An electrical power system is a network of interconnected electrical devices, which are used to generate, transmit, distribute and utilise the electrical power.
A typical electrical power system has following main components −
In the single line diagram, the GS represents the generating station where electrical power is produced by three-phase parallel connected alternators. The usual generation voltage is 11 kV (in India). For economic considerations the generation voltage is stepped up to high voltages (say 132 kV) using 3-phase transformers for the transmission.
The electrical transmission system is divided into two types depending on the voltage levels as −
The primary transmission system conveys electrical power from generating stations to the receiving stations (RS). In this, the electrical power at 132 kV (or higher voltages) is transmitted by 3-phase 3-wire overhead system to the receiving stations located at the outskirts of the city.
The part of power system between receiving stations and sub-stations form secondary transmission system. The primary transmission terminates at receiving stations. At the receiving stations, the voltage is reduced to 33 kV using a step-down transformer. From the receiving station, electrical power is transmitted at 33 kV by 3-phase 3-wire overhead system to various sub-stations located in the city.
The distribution system is part of power system network which distributes electrical power the utilization. The distribution of electrical power is done in stages as follows −
This is the part of power system circuit which connects the sub-stations to the distribution sub-station or distribution transformer located near the consumer’s premises. Since, the secondary transmission line terminates at the sub-stations where voltage is decreased from 33 kV to 11 kV. The 11 kV lines run along the road sides of the city. It may be noted that, big consumers are supplied power at 11 kV for handling with their own sub-stations.
The secondary distribution system delivers electrical power at a voltage for direct utilisation by the electrical appliances. The primary distribution lines are terminated at distribution substations located near the consumer’s localities, where the voltage is stepped down to 415 V, 3-phase 4-wire for secondary distribution. In secondary distribution system, the voltage between any two lines is 415 V and between any one line and neutral is 230 V. The secondary distribution consists of feeders, distributors and service mains.
Any device which utilises the electrical energy for doing work is known as electrical load −
The electrical loads are of two types −
Single phase loads – These loads are supplied by any one phase and neutral, some 1-phase loads are fans, coolers, AC, 1-phase motors, electric bulbs etc.
Three phase loads – such as 3-phase induction motor. The three phase loads are of two type viz. star connected loads and delta connected loads.