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1
Concept of Energy Transmission
Group Members
MadihaNaeem
MuqadsaIftikhar
Zunaib Ali
2
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.
3
Main Parts of Power System
Four Main Parts:
Generation System
Transmission System
Distribution System
Consumer (LOAD)
4
Simplified Diagram of Power System
5
Generating Station
Thermal Power Plant
Nuclear Power Plant
Hydro Power Plant
Gas Turbine Power Plant
Combined Cycle Power Plant
6
The commonly used power plants are:
One-Line Diagram of Generating Station
7
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
9
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.
10
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…
11
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.
12
Components of a Transmission Line
Conductor
Earth wire
Insulator
Transmission Tower
Wave trap and other hardware(Clamp, Spacer, Vibration dampers, connectors
etc.
13
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.
14
Selection of Transmission Voltage
Standard Voltage: 66,110,132, 220, 400 KV
Tolerances - ±10% up to 220 KV & ±5% for 400 KV
15
• Quantum of power to be evacuated
• Length of line
• Voltage regulation
• Power loss in Transmission
• Initial and operating cost
Selection Criterion
of Economic Voltage
16
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
18
• 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
20
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
21
EHV- Tower
22
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
23
Fig. Typical 765 KV Tower Structure
Height of Tower Structure
4321 hhhhH
24
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
25
Determination of Base Width
25
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
26
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
27
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:
28
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
29
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)(
2
VScmSpacing
Where-
V= Voltage of system in KV
S= Sag in cm
Spacing Between Conductor(Phases)
Mecomb's formula
VDE formula
30
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
31
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
32
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)
33
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.
34
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
21
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
35
• 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
36
Continuous Current Rating.
37
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.)
38
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
39
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
41
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
42
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
43
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
44
• 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
45