EE 1302 TRANSMISSION AND DISTRIBUTION

BY

M.VALAN RAJKUMAR, L/ EEE

N.P.R.

COLLEGE OF ENGINEERING AND TECHNOLOGY,

NATHAM.

ANNA UNIVERSITY TIRUCHIRAPPALLI, Tiruchirappalli

Regulations 2008

Curriculum

SEMESTER V

S. No. Subject Subject L T P C

Code

Theory

1 MG1301 Total Quality Management 3 0 0 3

2 EE1301 Electrical Machines II 3 1 0 4

3 EE1302 Transmission and Distribution

Engineering 3 1 0 4

4 EC1307 Digital Signal Processing 3 1 0 4

5 EC1308 Principles of Communication

Engineering 3 0 0 3

6 CS1312 Object Oriented Programming 3 0 0 3

Practical

7 EE1303 Electrical Machines II

Laboratory 0 0 3 2

8 EC1309 Digital Signal Processing

Laboratory 0 0 3 2

9 CS1313 Object Oriented Programming

Laboratory 0 0 3 2

Total 27

EE1302 – TRANSMISSION AND DISTRIBUTION ENGINEERING

L T P C

3 1 0 4

UNIT I TRANSMISSION SYSTEMS 9

Structure of electric power system – Various levels Generation, Transmission and distribution –

HVDC and EHV AC transmission – Comparison of economics of transmission – Technical

performance and reliability – Application of HVDC transmission system – FACTS (qualitative

treatment only) – TCSC – SVC – STATCOM – UPFC

UNIT II TRANSMISSION LINE PARAMETERS 9

Parameters of single and three phase transmission lines with single and double circuits –

Resistance, Inductance and Capacitance of solid, stranded and bundled conductors – Symmetrical and unsymmetrical spacing –

Transposition – Application of self and mutual GMD – Skin and proximity effects – Interference with neighboring communication

circuits – Typical configuration– Conductor types and electrical parameters of 400, 220, 110, 66 and 33 kV lines

UNITIII MODELLING AND PERFORMANCE OF TRANSMISSION LINES 9

Classification of lines – Short, medium and long line – Equivalent circuits, attenuation constant –

Phase constant – Surge impedance – Transmission efficiency and voltage regulation – Real and

reactive power flow in lines – Power-angle diagram – Surge-impedance loading – Loadability

limits based on thermal loading – Angle and voltage stability considerations – Shunt and series

compensation – Ferranti effect and corona loss

UNIT IV INSULATORS AND CABLES 9

Insulators – Types – Voltage distribution in insulator string and grading – Improvement of string

efficiency – Underground cables – Constructional features of LT and HT cables – Capacitance –

Dielectric stress and grading – Thermal characteristics

UNIT V SUBSTATION GROUNDING SYSTEM AND DISTRIBUTION SYSTEM 9

Types of substations – Bus-bar arrangements – Substation bus schemes – Single bus scheme –

Double bus with double breaker – Double bus with single breaker – Main and transfer bus – Ringbus – Breaker-and-a-half with

two main buses – Double bus-bar with bypass isolators – Resistanceof grounding systems – Resistance of driven rods, resistance of

grounding point electrode –Grounding grids – Design principles of substation grounding system – Neutral grounding

L: 45 T: 15 Total: 60

TEXT BOOKS

1. Gupta, B.R., “Power System Analysis and Design”, S.Chand, 2003

2. Singh, S.N., “Electric Power Generation, Transmission and Distribution”, Prentice Hall of

India, 2002

REFERENCES

1. Luces M. Fualkenberry, Walter Coffer, “Electrical Power Distribution and Transmission”,

Pearson Education, 1996

2. Hadi Saadat, “Power System Analysis”, Tata McGraw Hill Publishing Company, 2003

3. Wadhwa, C.L., “Electric Power Systems”, New Age International (P) Ltd., 2000

4. Turan Gonen, “Electric Power Distribution Engineering”, 2nd Edition, CRC Press, 2007

UNIT I TRANSMISSION SYSTEMS

Structure of electric power system – Various levels Generation, Transmission and distribution –

HVDC and EHV AC transmission – Comparison of economics of transmission – Technical

performance and reliability – Application of HVDC transmission system – FACTS (qualitative

treatment only) – TCSC – SVC – STATCOM – UPFC

• Various levels such as generation, transmission and distribution.

INTRODUCTION

• HVDC

High voltage direct current (HVDC) is used to transmit large amounts of power over long

distances or for interconnections between asynchronous grids. When electrical energy is required

to be transmitted over very long distances, it is more economical to transmit using direct current

instead of alternating current

• EHV AC transmission

Hydro-electric and coal or oil-fired stations are located very far from load centres for various

reasons which requires the transmission of the generated electric power over very long

distances.

This requires very high voltages for transmission. The very rapid strides taken by

development

of dc transmission since 1950 is playing a major role in extra-long-distance transmission,

complementing or supplementing EHV AC transmission.

Technical performance and reliability

Considerations in the design of a power line:

• • The amount of active power it has to transmit

• • The distance over which the power must be

carried

• • The cost of the power line

• • Aesthetic considerations, urban congestion,

ease of installation, expected load growth

• Application of HVDC transmission system

• HVDC Light, is utilising state of the art semiconductors,

control and cable insulation and can offer many new transmission opportunities as has been

demonstrated by actual projects above. It offers a lot of possibilities to enhance the power

systems.

• Wind power, even big parks, can easily be connected to the grid. In many cases HVDC Light

can give new opportunities to trade electric energy in the new deregulated markets.

• As HVDC Light has been developed to minimise environmental impact and impact on the

connecting grids, the licence procedure is generally more favourable than more traditional

solutions.

FACTS:

• TCSC

The Thyristor Controlled Series Capacitor (TCSC)

seems to be one of the members within the FACTS

family, beside the SVC that was established long ago,

which has attracted the most interest so far. One reason

may be that a distinctive quality of the TCSC concept is

that it uses an extremely simple main circuit topology.

The capacitor is inserted directly in series with the

transmission line and the thyristor controlled inductor is

mounted directly in parallel with the capacitor. Thus no

interfacing equipment like e.g. high voltage transformers

is required. This makes TCSC much more economical

than some other competing FACTS technologies

• SVC

• Static variable capacitor

Parallel-connected static var generator or absorber

Output is adjusted to exchange capacitive or

inductive current.

Maintain or control specific parameters of the electrical power system (typically bus voltage).

Thyristor-based Controllers

Lower cost alternative to STATCOM

• STATCOM

Static Synchronous Compensator (STATCOM)

• Parallel-connected static var compensator

• Capacitive or inductive output current controlled independently of the ac system voltage

• UPFC

Unified power flow controller (UPFC) is one of the FACTS devices, which can control

power system parameters such as terminal voltage, line impedance and phase angle. Therefore, it

can be used not only for power flow control, but also for power system stabilizing control.

UNIT-I

TRANSMISSION SYSTEM – INTRODUCTION

PART-A

1. What is meant by power supply system? (2)

2. What is meant by Transmission and Distribution system? (2)

3. What are the different types of Power supply system? (2)

4. What are the various components of power supply system? (2)

5. What are the different types of power plants? (2)

6. What are the different operating voltages used for generation, primary and

secondary transmission in AC power supply systems in India? (2)

7. Define feeder, distributor and service mains. (2)

8. List the advantages of high voltage transmission. (2)

9. State Kelvin’s law. (2)

10. What are the limitations of Kelvin’s law? (2)

PART-B

1. (i) Discuss various types of HVDC links. (8)

(ii) List out the main components of a HVDC system. (8)

2. (i) Draw and explain the structure of modern power systems with typical

voltage levels (13)

(ii) What is the highest voltage level available in India? (3)

3. (i) Explain the effect of high voltage on volume of copper and on efficiency. (8)

(ii) Explain why the transmission lines are 3 phase 3-wire circuits while

distribution lines are 3 phase 4-wire circuits. (8)

4. (i) Draw the model power system with single line representation. Show its

essential constituent sections. (6)

(ii) What are the AC transmission and distribution level voltages we have in

India? (4)

(iii) What are the different kinds of DC links? Draw relevant diagrams. (6)

5. (i) Explain why EHV transmission is preferred? What are the problems

involved in EHV AC transmission? (8)

(ii) With neat schematic, explain the principle of HVDC system operation. (8)

6. Explain about FACTS with neat diagram (16)

7. Explain TCSC and SVS systems (16)

8. Explain with neat diagram about STATCOM and UPFC (16)

9. (i) Compare EHVAC and HVDC transmission (8)

(ii) Explain the applications of HVDC transmission system (8)

UNIT II TRANSMISSION LINE PARAMETERS

Parameters of single and three phase transmission lines with single and double circuits –

Resistance, Inductance and Capacitance of solid, stranded and bundled conductors – Symmetrical

and unsymmetrical spacing – Transposition – Application of self and mutual GMD – Skin and

proximity effects – Interference with neighboring communication circuits – Typical configuration

– Conductor types and electrical parameters of 400, 220, 110, 66 and 33 kV lines

„ Parameters in the transmission line

„ resistance r, inductance L, capacitance C

„ L and C are due to the effects of magnetic and electric fields around the conductor

„ Overhead transmission line

„ ANSI voltage standard: 69kV, 115kV, 138kV, 161kV,

230kV, 345kV, 500kV, 765kV line-to-line

„ extra-high-voltage (EHV): >230kV, ultra-high-voltage

(UHV): ≥765kV

„ bundling: use more than one conductor per phase, usually used at voltage > 230kV

„ advantage of bundling: increase effective radius of line conductor, reduce electric field strength and

reduces corona power loss, audio loss and radio interference, and reduces line reactance

LINE RESISTANCE

„ Transmission line resistance

„ dc flow: resistance of solid round conductor is given

by Rdc

=ρl/A

„ ac flow: the current distribution is not uniform, the

current density is greatest at surface of the conductor, this is called skin effect, therefore, Rac

> Rdc

„ temperature: resistance increases when temperature increases „ Transmission line inductance

„ definition of inductance L: L=λ/I, λ is flux linkage

„ magnetic field density: Hx=I

x/2πx, x is the radius of circle, I

x induces magnetic field density H

x

INTERNAL INDUCTANCE

„ Derivation of internal inductance Lint

„ consider the flux linked by the portion x ≤ r of current

Ia

flowing inside a cylinder of radius x, the magnetic intensity:

∫ Hdl

= I

enclosed

2 2

„ Since Ia

= πx

I ,

πr 2

therefore 2πxH

x

πx

= I

πr 2

„ magnetic flux density Bx:

Bx=µ

oH

x=µ

oxI /2πr

2

„ µo

is the permeability of free space: 4π×10-7

H/m

„ since current flowing into the circuit of x is only a fraction of Ia, the effective turn is equivalent to the fraction

N = πx2/πr

2

∫

INTERNAL INDUCTANCE

„ Derivation of internal inductance L

int

„ πx2/πr

2 turns of the current I

a linked by flux:

„ dλx= (πx

2/πr

2) dφ

x = (πx

2/πr

2) (B

x×1dx)

= (πx2/πr

2) (µ

oxI/2πr

2) ×1dx = (µ

oI) x

3/(2πr

4) dx

„ total flux linkage in the inductor:

λint

=

µo

I r

2πr 4

0

x3dx =

µ

o I

8π

„ inductor due to the internal flux:

Lint

=µo/8π=(1/2)×10

-7 H/m

„ inductor Lint

is independent of the conductor radius r

∫

EXTERNAL INDUCTANCE

„ Derivation of external inductance L

ext

„ consider Hx

external to conductor at x>r, since the circle at radius x enclose entire current, Ix=I (

see Fig.4.4 ): Bx=µ

oH

x=µ

oI/2πx

„ the entire current I is linked by the flux outside the conductor, dλx=dφ

x = B

xdx*1=µ

oI/(2πx)*dx

„ external flux linkage between D1

and D2

:

λext

= µ

o I

2π

D2 1

dx D1 x

= 2 ×10−7

I ln D

2

D1

Wb/m

„ inductance between two points D

1 and D

2 due to the external flux:

Lext = 2 ×10

−7 ln

D

2

D1

H/m

UNIT- II

TRANSMISSION LINE PARAMETERS

PART-A

1. Define Skin effect. (2)

2. What is meant by proximity effect? (2)

3. Differentiate the stranded conductor and bundled conductor. (2)

4. List out the advantages of double circuit lines. (2)

5. Define - Self and mutual – G.M.D. (2)

6. What is meant by inductive interference? (2)

7. What is transposition of conductors? (2)

8. What is ACSR conductor? (2)

9. What is fictitious conductor radius? (2)

10. Define unsymmetrical and symmetrical spacing. (2)

PART-B

1. From the fundamentals derive an expression for inductance of a single phase

transmission system. (16)

2. Derive an expression for capacitances of a single phase transmission system

and discuss the effect of earth on capacitance with suitable equation. (16)

3. Derive an expression for inductance

i) Of a single-phase overhead line. (8)

ii) A conductor is composed of seven identical copper strands

each having a radius r. Find the self-GMD of the conductor. (8)

4. i) Derive an expression for the capacitance between conductors of a

Single phase overhead line. (8)

ii) Find the capacitance between the conductors of a single-phase 10 km

long line. The diameter of each conductor is 1.213cm. The spacing

between conductors is 1.25m. Also find the capacitance of each conductor

neutral. (8)

5. i) Derive the expression for inductance of a two wire 1Φ transmission line (8)

ii) Derive the expression for capacitance of a 1Φ transmission line (8)

6. i) What are the advantages of bundled conductors? (4)

ii) Derive the expression for capacitance of a double circuit line for hexagonal

spacing. (8)

iii) Why is the concept of self GMD is not applicable for capacitance? (4)

7. i) Explain clearly the skin effect and the proximity effects when referred to

overhead lines. (8)

ii) Write a short note on the inductive interference between power and

communication lines. (8)

8. i) Derive the expression for the capacitance per phase of the 3 Φ double circuit

line flat vertical spacing with transposition. (8)

ii) A 3 Φ overhead transmission line has its conductors arranged at the corners

of an equilateral triangle of 2m side. Calculate the capacitance of each line

conductor per km. Given the diameter of each conductor is 1.25cm. (8)

9. Find the capacitance per km per phase of a 3Φ line arrangement in a

horizontal plane spaced 8 metres apart. The height of all conductors above the

earth is 13 metres. The diameter of each conductor is 2.6 cm. the line is

completely transposed and takes the effect of ground into account. (16)

10. Discuss the concept of GMR and GMD in the calculation of transmission line

inductance. (16)

UNIT IV INSULATORS AND CABLES

Insulators – Types – Voltage distribution in insulator string and grading – Improvement of string

efficiency – Underground cables – Constructional features of LT and HT cables – Capacitance –

Dielectric stress and grading – Thermal characteristics

Insulators:

• An insulator, also called a dielectric, is a material that

resists the flow of electric current. An insulating material

has atoms with tightly bonded valence electrons. These

materials are used in parts of electrical equipment, also

called insulators or insulation, intended to support or

separate electrical conductors without passing current

through themselves.

• The term is often used more specifically to refer to

insulating supports that attach electric power

transmission wires to utility poles or pylons.

• Some materials such as glass or Teflon are very good

electrical insulators.

• A much larger class of materials, for example rubber-like

polymers and most plastics are still "good enough" to

insulate electrical wiring and cables even though they

may have lower bulk resistivity.

• These materials can serve as practical and safe insulators

for low to moderate voltages (hundreds, or even

thousands, of volts).

• Insulators are used for high-voltage power transmission

are made from glass, porcelain, or composite polymer

materials. Porcelain insulators are made from clay, quartz

or alumina and feldspar, and are covered with a smooth

glaze to shed dirt.

• Insulators made from porcelain rich in alumina are used

where high mechanical strength is a criterion.

• Porcelain has a dielectric strength of about 4-10 kV/mm.

Glass has a higher dielectric strength, but it attracts

condensation and the thick irregular shapes needed for

insulators are difficult to cast without internal strains.

• Some insulator manufacturers stopped making glass

insulators in the late 1960s, switching to ceramic

materials.

• Recently, some electric utilities have begun converting to

polymer composite materials for some types of

insulators.

• These are typically composed of a central rod made of

fibre reinforced plastic and an outer weathershed made

of silicone rubber or EPDM.

• Composite insulators are less costly, lighter in weight, and

have excellent hydrophobic capability. This combination

makes them ideal for service in polluted areas.

• However, these materials do not yet have the long-term

proven service life of glass and porcelain.

• Design

• Cap and pin insulator string (the vertical string of discs)

on a 275 kV suspension pylon.

• The electrical breakdown of an insulator due to excessive

voltage can occur in one of two ways:Puncture voltage is

the voltage across the insulator(when installed in its

normal manner) which causes a breakdown and

conduction through the interior of the insulator.

• The heat resulting from the puncture arc usually damages

the insulator irreparably.

• Flashover voltage is the voltage which causes the air

around or along the surface of the insulator to break

down and conduct, causing a 'flashover' arc along the

outside of the insulator.

• They are usually designed to withstand this without

damage. High voltage insulators are designed with a

lower flashover voltage than puncture voltage, so they

will flashover before they puncture, to avoid damage.

• Dirt, pollution, salt, and particularly water on the surface

of a high voltage insulator can create a conductive path

across it, causing leakage currents and flashovers.

• The flashover voltage can be more than 50% lower when

the insulator is wet. High voltage insulators for outdoor

use are shaped to maximise the length of the leakage

path along the surface from one end to the other, called

the creepage length, to minimize these leakage currents.

• To accomplish this the surface is molded into a series of

corrugations or concentric disk shapes. These usually

include one or more sheds; downward facing cup-shaped

surfaces that act as umbrellas to ensure that the part of

the surface leakage path under the 'cup' stays dry in wet

weather. Minimum creepage distances are 20-25 mm/kV,

but must be increased in high pollution or airborne sea-

salt areas.

Cables

• A cable is one or more strands bound together. Electrical

cables may contain one or more metal conductors, which

may be individually insulated or covered.

• An optical cable contains one or more optical fibers in a

protective jacket that supports the fibers.

• Mechanical cables such as wire rope may contain a large

number of metal or fiber strands.

• Electrical cables may be made flexible by stranding the

wires. The technical issue is to reduce the skin effect

voltage drop while using with alternating currents.

• In this process, smaller individual wires are twisted or

braided together to produce larger wires that are more

flexible than solid wires of similar size. Bunching small

wires before concentric stranding adds the most

flexibility.

• A thin coat of a specific material (usually tin-which

improved striping of rubber, or for low friction of moving

conductors, but it could be silver, gold and another

materials and of course the wire can be bare - with no

coating material) on the individual wires.

• Tight lays during stranding makes the cable extensible

(CBA - as in telephone handset cords).

• Bundling the conductors and eliminating multi-layers

ensures a uniform bend radius across each conductor.

• Pulling and compressing forces balance one another

around the high-tensile center cord that provides the

necessary inner stability.

• As a result the cable core remains stable even under

maximum bending stress.

Cables can be securely fastened and organized, such as using

cable trees with the aid of cable ties or cable lacing.

Continuous-flex or flexible cables used in moving applications

within cable carriers can be secured using strain relief devices

or cable ties.

Copper corrodes easily and so should be layered with

Lacquer

UNIT-IV

INSULATORS AND CABLES

PART-A

1. What is the purpose of insulator? (2)

2. What is the main purpose of armouring? (2)

3. What is meant by efficiency of an insulator string? (2)

4. List out various types of insulators used for overhead transmission lines. (2)

5. Mention the advantages of the pin type insulator. (2)

6. What are the main causes for failure of insulators? (2)

7. What are the different tests that are conducted on an insulator? (2)

8 What are the methods for improving string efficiency? (2)

9. Write short notes on puncture test. (2)

10. Define impulse ratio. (2)

PART-B

1. Discuss any two methods to increase the value of string efficiency, with

suitable sketches. (16)

2. Explain any two methods of grading of cables with necessary diagrams. (16)

3. i) What are different methods to improve string efficiency of an insulator? (8)

ii) In a 3-unit insulator, the joint to tower capacitance is 20% of the capacitance

of each unit. By how much should the capacitance of the lowest unit be

increased to get a string efficiency of 90%. The remaining two units are left

unchanged. (8)

4. i) Derive the expression for insulator resistance, capacitance and electric

stress in a single core cable.Where is the stress maximum and minimum?(8)

ii) A single core 66kv cable working on 3-phase system has a conductor

diameter of 2cm and sheath of inside diameter 5.3cm. If two inner sheaths

are introduced in such a way that the stress varies between the same

maximum and minimum in the three layers find:

a) position of inner sheaths

b) voltage on the linear sheaths

c) maximum and minimum stress (8)

5. i) Draw the schematic diagram of a pin type insulator and explain its

function. (8)

ii) A 3 phase overhead transmission line is being supported by three disc

insulators. The potential across top unit (i.e. near the tower) and the middle

unit are 8kV and 11kV respectively. Calculate,

a) The ratio of capacitance between pin and earth to the self capacitance of

each unit (4)

b) Line Voltage (2)

c) String Efficiency (2)

6. i) Describe with the neat sketch, the construction of a 3 core belted type

cable. (8)

ii) A conductor of 1cm diameter passes centrally through porcelain cylinder of

internal diameter 2 cms and external diameter 7cms. The cylinder is

surrounded by a tightly fitting metal sheath. The permittivity of porcelain is 5

and the peak voltage gradient in air must not exceed 34kV/cm. Determine

the maximum safe working voltage. (8)

7. i) What are the various properties of insulators? Also briefly explain about

suspension type insulators. (8)

ii) Calculate the most economical diameter of a single core cable to be used

on 132kV, 3 phase system. Find also the overall diameter of the insulation,

if the peak permissible stress does not exceed 60kV/cm. also derive the

formula used here. (8)

8. i) Briefly explain about various types of cables used in underground system.(8)

ii) A string of 4 insulator units has a self capacitance equal to 4 times the pin to

earth capacitance. Calculate,

a) Voltage distribution as a % of total voltage

b) String efficiency (8)

9. i) Give any six properties of a good insulator. (4)

ii) With a neat diagram, explain the strain and stay insulators. (4)

iii) A cable is graded with three dielectrics of permittivities 4, 3 and 2. The

maximum permissible potential gradient for all dielectrics is same and

equal to 30 kV/cm. The core diameter is 1.5cm and sheath diameter is

5.5cm. (8)

10. i) Explain the constructional features of one LT and HT cable (8)

ii) Compare and contrast overhead lines and underground cables. (8)

UNIT V SUBSTATION GROUNDING SYSTEM AND

DISTRIBUTION SYSTEM

Types of substations – Bus-bar arrangements – Substation bus schemes – Single bus scheme –

Double bus with double breaker – Double bus with single breaker – Main and transfer bus –Ring

bus – Breaker-and-a-half with two main buses – Double bus-bar with bypass isolators –

Resistance of grounding systems – Resistance of driven rods, resistance of grounding point

electrode –Grounding grids – Design principles of substation grounding system – Neutral

grounding

UNIT-V

SUBSTATION GROUNDING SYSTEM AND DISTRIBUTION SYSTEM

PART-A

1. What is substation? (2)

2. What is earth resistance? (2)

3. What are the classifications of substation according to service? (2)

4. What are the types of transformer substations? (2)

5. What are the factors to be considered for busbar design? (2)

6. What is neutral grounding or neutral earthing? (2)

7. What are the equipments used in a transformer substation? (2)

8. What are the different types of bus bar arrangements in substations? (2)

9. What is bus bar? (2)

10. What are the materials mainly used in busbars? (2)

PART-B

1. With a neat sketch explain double bus with double breaker and double bus

with single breaker. State their advantages and disadvantages. (16)

2. Explain the following:

(i) Neutral grounding

(ii) Resistance grounding. (16)

3. Explain about the various types of substations (16)

4. Write short notes on

I. Sub mains (4)

II. Stepped and tapered mains (12)

5. Explain the substation bus schemes. (16)

6. Write short notes on

i. Busbar arrangement in substation (8)

ii. Grounding grids (8)

7. i) Explain the design principles of substation grounding system. (8)

ii) Explain the equipments in a transformer substation.