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