Electric Power Distribution System Basics

Electrical power is dominant as it is relatively much easier to transmit and distribute than other forms of energy such as mechanical. Imagine transmitting mechanical energy to just 20 feet of distance. Isn't it much easier to use wires instead of belts, chains or shafts?
We have seen how electrical energy is generated in generating stations and how it is transmitted over long distances through transmission networks. Now, let's see how electrical power is distributed to the consumers.

Power Distribution System

A distribution substation is located near or inside city/town/village/industrial area. It receives power from a transmission network. The high voltage from the transmission line is then stepped down by a step-down transformer to the primary distribution level voltage. Primary distribution voltage is usually 11 kV, but can range between 2.4 kV to 33 kV depending upon region or consumer.
A typical power distribution system consists of -
  • Distribution substation
  • Feeders
  • Distribution Transformers
  • Distributor conductors
  • Service mains conductors
Along with these, a distribution system also consists of switches, protection equipment, measurement equipment etc.
Distribution feeders: The stepped-down voltage from the substation is carried to distribution transformers via feeder conductors. Generally, no tappings are taken from the feeders so that the current remains same throughout. The main consideration in designing of a feeder conductor is its current carrying capacity.
Distribution transformer: A distribution transformer, also called as service transformer, provides final transformation in the electric power distribution system. It is basically a step-down 3-phase transformer. Distribution transformer steps down the voltage to 400Y/230 volts. Here it means, voltage between any one phase and the neutral is 230 volts and phase to phase voltage is 400 volts. However, in USA and some other countries, 120/240 volts split-phase system is used; where voltage between a phase and neutral is 120 volts.
Distributors: Output from a distribution transformer is carried by distributor conductor. Tappings are taken from a distributor conductor for power supply to the end consumers. The current through a distributor is not constant as tappings are taken at various places throughout its length. So, voltage drop along the length is the main consideration while designing a distributor conductor.
Service mains: It is a small cable which connects the distributor conductor at the nearest pole to the consumer's end.
simple radial AC power distribution
The above figure shows a simple radial AC power distribution system. The figure does not show other equipment like circuit breakers, measuring instruments etc. for simplicity purpose.

Primary distribution

It is that part of an AC distribution system which operates at somewhat higher voltages than general residential consumer utilization. Commonly used primary distribution voltages in most countries are 11 kV, 6.6 kV and 3.3 kV. Primary distribution handles large consumers such as factories and industries. It also feeds small substation from where secondary distribution is carried out. Primary distribution is carried out by 3-phase, 3-wire system.

Secondary distribution

This part directly supplies to the residential end consumers. Domestic consumers are fed with single phase supply at 230 volts (120 volts in USA and some other countries). Three phase supply may also be provided at 400 volts for big properties, commercial buildings, small factories etc. Secondary transmission in most countries is carried out by 3-phase, 4-wire system.

Classification of power distribution systems