Electric Power Distribution System Basics

What is a Distribution System?

The part of the power system that distributes electric power for local use is called as distribution system. Generally, a distribution system is the electrical system between the substation fed by transmission system and the consumer’s meters. A typical distribution system is shown in the figure.

Components of Distribution System

  • Distribution Sub-Station – A distribution sub-station is the electrical system which transfers power from transmission system to the distribution system of an area.

  • Feeders – A feeder is a conductor which connects the distribution sub-station to the area where power is to be distributed. The current in a feeder remains the same throughout its length because no tapings are taken from it. The main consideration in the design of a feeder being its current carrying capacity.

  • Distribution Transformers – The distribution transformer is a step-down transformer in which primary and secondary are delta and star connected respectively. It is also termed as service transformer. The output voltage of distribution transformer is 440 V in 3-phase system whereas 230 V in 1-phase system in India.

  • Distributor – A distributor is a conductor from which tapings are taken for supply to the consumers. Due to the taping is done at various places in a distributor, the current being not same throughout its length. The main design consideration of a distributor is the voltage drop across its length because the statutory limit of voltage variations is ± 6 % of rated voltage at the consumer’s terminals.

  • Service Mains – Service Mains is a small cable which connects the distributor to the consumer’s meter.

Classification of Distribution System

Classification based on the nature of current

  • DC distribution system

  • AC distribution system

Classification based on the type of construction

  • Over-head system

  • Under-ground system

Classification based on the scheme of connection

  • Radial system

  • Ring main system

  • Inter-connected system

Types of AC Distribution System

Primary Distribution System

The primary distribution system is the part of AC distribution system which operates at voltages slightly higher than general utilization. The voltage used for primary distribution depends upon the amount power to be transferred and distance of substation required to be fed. The commonly used primary distribution voltages are 11 kV, 6.6 kV and 3.3 kV. The primary distribution is done by 3-phase 3-wire system because of economic considerations.

Secondary Distribution System

The secondary distribution system includes those ranges of voltage at which consumer utilises the electrical energy. In India, the secondary distribution employs 440V (3-phase) & 230V (1-phase), 3-phase 4-wire system.

Requirements of a Distribution System

Some of the requirements of a good distribution system are −

  • Proper Voltage – The voltage variations at consumer’s terminals should be as low as possible. The statutory limit of voltage variations is ± 6 % (India) of the rated voltage at consumer’s terminals.

  • Availability of Power on Demand – The electric power must be available to the consumers in any amount that they may require from time to time.

  • Reliability – The modern industry is almost dependent on electric power for its operation. This calls for reliable service as much possible.