ac transmission line
Post on 16-Feb-2016
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AC Transmission Line
By: Adisu Teshale
Outlines• AC and DC Supply System• Over head and Underground Txn. & Dxn.
System• AC Overhead Transmission line• Conductor (Resistance, Inductance,
Capacitance of Txn. line)• Insulator of overhead Transmission Line• Mechanical Design of Overhead Txn. line
Advantage of DC Transmission over AC
• There are two conductors used rather than three/more.
• There are no Inductance
• There is no Skin effect
• Small cross sectional area conductor required compared to AC.
• DC requires less insulation than AC.
• There is no interference with communication system.
• Corona losses are very low.
• No difficulties in synchronizing and stability problems.
• DC is more efficient than AC system
• DC requires less insulation than AC system.
Disadvantages of DC Transmission
• Due to commutation problem, Electric power can’t be
produce at High (DC) Voltage.
• For High Voltage transmission, we can not step the level of DC
Voltage (As Transformer can not work on DC)
• The Switches and Circuit breakers used in DC is a limited
capacity and cost.
• The level of DC Voltage can not be change easily
Advantages of AC Transmission System
• The Level of AC voltage may be increased or decreased step
up and Step down transformers.
• AC Circuit breakers is cheap than DC Circuit breakers.
• The repairing and maintenance of AC sub station is easy and
inexpensive than DC Substation.
• Nowadays, the generation, transmission and distribution
mostly is in AC.
Disadvantages of AC System
• In AC line, the size of conductor is grater than DC Line hence the Cost
of AC Transmission lines are greater than DC.
• Due to Skin effect, the losses in AC system are more.
• In AC Lines, due to Capacitance &inductance effect, there is
continuously power losses.
• More insulation required in AC System
• Also corona Losses occur In AC System,
• There is telecommunication interference in AC System.
• There are stability and synchronizing problems in AC System.
• DC System is more efficient than AC System.
Overhead vs. Underground Transmission line• Cost: Initial cost of underground is 4 to 15 times higher than overhead. While
running & maintenance cost of over head is higher.
• Flexibility: Overhead is more flexible than underground
• Reliable: Under ground is reliable than Overhead line
• Frequency of fault: frequency of fault is higher in OH. However fault location
is difficult in UG Txn. Line.
• Voltage level: the working voltage of UG Txn line is limited to 66KV.
• Interference to communication: OH line highly exposed to radio interference
& storms.
• Safety & Appearance: UG is more safe & good appearance.
Overhead vs. Underground
Underground Transmission line
• Depending on their Voltage level and types of insulators, Under
ground cables are classified as:
i. Low Tension cable – support up to 1KV
ii. High Tension cable – support up to 11KV
iii. Super Tension cable – support up to 33KV
iv. Extra High Tension cable – support up to 66KV
v. Extra super Tension cable – support > 66KV
Due to voltage limitation and cost , Under Ground systems are
recommended for Distribution system.
Overhead Transmission line
• High-voltage overhead transmission lines are a reliable, low cost,
easily maintained and most commonly used method to transport
bulk electricity across long distances.
• Generally main components of Over head Transmission line are:
- Conductor material
- Line support (pole, tower)
- Insulator
- Cross arms
- Miscellaneous
Conductor materials of OH Txn. line- Copper is an ideal conductor to be used in OH Txn. Line due to its cost.
Hence today Alloy of Aluminum is widely used.
- The most commonly used conductors in OH Txn. Line are:
• All Aluminum conductor (AAC)
• All Aluminum Alloy conductor (AAAC) conductor
• Aluminum conductor steel reinforced (ACSR) conductor
• Aluminum conductor Alloy reinforced (ACAR) conductor
• New Technology Conductors ( Trapezoidal, ACSS, INVAR, Self Damping,
Vibration Resistant…. are used)
All conductors are stranded type.
Electrical properties of OH Conductors
• behaves
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