Power system single line diagram

Single line diagram of typical AC supply system (single line diagram of power system from generation to distribution)

Single line diagram of power system short note :

The extensive network of conductors between the power station and the loads (consumers) can be mainly divided into two parts, are transmission system and distribution system.

 Each system can be furthered sub-divided into two :-- 

1. Primary transmission and Secondary transmission
2. Primary distribution and Secondary distribution

The below figure shows the layout of a typical a.c. power supply system by a single line diagram 

Single line diagram of typical AC supply system

NOTE: It is not necessary that all the power supply scheme include all the stage that are shown in above diagram.

Let's see what are the stages are in a single diagram of the power system and how they operate.

(i) Generating station : The very first step of a single line diagram (i.e. typical AC supply system) is a generating station where electric power is produced by 3-phase synchronous generators (alternators) operating in parallel. The commonly generating voltage is 11 kV (It may be 6.6 kV or even 33 kV in certain cases). For economy purpose the generating voltage that is 11 kV is stepped up to 132 kV or more(depend upon length of the transmission line and amount of power) in the transmission line at the generating station by the help of 3-∅ transformers.

The transmission of electric power at high voltages has various advantages includes : 

• The saving of conductor material
• High transmission efficiency

(But there is a limit to which the transmission voltage can be increased)

In general the primary transmission is carried out at 66 kV, 132 kV, 220 kV or 400 kV.

(ii) Primary transmission : The electric power at 132 kV from the generating station is transmitted by 3-∅, 3 wire overhead system to the outskirt of the city. This process or transmission of power is called primary transmission.

(iii) Secondary transmission : The primary transmission line terminates at the RS(receiving station) which generally placed at the outskirts of the city. At the RS(receiving station) the voltage is brought down to 33 kV by step-down transformer. From RS the electric power is transmitted at 33 kV by 3-∅, 3-wire overhead system to various sub-stations(SS) situated at specific points in the city. This forms the secondary transmission.

(iv) Primary distribution : The secondary transmission ends at the SS(sub-station) where voltage is brought down from 33 kV to 11 kV, 3-phase, 3-wire. The 11 kV lines run along the main road sides of the city. This forms the primary distribution. In some cases the large consumers are supplied at power 11 kV for further handling with their own sub-stations. Example: Colleges.

(v) Secondary distribution : The electric power from primary distribution line (11 kV) is delivered to DS (distribution sub-station). The sub-station is situated or located at the consumers localities and step down the voltage to 400 V, 3-∅, 4-wire for secondary distribution. The voltage between the two phases (P-P) is 400 V and between phase and neutral (P-N) is 230 V. The 1-∅ residential load is connected between any one phase and neutral, whereas 3-∅, 400 V motor load is connected across 3-∅ lines directly.

The secondary distribution system is consist of : 

• Feeders
• Distributors and
• Service mains

Important Points :

1. Classification of transmission system :

• Primary transmission 
• Secondary transmission

2. Classification of distribution system :

• Primary distribution
• Secondary distribution

--- Describe the basic structure of a single line diagram.

--- What is single line diagram in power system ?