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Concept of Energy Transmission

Group Members

MadihaNaeem

MuqadsaIftikhar

Zunaib Ali

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COMSATS INSTITUTE OF INFORMATION & TECHNOLOGY, ABBOTTABAD

Purpose of Electrical Transmission System

The purpose of the electric transmissionsystem is the efficient interconnection ofthe electric energy producing powerplants or generating stations with theloads.

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Main Parts of Power System

Four Main Parts:

Generation System

Transmission System

Distribution System

Consumer (LOAD)

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Simplified Diagram of Power System

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Generating Station

Thermal Power Plant

Nuclear Power Plant

Hydro Power Plant

Gas Turbine Power Plant

Combined Cycle Power Plant

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The commonly used power plants are:

One-Line Diagram of Generating Station

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Fig.1: Simplified Connection Diagram

Main Parts of Generating Station

Types based on Insulators

8Fig.2: CB Diagram

Circuit Breaker (CB)

A circuit breaker is an automatically operated electrical switch, designed to protect an electrical circuit from damage caused by faultcurrent or short circuit

Oil Circuit Breaker

Air Circuit Breaker

SF6 Circuit Breaker

Vacuum Circuit Breaker

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Type Medium Voltage, Breaking Capacity

Air break Circuit Breaker Air at atmospheric pressure(430 – 600) V– (5-15)MVA

(3.6-12) KV - 500 MVA

Miniature CB. Air at atmospheric pressure (430-600 ) V

Tank Type oil CB. Dielectric oil (3.6 – 12) KV

Minimum Oil CB. Dielectric oil (3.6 - 145 )KV

Air Blast CB.Compressed Air

(20 – 40 ) bar

245 KV, 35000 MVA

up to 1100 KV, 50000 MVA

SF6 CB. SF6 Gas

12 KV, 1000 MVA

36 KV , 2000 MVA

145 KV, 7500 MVA

245 KV , 10000 MVA

Vacuum CB. Vacuum 36 KV, 750 MVA

H.V.DC CB. Vacuum , SF6 Gas 500 KV DC

Table.1: Circuit Breaker Description

Main Parts of Generation Station…

Disconnect Switch

Provides visible circuit separation andpermits CB maintenance. It can beoperated only when the CB is open i.e. inno-load condition.

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Surge Arrester

Used for protection against lightning andswitching over-voltages. They are voltagedependent, nonlinear resistors (Varistors).The arrester provides a low-impedance pathto ground for the current from a lightningstrike or transient voltage and then restoresto a normal operating condition.

Main Parts of Generation Station…

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Current Transformers (CT) and Potential Transformers (PT)

Used to lower the magnitude of the current and voltage to be measured.

In case of normal meters, to measure current and voltage in a high voltage circuit at 220kV, properly insulated meters are needed to withstand that voltage. The meters will be very big for that purpose.

The CT and PT is used to solve this problem. The CT and PT works on the principle of transformer and lowers the current and/or voltage at a lower value which can be safely and easily measured.

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Components of a Transmission Line

Conductor

Earth wire

Insulator

Transmission Tower

Wave trap and other hardware(Clamp, Spacer, Vibration dampers, connectors

etc.

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Design Methodology

• Gather preliminary line design data and available climatic data

• Select reliability level in terms of return period of design

• Calculate climatic loading on components

• Calculate loads related to safety during construction and maintenance

• Select appropriate correction factors, if applicable, to the design components such as use factor, strength factors related to numbers of components, quality control, and the characteristic strength.

• Design the components for the above loads and strength.

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Selection of Transmission Voltage

Standard Voltage: 66,110,132, 220, 400 KV

Tolerances - ±10% up to 220 KV & ±5% for 400 KV

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• Quantum of power to be evacuated

• Length of line

• Voltage regulation

• Power loss in Transmission

• Initial and operating cost

Selection Criterion

of Economic Voltage

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1506.15.5*

KVALE

*

Economic Voltage of Transmission of Power

KVA=Power to be transferred

E = Transmission voltage (KV) (L-L).

L = Distance of transmission line in KM

Typical Transmission Voltage Levels

Voltages Level Range (KV)

Maximum Length (Miles)

High Voltage 100 to 230 200

Extra High Voltage 230 to 800 400 to 500

Ultra High Voltage Above 800 1300

Types of Towers

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• Used on straight runs and up to 2° line diversionType A Tower (Tangent Tower

with suspension string)

• Used for line deviation from 2° to 15°Type B Tower (Small Angle Tower with tension string)

• Used for line deviation from 15° to 30°.Type C Tower (Medium Angle

Tower with tension string ).

• Used for line deviation from 30° to 60°Type D Tower (Large angle tower with tension string)

• Used for line termination & startingType E Tower (Dead End

Tower with tension string)

• Used for transposition of towerTransposition Tower

• Used for River crossing, Mountain crossing etc.Suspension Tower (Span

≈ 1000 m)

Suspension Tower Transposition Tower Tension Tower

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Different Types of Towers

Selection of Tower Structure

Single circuit Tower/ double circuit Tower

Length of the insulator assembly

Minimum clearances to be maintained between conductors, and between conductors and tower

Location of earth wire/wires with respect to the outermost conductor

Mid-span clearance required from considerations of the dynamic behavior of conductors and lightning protection of the line

Minimum clearance of the lowest conductor above ground level

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EHV- Tower

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Shield conductors: • Two grounded shield

conductors protect the phase conductors from lightning.

Tower: • The figure shows a lattice,steel tower.

Insulator: • V strings hold four

bundled conductors in each phase

Conductor: • Each conductor is

stranded, steel reinforced aluminum cable.

Foundation and grounding:

• Steel-reinforced concrete foundation and grounding electrodes placed in the ground

Tower Design

Tower height

Base width

Top damper width

Cross arms length

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Fig. Typical 765 KV Tower Structure

Height of Tower Structure

4321 hhhhH

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Height of tower is determine by-

h1=Minimum permissible ground clearance

h2=Maximum sag

h3=Vertical spacing between conductors

h4=Vertical clearance between earth-wire and top conductor

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Determination of Base Width

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Ryle

Formula

The base width(at the concrete level) is the distance between the centre of gravity at one corner leg and the centre of gravity of the adjacent corner leg.

A particular base width which gives the minimum total cost of the tower and foundations

An applied force that causes a structure to turn over

The ratio of base width to total tower height for most towers is generally about one-fifth to one-tenth.

Spacing and Clearances

Ground Clearances

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K*0.3055.182CL

33

33VK

Where-

S.No. Voltage level Ground clearance(m)

1. ≤33 KV 5.20

2. 66 KV 5.49

3. 132KV 6.10

4. 220 KV 7.01

5. 400 KV 8.84

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Clearance for Power Line Crossings

•Minimum clearances between the conductors of a power line and telecommunication wires are

Crossing over telecommunication lines

Voltage Level Minimum Clearance(mm)

≤33 KV 2440

66KV 2440

132 KV 2740

220 KV 3050

400 KV 4880

• 3.05m above maximum flood level.Crossing over

rivers:

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Power line Crossing another Power line

System Voltage Level Clearance(m)

≤ 66 KV 2.40

132 KV 2.75

220KV 4.55

400 KV 6.00

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SW

DVcmSpacing 010.43048.0)( *

Where-

V= Voltage of system in KV

D= Diameter of Conductor in cm

S= Sag in cm

W= weight of conductor in Kg/m

20005.7)(

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VScmSpacing

Where-

V= Voltage of system in KV

S= Sag in cm

Spacing Between Conductor(Phases)

Mecomb's formula

VDE formula

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Still's formula

8.27

2

*814.108.5)(l

VcmSpacing

Where-

l = Average span length(m)

NESC formula

2681.3*762.0)(

LSVcmSpacing

Where-

V= Voltage of system in KV

S= Sag in cm

L= Length of insulator string in cm

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Swedish formula

EScmSpacing *7.05.6)(

Where-

E= Line Voltage in KV

S= Sag in cm

French formula

5.10.8)(

ELScmSpacing

Where-

E= Line Voltage in KV

S= Sag in cm

L= length of insulating string(cm)

Clearances b/w Conductors

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SYSTEM

VOLTAGE

TYPE OF

TOWER

Vertical spacing b/w

conductors(mm)

Horizontal spacing b/w

conductors(mm)

66 kV

SINGLE

CIRCUIT 1080 4500

DOUBLE

CIRCUIT 2060 5550

132 KV

SINGLE

CIRCUIT 4200 7140

DOUBLE

CIRCUIT 3965 7320

Types of Conductors

• AAC(All Aluminium Conductor)

• AAAC(All Alloy Aluminium Conductor)

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Fig. AAC Conductors Fig. AAAC Conductors

• ACSR Conductor(Aluminium Conductor Steel Reinforced)

Table: Technical Data of ASCR Conductors Commonly used in EHV Transmission By Wapda.

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CodeWords

StrandingAluminum SteelConductor Core

No/mm No/mm

Weight Per KmAluminum Steel

kg/km kg/km

WeightCompleteConductor

kg/km

DiameterComplete SteelConductor Core

mm mm

CrossArea of

Aluminum

mm2

Area ofCompleteConductor

mm2

RatedUltimateStrength

kg

D.CResistance

at 200

ohm/km

Gopher

Rabbit

Dog

Hare

Osprey

Cuckoo

Zebra

Moose

Panther

6/2.36

6/3.35

6/4.72

6/4.72

16/4.465

24/4.62

54/3.18

54/3.53

30/3

1/2.36

1/3.35

7/1.57

1/4.72

¼.465

7/3.08

7/3.18

7/3.53

7/3

72

145.1

288.1

288.1

777

1116

1182

1463

588

34.1

68.8

106.2

136.5

121.8

407.6

439

535

387

106

214

394

425

898.8

15424

1621

1998

976

7.08

10.05

14.15

14.16

22.23

27.72

28.62

31.77

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2.36

3.35

4.71

4.72

4.465

9.24

3.18

3.53

3

26.25

52.88

104.98

105

281.9

402.33

428.9

528.5

212.1

30.62

61.69

118.53

122.5

297.56

454.48

484.59

597.0

261.5

980

1875

3225

3225

6220

12385

13000

16224

9150

1.093

0.543

0.273

0.273

0.123

0.072

0.0686

0.0559

0.07311

Fig. ASCR Conductors

400kv - 'Moose' ACSR

220kv - 'Zebra' ACSR

132kv - 'Panther' ACSR

Selection of Conductor Size

• Tensile Strength(For Tension)

• Strain Strength(For Vibration)

Mechanical Requirement

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• Mechanical Requirement

• Electrical Requirement

Use vibration damper for vibration control.

• Continuous current rating.

• Short time current carrying rating.

• Voltage drop

• Power loss

• Minimum dia to avoid corona

• Length of line

• Charging current

ElectricalRequirement

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Continuous Current Rating.

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The designated RMS alternating current in amperes which a conductor will carry continuously in free air without tripping or exceeding temperature limits.

This current rating can be adjusted for specific ambient temperature without exceeding the normal allowable maximum temperature a line trap can withstand.

The normal continuous current rating of line traps is per manufacturer’s nameplate and based at 40°C ambient temperature.

Short Time Rating

Where A=area of conductor(mm2)

IF= fault current(KA)

t= fault duration(1 sec.)

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tIA F **58.7

According to short time rating conductor size is given by-

Corona

• V0= corona starting voltage, KV(rms)

• r= radius of conductor in cm

• D= GMD equivalent spacing b/w conductors in cm

• m= roughness factor

= 1.0 for clean smooth conductor

=0.85 for stranded conductor

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r

Dn

r

rmV log

)3.01(1.210

A corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor that is electrically energized.

Visual corona voltage in fair weather condition is given by-

INSULATOR

Insulator are required to support the line conductor and provide clearance from ground and structure.

Insulator material-

• High grade Electrical Porcelain

• Toughened Glass

• Fiber Glass

Choice of insulator material is govern by availability, price and ease of maintenance.

Porcelain insulator are largely used in Pakistan.

Earth Wire

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Earth wire provided above the phase conductor across the line and grounded at every tower.

• It shield the line conductor from direct strokes

• Reduces voltage stress across the insulating strings during lightning strokes

Design criterion:

• Shield angle

• 25°-30° up to 220 KV

• 20° for 400 KV and above

• Duration should be consider as 200 µ-sec

• Earth wire should be adequate to carry very short duration lightning surge current of 100 KA without excessive over heating

• Safe temp rise limited to 300°C

tIA 5A= Area(in mm2) of cu conductor

I =current in KA

t = Time insecond

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System voltage Mid span clearance(m)

≤ 66 KV 3.0

110 KV 4.5

132 KV 6.1

220 KV 8.5

400 KV 9.0

Mid span clearance:

• Direct distance b/w earth wire and top power conductor.

Following value of mid span clearance should be considered

Tower Grounding

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Used to reduce earth wire potential and stress on insulators at the time of stroke and also for safety

• Tower footing resistance will be 10Ω and should not be more than 20 Ω under any condition throughout the year

• Earth resistance depend upon soil resistivity(general 100 Ω-m)

Method of Tower Grounding

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• One or more conductor are connected to tower lags and buried in back filled of tower foundation.

• Used where soil resistivity is low

Buried Conductor

•A length of wire/ Strip of 50 m is buried horizontally at depth of 0.5 m bellow ground. This wire is connected to tower lags.

•Used when earth resistance is very high and soil conductivity is mostly confined to upper layer)

Counterpoise Wire

• Pipe/Rod of 3 to 4 m is driven into ground near the tower and top of rod is connected to tower by suitable wire/strip

• Used where ground conductivity increase with depth

Rod Pipe

• Pipe/Rod of 3 to 4 m are buried in treated earth pits and top of rod is connected to tower by suitable wire/strip.

• Used in very high resistivity near tower

Treated Earth Pits

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