Introduction to Electric Power Supply System

Introduction to Electric Power Supply System

An electric power system consists of three principal components: the power station, the transmission lines and the distribution systems.

The ever increasing use of electric power for domestic, commercial & industrial purposes necessitates to provide bulk electric power economically. This is achieved with the help of suitable power producing units, known as power plants or electric power generating stations.

There is a large network of conductors between the power plant & consumers. This network can be broadly divided into two parts. viz. transmission and distribution of electrical energy.

Layout of Electric Power System

The electrical system consisting of generation, transmission and distribution of electric power is called electric power system.

Figure shows the one-line diagram of a very simple electric power system. The power stations are located at favorable places, generally quite far away from the consumers. Owing to economical reasons, electric power has to be transmitted through transmission lines at a very high voltage. Therefore voltage at power station is stepped to a high value (say 220kV) by the step-up transformer. The electric power is carried by the transmission lines at this high voltage. At the outskirts of the city, this voltage is reduced considerably (say 11kV) by a step-down transformer (also called transmission sub-station). The feeders carry the power to distribution sub-station. Feeders should not be tapped for direct supply. At the distribution sub-station, the step-down transformer reduces the voltage to 400V. Distributors are used to supply power to various consumers to meet their load demands as and when required.

Electric Supply System

The conveyance of electric power from a power station to consumers' premises is known as electric supply system.

Now-a-days, 3-phase, 3-wire a.c. system is universally adopted for generation and transmission of electric power, distribution of electric power is done by 3-phase, 4-wire a.c. system.

Typical A.C. Power Supply Scheme

The large network of conductors between the power station and the consumers can be divided into two parts viz., transmission system and distribution system. Each part can further divided into two- primary transmission and secondary transmission and primary distribution and secondary distribution.

(i) Generating station- In fig below G.S. represents the generating station where electric power is produced by 3 phase alternators. The transmission of electric power at high voltage has several advantages including the saving of conductor material and high transmission efficiency.

(ii) Primary transmission- The electric power at 132kV is transmitted by 3-phase, 3-wire overhead system to the outskirts of the city. This forms the primary transmission.

(iii) Secondary transmission- The primary transmission line terminates at the receiving station (RS) which usually lies at the outskirts of the city. At the receiving station, the voltage is reduced to 33 kV by step-down transformers.

(iv) Primary distribution- The secondary transmission line terminates at the substation (SS) where voltage is reduced from 33kV to 11kV, 3-phase, 3-wire.

(v) Secondary distribution. The electric power from primary distribution line (11kV) is delivered to distribution sub-stations (DS). These sub-stations are located near the consumers' localities and step down the voltage to 400V, 3-phase, 4-wire for secondary distribution. The voltage between any two phases is 400V and + between any phase and neutral is 230V.

Interconnected Grid System

The connection of several generating stations in parallel is known as interconnected grid system.

Some of the advantages of interconnected system are listed below:

(i) Exchange of peak loads. If the load curve of a power station shows a peak demand that is greater than the rated capacity of the plant, then the excess load can be shared by other stations interconnected with it.

(ii) Use of older plants. The interconnected system makes it possible to use the older and less efficient plants to carry peak-loads of short durations. Although such plants may be inadequate when used alone, yet they have sufficient capacity to carry short peaks of 103 when interconnected with other modern plants.

(iii) Ensures economical operation. Sharing of load among the stations is arranged in such a way that more efficient stations work continuously throughout the year and the less efficient plants work for peak load hours only.

(iv) Increases diversity factor. Maximum demand on the system is much reduced as compared to the individual maximum demands on different stations.

(v) Reduces plant reserve capacity. Every power stations is required to have a standby unit for emergencies.