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Electrical Transmission and Distribution Systems

Updated on January 6, 2015

The electrical energy is generated at generating station is conveyed to the consumers, through a network of transmission and distribution systems.


Transmission system:

The energy or power can be transferred from a power station to loads (i.e. consumers) is known as transmission.

They are types of transmissions

AC transmissions system

DC transmissions system and

The overhead (OH) lines and underground systems,They are:

  • DC-2 wire system
  • DC-3 wire system
  • AC 1-phase 2 wire system
  • AC 3-phase 3wire system

Over head (OH) system:

The transmission line which is placed above earth surface or the cross arms of towers is known as OH system.

Underground system:

The transmission line placed below the earth (or) ground is known as the underground system.

Components required for the OH lines:

  • Cross arms
  • Earth wires
  • Danger plates
  • Guard wires.etc

  • Towers
  • Conductors
  • Insulators
  • Poles

Towers (or supports):

The conductors are carried from one place to another place by poles with the help of cross arms and insulators.

Types of poles:

  • Wooden poles (upto 20kv)
  • Steel poles (upto 33kv)
  • Reinforced cement concrete ( RCC poles) (upto 11kv)
  • Towers (above 11kv)

Tower: Towers are placed according to the voltage should be transmitted. (Above the 132kv)

Types of Towers:

  • Rigid self supporting lattice towers
  • Semi-flexible towers
  • Flexible towers (guyed tower)



It is one of the most important materials in the transmission and distribution system of electrical power most of capital (money invested). All the conductors are used for OH lines. Generally in a standard conductor there is one centre wire and around, they are successive layers of wires. Therefore, the numbers of individual wires are calculated by the formula is 3n (n+1) +1 where n = 1, 2, 3, 4…

Materials used for conductors:

The most commonly used materials in OH lines are

They are

  • Copper
  • All aluminum standardized conductors (AAC or AASC)
  • Aluminum Conductor Steel Reinforced (ACSR)
  • Galvanized Steel-Cadmium Copper

Among these materials commonly used material is ACSR conductor.



The function of insulator is to provide insulation resistance for the leakage current from OH line to Earth.

Materials used in insulators:

  • Porcelain
  • Glass
  • Steatite
  • Various artificial materials

Different types of Insulators:

  • Pin type Insulators
  • Suspension type
  • Strain type
  • Shackle type

DC transmissions:

The first electrical central station was built by Thomas Alva Edison at Pearl Street in New York. And it was begun in the year of 1882 with operating DC voltage at 110Volts. Nowadays, modern civilization depends on the consumption of electrical energy for agricultural, commercial, domestic and social purpose. The transmission system is called as DC transmission system. It supplies the bulky power transmission over a long distance.

Types of transmission system:

The types of transmission system are depends up on the rating of voltages. They are

  1. High voltage AC transmission (H VAC)- upto 220KV
  2. Extra- High voltage AC transmission (EH VAC)- 220 to 760KV
  3. Ultra- High voltage AC transmission (UH VAC)- above 760KV
  4. High voltage DC transmission (HVDC)- upto 600KV

Principles of HVDC:

It is mainly consists of rectifier (converter) and inverter for converting AC to DC and DC to AC at receiving end and sending end. The role of rectifier and inverter station can be reversed by suitable converter control. It requires mainly converter stations, transformers, AC substation at starting end, smoothing reactors, electrode line, earth electrode, HVDC transmission line, inverter stations etc...

HVDC system which are working condition throughout the world:

1. New Zealand (back to back) - 250KV, 600MW

2. Pacific intertie (US)-400KV, 1600MW

3. Itaipu (Britain)-300KV, 1575 MW

4. Black water (Back to back) (US)-56KV, 200MW

5. Sakuma (back to back) (Japan)-250KV, 300MW

Types of HVDC systems

  1. Mono polar HVDC Transmission system
  2. Bipolar HVDC Transmission system
  3. Homo polar HVDC Transmission system
  4. Back to Back coupling system
  5. Multi terminal HVDC Transmission system

Advantages of AC transmission:

  • Maintenance of ac substation is simple
  • Low voltage can be easily step up or step down by using transformer
  • Electrical power can be generated at high voltages

Advantages of DC transmission:

  • It requires two conductors only
  • There is no losses
  • Easily constructed

Distribution system:

The conductor carries the current or electrical energy is conveyed from a bulk power source (substation) to the consumer or load is called distribution.

There are two types of distribution systems, They are:

1. Primary distribution (high voltages)

2. Secondary distribution (low voltages)

Primary distribution:

It is defined as the power is conveyed for distribution and also to the back to back supply consumers (voltages, 11KV, 6.6KV, 3.3KV Etc..) is known as primary distribution.

Secondary distribution:

It is placed between the primary distribution system and consumers and are supply the power to at different consumers are loads (voltages, 440volts, 230volts) is known as secondary distribution.


Types of distribution systems

Under scheme connection

1. Radial distribution system

2. Ring distribution system

3. Parallel distribution system

4. Inter connection distribution system


The inter connection of operates, used for step down or step up voltage for consumer utilized level that place is called substation.

The substation consists of the Transformers, Conductors, Feeders, Relays, Bus bars, Insulators, Fuses, current transformers (CT), potential transformers (PT) etc..


Types of sub stations:

  1. Static sub stations
    • Primary sub station
    • Secondary sub station
    • Distribution system
  2. Indoor sub stations
  3. Outdoor sub stations
    • Pole mounted (11kv/440volts)
    • Plinth mounted (11kv/440volts)
  4. Underground sub stations

Layouts of sub stations:

1.33/11kv sub station

2.132/11kv sub station



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