considerably with the increase in line voltage.

DISADVANTAGES
Corona Loss & radio interference
Line Supports
Erection Difficulties
Need of Insulation
Generates Electrostatic effects

Components of HVDC transmission
system

Comparison of
HVAC &HVDC
HVDC Transmission System HVAC Transmission System
Low losses.
Losses are high due to the skin
effect and corona discharge
Better Voltage regulation and Control
ability.
Voltage regulation and Control ability is
low.
Transmit more power over a longer
distance.
Transmit less power compared to a
HVDC system.
Less insulation is needed. More insulation is required.
Reliability is high. Low Reliability.
Asynchronous interconnection is
possible.
Asynchronous interconnection is not
possible.
Reduced line cost due to fewer
conductors.
Line cost is high.
Towers are cheaper, simple and narrow. Towers are bigger compared to HVDC.

Disadvantages of HVDC Transmission
•Converters with small overload capacity are used.
•Circuit Breakers, Converters and AC filters are expensive especially for small distance
transmission.
•No transformers for altering the voltage level.
•HVDC link is extremely complicated.
•Uncontrollable power flow.

Application of HVDC Transmission
•Undersea and underground cables
•AC network interconnections
•Interconnecting Asynchronous system

Module VI:
Components Of Distribution System.
 Introduction.
 Classification of distribution system.
 Connection schemes of distribution system.
• A.C. distribution calculations

INTRODUCTION
Electric Power Distribution System states that part of power system which distributes electric
power for local use is known as distribution system.

Classification of Distribution Systems:
Nature of current:
(a) d.c. distribution system
(b) a.c. distribution system.
Type of construction:
(a) overhead system
(b) underground system. The overhead system is generally employed for distribution as it is 5 to
10 times cheaper than the equivalent underground system. In general, the underground system is
used at places where overhead construction is impracticable or prohibited by the local laws.
Scheme of connection: (a) radial system (b) ring main system (c) inter-connected system.

CONNECTION SCHEMES OF
DISTRIBUTION SYSTEM
( i) Radial System

The ring main system has the following advantages :
( a) There are less voltage fluctuations at consumer’s terminals.
( b) The system is very reliable as each distributor is fed via *two feeders. In the event of fault on any
section of the feeder, the continuity of supply is maintained. For example, suppose that fault occurs at any
point F of section SLM of the feeder. Then section SLM of the feeder can be isolated for repairs and at the
same time continuity of supply is maintained to all the consumers via the feeder SRQPONM.

The interconnected system has the
following advantages :
( a) It increases the service reliability.
( b) Any area fed from one generating station during peak load hours can be fed from the other
generating station. This reduces reserve power capacity and increases efficiency of the system.

Module VII: Underground Cables.
• Introduction & requirements.
• Classification of cables.
• Cable conductors.
• Cable insulation, Metallic sheathing & mechanical protection

Requirements of Underground Cable
The conductor used in underground cable shall be tinned copper or aluminum conductor of
high conductivity. Stranding is very important to provide flexibility and increase current carrying
capacity.
The size of conductor shall be sufficient enough to carry load current without heating and
appreciable voltage drop. The voltage drop shall be within the permissible range.
The cable must have proper thickness of insulation to provide high degree of safety and
reliability at operating voltage.
It must have been provided with suitable mechanical protection to withstand rough handling
during lying of cable.
The material used in the manufacture of cable should be such that there is complete chemical
and physical stability throughout.

Module VIII: Substations.
• Introduction.
• Classification of indoor & outdoor sub-stations.
• Selection & location of site.
• Main connection schemes.
• Equipment’s circuit element of substations.

INTRODUCTION
Substations serve as sources of energy supply for the local areas of distribution in which these
are located.
Their main functions are to receive energy transmitted at high voltage from the generating
stations, reduce the voltage to a value appropriate for local distribution and provide facilities for
switching

Classification of Substations:
Classification of Substations on the Basis of Nature of Duties:
1. Step-Up or Primary Substations: Associated with generation
2. Primary Grid Substations: Associated with Transmission
3. Step-Down or Distribution Substations: Associated with Distribution

Classification of Substations on the Basis
of Service Rendered:
1. Transformer Substations:
2. Switching Substations:
3. Converting Substations:

of Operating Voltage:
1. High Voltage Substations (HV Substations) involving voltages between 11 kV and 66
kV.
2. Extra High Voltage Substations (EHV Substations) involving voltages between 132 kV
and 400 kV.
3. Ultra High Voltage Substations (UHV Substations) operating on voltage above 400 kV.

of Importance:
1. Grid Substations:
2. Town Substations:

Classification of Substations on
the Basis of Design:
1. Indoor Type Substations
2. Outdoor Substations:
(a) Pole Mounted Substations
(b) Foundation Mounted Substations

Selection and Location of Site for a
Substation:
1. Type of Substation
2. Availability of Suitable and Sufficient Land
3. Communication Facility
4. Atmospheric Pollution
5. Availability of Essential Amenities to the Staff
6. Drainage Facility

Main Electrical Connections:
The main connection scheme is drawn keeping in view the following factors:
(i) General bus-bar arrangement,
(ii) Operating voltage,
(iii) Number of incoming and outgoing lines,
(iv) Number of transformers,
(v) Safety to equipment,
(vi) Safety to operating personnel, and
(vii) Future extension requirement.

Equipment’s circuit element of
substations

Transmission & distribution of electrical power

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