Electric Supply System | A.C. Power Supply Scheme & its Single Line Diagram

What is Electric Supply System?

The carrier of electrical power from power generating station to the consumer’s premises for its utilisation is called the electrical supply system.

The whole electric supply system is segmented into three principle components, viz. −

  • Power generating station

  • Transmission system

  • Distribution system

The electric power is produced at the power generating station which are located quite away from the consumers. Then, the power is transmitted over large distances to load centres with the help of conductors called transmission lines. Finally, it is distributed to a large number of consumers through a distribution system.

Types of Electric Supply System

Depending upon the types of supply, the electric supply system is of following types −

  • AC Supply System
  • DC Supply System

Based on the method used for transmission and distribution of electric power, the electric supply system is of two types viz. −

  • Overhead Supply System
  • Underground Supply System

Nowadays, the 3-phase 3-wire AC system is universally adopted for generating and transmission of electric power while the 3-phase 4-wire AC system is used for distribution.

AC Power Supply Scheme and Single Line Diagram

The network of conductors between the electric power station and the consumers can be divided into two parts and each part can be further sub-divided into two parts as −

  • Transmission System
    • Primary Transmission System
    • Secondary Transmission System
  • Distribution System
    • Primary Distribution System
    • Secondary Distribution System

The single line diagram (SLD) of a typical electric AC supply system is shown in the figure. The various parts of the electrical supply are described below −

  • Generating Station – In the SLD, the GS represents the generating station where the electric power is produced by 3-phase alternators operating in parallel. The usual generating voltage is 11 kV due to some technical considerations.

  • Primary Transmission – The electric power produced at 11 kV is stepped upto 132 kV at the generating station with the help of 3-phase transformers. Then, the electric power at 132 kV is transmitted by 3-phase 3-wire overhead transmission system to the outskirts of the city. This part of electric supply system forms the primary transmission.

  • Secondary Transmission – The primary transmission lines terminate at the receiving station (denoted by RS). At the receiving station, the voltage is stepped-down to 33 kV using 3-phase step down transformers. From the RS, the electric power is transmitted at 33 kV by 3-phase 3-wire overhead transmission system to various sub-stations (denoted by SS) located at the strategic points in the city. This part of electric supply system called the secondary transmission.

  • Primary Distribution – The secondary transmission lines terminate at the sub-stations where the voltage is further reduced from 33 kV to 11 kV. The 11 kV distribution lines run along the road sides of the city. This segment of the supply system forms the primary distribution. It is important to note that the large consumers (demand > 50 kW) are generally supplied the electric power at 11 kV for further handling with their own sub-stations.

  • Secondary Distribution – The electric power at 11 kV, i.e., from primary distribution lines is delivered to distribution substations (represented by DS). These DS are located near the consumer’s localities and step-down the voltage to 440 V for secondary distribution. Here, the voltage between any two phases is 440 V while the voltage between any one phase and the neutral is 230 V. Therefore, the secondary distribution system consists of feeders, distributors and services mains.